Astronomical Activities in India (Research and not spacecraft/equipment)

[Indx TechStyle]

https://arxiv.org/pdf/1810.02217.pdf

Amazing!!

 

[Indx TechStyle]

Nov 30, 2018
AstroSat Picture of the Month Nov, 2018
Hot Ultraviolet stars in the Globular Cluster NGC 288

This month, APOM presents ultraviolet images of the globular cluster NGC 288, located at a distance of around 30,000 light years in the constellation Sculptor. This cluster was first described by John Dreyer in 1888. This is the second globular cluster in the APOM series, the first one being NGC 1851, presented a year ago. A globular cluster is a very large group of stars formed from the same cloud of gas and dust at nearly the same time. They are called globular because of the spherical distribution of stars, and each cluster is held together as a single entity by the gravity of the star members. Globular clusters are few of the oldest known objects in our galaxy. The stars in NGC 288 are believed to be nearly 12.6 billion years old.

The Ultraviolet Imaging Telescope (UVIT) on AstroSat has been used to image the stars in NGC 288 in the ultraviolet light, by a group of researchers from the Indian Institute of Astrophysics, Bangalore, and the National Research Council of Canada. The number of stars seen in the far-ultraviolet light are fewer than those seen in the near-ultraviolet light, and the reason for this is that only the hottest stars are seen in the far-ultraviolet.

Since globular clusters have very old stars, many of the heavier ones have already evolved to later phases of stellar lives (eg. red giant stars, horizontal branch stars). About 115 hot horizontal branch stars having surface temperature nearly twice that of the surface of Sun have been detected in NGC 288 using the near and far-ultraviolet filters of UVIT. A couple of very hot stars (extreme horizontal branch stars) whose whose surface temperatures are nearly five times that of the Solar surface have also been found. Using a combination of ultraviolet and optical light, these researchers have also identified 68 blue stragglers in this cluster. A blue straggler is a star formed when two stars either merge or transfer mass between them. Such stars have been mostly found in globular clusters where the star density is high. The capabilities of UVIT have made it easy for the researchers to see and inspect hot stars towards the cluster individually, allowing them to study the properties of each star, such as the mass and temperature.

The paper describing their results can be downloaded here.

Click here for the press story from India Science Wire and here for the entire APOM archive.

Courtesy: ISRO - Government of India

 

[Indx TechStyle]

AstroSat unravels how hot stars evolve in Globular Clusters in the Milky Way
Indian astronomers have peered deep into the Globular Cluster NGC 288, using the Ultraviolet Imaging Telescope (UVIT) on AstroSat to uncover the nature of special kinds of stars.

The image of this cluster is the AstroSat Picture of the Month for November. AstroSat is India's first dedicated multi-wavelength space observatory launched by the Indian Space Research Organisation in 2015.

Galaxies are made of innumerable stars, some of which are single, like our Sun while others are grouped together in clusters. Our Milky Way has over 150 Globular Clusters. These are clusters of hundreds of thousands of stars held together by their own gravity, packed into a size of only few tens of light years. They orbit the centre of the Milky Way as one unit. NGC 288, located around 30000 light years from the earth is one such Globular Cluster that was imaged by UVIT in three different 'ultraviolet colours'.

“Stars are not unchanging objects. They are born, they evolve through various stages of their life, and then they die,” explains Snehalata Sahu, a Ph.D. student at Bangalore-based Indian Institute of Astrophysics and lead author of research paper on NGC 288 published in Monthly Notices of the Royal Astronomical Society.

Stars of different masses go through different paths in their evolution. As they do so, their intrinsic luminosity as well as their surface temperature (and hence colour) changes in different ways depending on their mass. Astronomers name different types of stars which are in various phases of their evolution, by their location in this luminosity versus temperature plot. This study has thrown up interesting results for two such kinds of stars, the Blue Horizontal Branch (BHB) stars and the Blue Straggler (BS) stars in the cluster NGC 288.

The picture of NGC 288 in the optical (left side; credit: ESO/DSS) shows numerous sun-like cooler stars and it is hard to locate hot stars. The image of the same cluster in the ultraviolet (right side, taken by UVIT, yellow is the near-UV and white is the far-UV image; credit: Snehalata Sahu) shows only hot stars as the cooler stars become undetectable.

" Stars are not unchanging objects. They are born, they evolve through various stages of their life, and then they die " : Snehalata Sahu
All stars in a Globular Cluster are born together, and are of the same age. They are also about the same distance from us. “Hotter stars are bluer and are bright in the ultraviolet (UV), where they are best studied,” said Professor Annapurni Subramaniam, a co-author of the paper.

UVIT has the unique capability of imaging a large field of view with high resolution. “This helped us not only image the entire cluster up to its outer most regions but also isolate every individual hot star even in the crowded centre of the cluster,” she added. This gave researchers advantage over previous ultraviolet studies of such clusters. They could measure near-UV and far-UV brightness of 115 HB stars, 68 BS stars and 2 Extreme HB (EHB) stars in NGC 288. Combining this data with already known optical properties of these stars, they could accurately model each of these UV-bright stars individually, and derive their temperatures as well.

“We first looked at the UV properties of the bluer HB stars and could easily distinguish between those stars cooler than 11500 degrees and those that are hotter,” said Sahu. “This change in stellar properties at 11500 degrees is due to atomic diffusion, which alters chemical make-up of outer layers of the star. We have now shown that this break can be easily identified from ultraviolet measurements of these stars and this method can now be used for other clusters as well.”

Professor Kameswara Rao, a co-author of the study, explained that the team could calculate the mass and size of the EHB stars as well. “We were able to identify these as sub-dwarf stars, about five times smaller than our Sun, but more than five times hotter. This was possible mainly because we could measure their ultraviolet properties”.

The location of stars within a globular cluster changes with time as heavier ones settle into the centre. Blue Straggler or BS stars are interesting in themselves. In crowded centres of Globular Clusters, some stars swallow matter from a companion star and sometimes two stars merge to form a new one too. Astronomers believe that BS stars are formed this way, and that’s why their ultraviolet properties are so different. By calculating the mass of each BS star from their ultraviolet data, and by studying their locations in the cluster, researchers could infer the nature of the motion of the stars within the cluster, otherwise known as its dynamical age.

“We have now shown that ultraviolet imaging of Globular Clusters is the easiest and clearest way to study the properties of some of these hot stars with peculiar properties. The superior resolution and large field of view of UVIT will prove crucial in understanding the evolution of these kinds of stars,” said Rao.

The study was conducted by Snehalata Sahu, Annapurni Subramaniam and Kameswara Rao from the Indian Institute of Astrophysics, along with Patrick Cote and Peter Stetson from the National Research Council of Canada.

AstroSat Pictures of the Month (APOM) is a joint initiative of the Public Outreach and Education Committee (POEC) of the Astronomical Society of India and AstroSat Training and Outreach Team. APOM helps in bringing higher energy sky seen by AstroSat closer to people through images. All APOMs are archived at click here India Science Wire.

Source: vigyanprasar.gov.in

 

[Indx TechStyle]

APOM Archives - The Public Outreach & Education Committee of the Astronomical Society of India

 

[Indx TechStyle]

Jan 31, 2019
AstroSat Picture of the Month of Jan, 2019

The 97 minute orbit of AstroSat around the Earth. The orbit is roughly equatorial (top right), inclined at around 6 degrees to it (top left). This results in each orbit being slightly displaced from the previous one (bottom). Pic Credits: Leo Jackson John, Operation Director, AstroSat, ISTRAC, ISRO

When seeing images from AstroSat, have you ever wondered where exactly is the satellite, how does it move, and how do astronomers get their hands on the data? This month's APOM is here to answer those questions for you.

AstroSat was launched by ISRO on 28 September, 2015 from Sriharikota https://www.isro.gov.in/about-isro/satish-dhawan-space-centre-sdsc-shar, on board the PSLV-C30 into its current orbit. This is a low-earth equatorial orbit, at a height of 650 km above the Earth. This orbit is not exactly over the equator, but is inclined at angle of about 6 degree to it. In the top left image, the green line marks the equator and the yellow line marks the orbit of AstroSat and the top right image is a view from over the north pole. But why was this orbit chosen?

Our Earth has a magnetic field (https://en.wikipedia.org/wiki/Earth's_magnetic_field), which behaves overall like a bar magnet (http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/MagEarth.html), with its poles a few degrees from the poles defined by our rotation. These magnetic fields trap charged particles within them, which form the Van Allen belts (https://www.space.com/33948-van-allen-radiation-belts.html). These belts are much closer to the Earth over the southern Atlantic Ocean. An equatorial orbit reduces the effect of this South Atlantic Anomaly https://en.wikipedia.org/wiki/South_Atlantic_Anomaly, on AstroSat which carries very sensitive instruments. Making the inclination exactly zero requires more resources and hence a 6 degree inclination was chosen.

AstroSat takes about 97 minutes to orbit the Earth once. Hence, it will not pass directly overhead the same point in successive orbits. Each orbit, therefore, will be slightly shifted with respect to the previous one. The individual orbits shown in red in the bottom image mark orbits. One orbit per week is plotted for clarity. Data is beamed down from an antenna on the satellite once every orbit, when it passes over India. The data is received by ISRO's dedicated Indian Deep Space Network http://isrohq.vssc.gov.in/VSSC_V4/index.php/ground-segment/82-chandrayaa... antenna in Byalalu https://en.wikipedia.org/wiki/Indian_Deep_Space_Networknear Bengaluru (marked as BLR). All orbits of AstroSat fall within the visibility of this antenna, which is marked by a circle centred at Byalalu. ISRO can also use an antenna in Indonesia, marked BIK, to monitor the satellite when needed. All the command, control and tracking of AstroSat is done by ISTRAC https://www.isro.gov.in/about-isro/isro-telemetry-tracking-and-command-n... in Bengaluru.

As of 30 January 2019, AstroSat has completed more than 18,000 orbits around the Earth, acting as our high energy eye, uncovering the nature of neutron stars, black holes, hot star, and many strange celestial objects. May it continue to do so for many more orbits!

 

[Indx TechStyle]

Mar 08, 2019
Astrosat Picture of the Month of Feb, 2019

New population of Ultraviolet stars in the Globular Cluster NGC 2808

The Sun is a constant presence in our lives and is about 5 billion years old. But will the Sun itself change in the millions of years to come? Any such change will occur so far into the future, that astronomers need to look to alternate places in the sky to understand this. Globular clustersare the best laboratories to study the fate of stars. This month, APOM brings forth a globular cluster called NGC 2808 located at a distance of about 47,000 light years in the constellation Carina. This is the third globular cluster in APOM, after NGC 1851 and NGC 288.

NGC 2808 is one of the most massive globular clusters that we know, with a stellar membership of more than a million stars. Being nearly 11 billion years old, stars like the Sun and heavier stars have evolved to later stages of evolution. Due to the large number of stars present in globular clusters, stars with different masses, and in different evolutionary stagescan be studied together. This is because it is believed that all stars in the cluster formed from the same material at approximately the same time. NGC 2808 is unique because very recent optical studies have shown that it houses many distinct populations of stars (five in this case) within it, the maximum found in any globular cluster till date. Stars at the same evolutionary stage but having similar masses in this cluster seem to have other properties (eg. brightness, material from which it is made) that are slightly different. These are then said to belong to different populations.

The stars that are bright in ultraviolet in this globular cluster have been studied using UVIT on-board AstroSat by a group of researchers from the Indian Institute of Space science and Technology (IIST), Trivandrum and Tata Institute of Fundamental Research, Mumbai. Using ultraviolet light from different wavebands (filters), these authors have identified stars belonging to later stages of stellar evolution, eg. Horizontal Branch stars, hot stars that have passed through the Asymptotic Giant Branch phase. They have also established the presence of four different populations of stars that are seen in the UV, including a new population for the first time. These UV populations of stars are related to the five groups of optical stars mentioned above. Earlier studies had shown the presence of a certain group of UV stars called the Red Horizontal Branch stars in the cluster. The current study has utilized the capabilities of UVIT to report that it is not one group, but rather a mixture of two different populations. This study of the UV populations in the cluster would help in refining our understanding of the formation of multiple populations in globular clusters.

The paper describing the results is accepted for publication by the Monthly Notices of Royal Astronomical Society and can be found here.

Accompanying science story is here.

 

[Indx TechStyle]

Apr 03, 2019
AstroSat Picture of the Month of March, 2019
Sharing the excitement of India's first dedicated space observatory, every month!

A star relives its youth while dancing with its wizened companion in NGC 5466.

This month we bring you yet another Globular Cluster, NGC 5466, located around 52000 light years from us in the constellation Bootes. However, we are going to turn our attention away from the cluster itself, and look at one particular star. This star, called NH 84, is a very special kind of star, and is what astronomers call a Blue Straggler Star, or BSS. Why are these special and how does it relive its youth?

If you have read our previous APOMs on Globular Clusters (here, here and here), you may remember that almost all stars in a cluster are born together at the same time. You may also recall that stars are born, live sedately for a long time, and then die in various spectacular ways. The more massive a star is, the faster it will evolve, and the faster it will die. More massive stars are also usually bluer and hotter, whereas less massive stars are redder and cooler. If we start with a bunch of stars that are born at the same time, like in a Globular Cluster, then as time goes by, we expect to see less and less hot blue stars, since they would have died already. Instead, we would only see the cooler, redder and older stars. Which is why astronomers were very surprised when, in 1953, Allan Sandage found young hot blue starsin old star clusters. How did these stars retain their youth in the face of time? The answer was very surprising indeed, and involved two stars instead of one.

The most common way this happens is in binary star systems, i.e., two stars orbiting each other. Snehalata Sahu of the Indian Institute of Astrophysics and her colleagues imaged the cluster NGC 5466 using the UltraViolet Imaging Telescope on AstroSat and identified many Blue Straggler Stars. In particular, they looked at one of them, NH 84, carefully and discovered that it had to be such a binary system. The bright star was a BSS which had swallowed up material from its companion star, and become more massive and bluer, reliving its youth. The poor companion, though, continued on to become a very hot and dense White Dwarf. How did these astronomers know that the companion is a White Dwarf? They deduced this based on the brightness of NH 84 that they measured in the ultraviolet wavelengths, which is where the White Dwarf shines the most. The BSS itself has a surface temperature of 8000 Kelvin, is about as massive as our Sun, and about 45% bigger. The White Dwarf, on the other hand, is 32000 Kelvin, is about half as massive as our Sun, but only 2% of its size!

This is only the second such BSS–White Dwarf pair that astronomers have found in Globular Clusters. Recently, another team led by Subramaniam had discovered, using the UVIT, another binary system where a BSS was orbiting an evolved aged star whose youth it had stolen. This latest discovery was possible because of the superior resolution and sensitivity of AstroSat in the ultraviolet. The authors are now chasing after the other Blue Straggler Stars in this cluster. Let us wait and see what discoveries await them.

The paper describing the results is accepted for publication by the Astrophysical Journal and can be found here. The accompanying science story, through India Science Wire, is here.

 

[Karthi]

3.6m Optical Telescope at Devasthal, Nainital.


some of the science drivers of DOT are

• Studying magnetic field structure of stars
• Studying chemical evolution of Milky -Way via stellar ISM abundance
• The search for extra solar planets
• Kinematics of stars in the outer region and the halo of the Milky-Way
• Polarization properties of BL-Lac and other galactic nuclei
• NIR spectroscopy and narrow band imaging of the galactic HII regions and star clusters to understand the formation of stars and their evolution in different environment
• Optical spectroscopy and deep time resolved imaging of galactic X-Ray binaries to understand the kinematics of accretion disk
• NIR spectroscopy of debris around stars to understand planet formation

 

[Karthi]

Picture of Saturn obtained during the first night operation of DOT. The seeing of this image was close to 1”.

The camera

 

[Indx TechStyle]

Mystery of a peculiar star SU Lyn resolved by PRL scientists using AstroSat

In a breakthrough, a team of astronomers from the Physical Research Laboratory (PRL), Ahmedabad used the data from Ultra-Violet imaging telescope (UVIT) onboard India’s Astrosat space observatory to resolve the nature of a peculiar star named SU Lyn. They have utilized, for the first time, the UV spectroscopy capability of UVIT and in the process, have thrown new light on the class of stellar objects known as symbiotic stars.

SU Lyn had long been known as an ostensibly unremarkable red giant star – a class of very large and cool stars, which form at the final stages of stellar evolution. However, it was noticed in 2016 that hard X-ray emission was emanating from SU Lyn. This raised the suspicion that the star harboured a hidden, hot companion assumed to be a white dwarf – an end-product when stars of intermediate-mass die. White dwarfs can be as massive as the Sun, yet they have a size similar to the size of the Earth.

The suggestion that SU Lyn could likely host a white dwarf posed a challenge for our understanding of such systems. Binary stellar systems consisting of a white dwarf and a red giant are known as symbiotic systems. In a symbiotic system, the white dwarf and red giant's interaction gives rise to several complex physical phenomena such as an accretion disk, jets, ionized symbiotic nebula, interaction of stellar winds to name a few. Due to this, Symbiotics are considered as one of the most intriguing astrophysical laboratories. A schematic picture of a typical symbiotic system and its various constituents are shown in Figure-1. These symbiotic systems have traditionally been identified and characterized by the presence of intense emission lines of several high ionization species observed in their optical spectra using ground-based telescopes. However, the optical spectrum of SU Lyn was devoid of these lines, raising a question mark on its symbiotic nature.

A more definite way to establish the presence of a white dwarf is through ultra-violet (UV) observations since white dwarfs are hot and emit radiation mostly in the UV range. UV radiation, however, cannot penetrate the Earth’s atmosphere and can only be detected using space-based UV telescopes and instruments. But at present, there are few UV telescopes in space and UV telescopes with spectroscopic capability are even rarer.

This is where India’s Astrosat space observatory and one of its payloads UVIT – the Ultra-Violet Imaging Telescope – played a crucial role. Instruments onboard the observatory are capable of recording the UV spectrum of stars, a feature that proved extremely useful. The PRL team had been observing SU Lyn since 2016 with various Indian observing facilities and a suite of instruments, most notably with the UVIT. From the ground, the star was observed with the HESP instrument on the IIA-HCT telescope, with the indigenous in-house developed MFOSC-P spectrograph and with the Near-Infrared Camera and Spectrometer on the PRL 1.2 m telescope at Mount Abu.

The Astrosat-UVIT spectrum of SU Lyn with the emission lines identified. Archival spectra of three other symbiotic systems (ER Del, SY Mus and AS 210) are also shown for comparison.

The Far-UV (1300-1800 Angstroms) spectrum of SU Lyn, obtained with the Astrosat-UVIT instrument, showed emission lines of silicon (Si IV), carbon (C IV), oxygen (OIII), and nitrogen (N III) in a spectrum typical of symbiotic stars (figure-2). The high-resolution optical spectrum also shows the weak presence of few emission lines, which are typically seen in the optical spectrum of symbiotic stars. The UV spectrum, complemented by optical and NIR spectra, thus, confirms the symbiotic nature of SU Lyn. Using a simple theoretical model to fit the UV observations, it was further shown that the white dwarf in SU Lyn is orders of magnitude less luminous (0.16 solar luminosity) compared to a white dwarf in a traditional symbiotic system (~100-1000 solar luminosity). Instead, the symbiotic phenomenon is predominantly powered by the relatively weaker UV radiation from the accretion disk (0.66 solar luminosity) around the white dwarf. This is the reason that the emission lines are weak in the optical spectrum and why the symbiotic nature of SU Lyn could not be established from ground-based observations earlier.

The resolution of the nature of SU Lyn is a significant result for stellar astronomy. There are only a few hundred symbiotic systems known in our Galaxy. This is in contrast with their predicted population of several hundred thousand. The presence of intense emission lines in low-resolution optical spectra has always been the traditional way to identify and discover symbiotic stars. However, these traditional methods would fail to detect the SU Lyn type of Symbiotics. These recent results by the PRL team have firmly established the existence of SU Lyn type symbiotic systems. It is highly probable that many more symbiotic stars like SU Lyn can exist which have so far evaded the detection by conventional methods. And this could be a reason why a smaller than expected number of symbiotic systems have been discovered so far.

It is equally important to note that these results are derived from a lesser-known spectroscopic capability of the UVIT instrument, which is preliminarily designed as an imaging instrument.

Reference :
Vipin Kumar, Mudit K Srivastava, Dipankar P K Banerjee, Vishal Joshi, UV spectroscopy confirms SU Lyn to be a symbiotic star, Monthly Notices of the Royal Astronomical Society: Letters, Volume 500, Issue 1, January 2021, Pages L12–L16; https://doi.org/10.1093/mnrasl/slaa159

 

[sorcerer]

AstroSat’s Ultraviolet Imaging Telescope spots rare ultraviolet-bright stars in a massive intriguing cosmic dinosaur in the Milky Way
Posted On: 21 JAN 2021 3:42PM by PIB Delhi



Astronomers exploring the massive intriguing globular cluster in our Galaxy called NGC 2808 that is said to have at least five generations of stars have spotted rare hot UV-bright stars in it. These stars whose inner core is almost exposed, making them very hot, exist in the late stages of evolution of a Sun-like star. It is not clear how these stars end their lives as not many of them are detected in these fast-evolving phases, making their study crucial.


Motivated by the fact that old globular clusters referred to as dinosaurs of the universe present excellent laboratories where astronomers can understand how stars evolve through various phases between their birth and death, scientists at the Indian Institute of Astrophysics (IIA) an autonomous institute of the Department of Science & Technology, Government of India, looked out for NGC 2808.


With spectacular ultraviolet images of the cluster from Ultraviolet Imaging Telescope (UVIT) onboard India’s first multi-wavelength space satellite, AstroSat, they distinguished the hot UV-bright stars from the relatively cooler red giant and main-sequence stars which appear dim in these images. The findings of this study have been accepted for publication in the journal ‘The Astrophysical Journal’.


The team of scientists comprising Deepthi S. Prabhu, Annapurni Subramaniam and Snehalata Sahu from IIA combined the UVIT data with observations made using other space missions such as the Hubble Space Telescope and the Gaia telescope along with ground-based optical observations. About 34 UV-bright stars were found to be members of the globular cluster. From the data, the team derived the properties of these stars such as their surface temperatures, luminosities and radii.


One of the UV-bright stars was found to be about 3000 times brighter than the Sun with a surface temperature of about 100,000 K. The properties of these stars were then used to place them on what astronomers call the Hertzsprung-Russel (HR) diagram along with theoretical models to throw light on the characteristics of their parent stars and to predict their future evolution. Most of the stars were found to have evolved from a solar stage called the horizontal branch stars with hardly any outer envelope. Thus they were bound to skip the last major phase of life called the asymptotic giant phase and directly become dead remnants or white dwarfs.


Such UV-bright stars are speculated to be the reason for the ultraviolet radiation coming from old stellar systems such as elliptical galaxies which are devoid of young blue stars. Hence, it is all the more important to observe more such stars to understand their properties.

Figure 1: A false colour image of the globular cluster NGC 2808 obtained using AstroSat/UVIT. The stars as seen using far-UV (FUV) filter are shown in blue colour, and the yellow colour is used to show the stars observed in near-UV (NUV).

 

[Chinmoy]

"worlds first" , wonder y media hype up things like this.

Actually it is the first INTERNATIONAL LMT. But also it is the only LMT in world today.

First light image by ILMT on May 2022.

 

[Indx TechStyle]

From ISRO/IUCAA (Inter-University Centre for Astronomy and Astrophysics) - URL embedded in main text (No Copyvio intended and content has been copied for purpose of archival)
Witnessing the ‘live’ formation of dwarf galaxies with AstroSat's ultraviolet-eye

Figure 1: The background is a 3-color optical image taken by the Hubble Space Telescope. The small box (left) shows a sample dwarf galaxy that was observed with the Ultraviolet Imaging Telescope on AstroSat.​

AstroSat detected extremely blue star-forming clumps on the galaxy's outer boundary (3-color UV-optical image shown in zoomed-in box).​

• Detecting massive young star-forming complexes beyond the visible boundary of faraway dwarf galaxies using the Ultraviolet Imaging Telescope on AstroSat.
• The discovery was made by an international team of astronomers from India, the USA and France, led by Dr. Kanak Saha, Professor of Astronomy, at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune and was published in Nature.
• The discovery is an important part of a Ph.D. thesis of a research scholar, Mr. Anshuman Borgohain, the lead author of the article, from Tezpur University, Assam.
• This is also a success story of the Associateship programme for faculty and students of Indian Universities run by IUCAA, Pune on behalf of the University Grants Commission, Ministry of Education.

Galaxies are the basic building blocks of the Universe- they come in all sizes. Our galaxy, the Milky Way, is one of the giant galaxies-with billions of stars, but little current star formation. Giant galaxies such as ours are surrounded by tens of dwarf galaxies- irregular in shape, often forming stars. As we look backwards in time we see that galaxies were smaller and more irregular (since light takes time to travel, a galaxy seen 3 billion light-years away from a Universe that is 3 billion years younger). How these dwarf and giant galaxies assemble their stars and evolve into modern-day galaxies, like our own Galaxy, is still one of the major puzzles.

A recent study by a team of scientists using AstroSat (India's first dedicated multi-wavelength space observatory) shows how the star-forming complexes in the outskirts of a dwarf galaxy migrate towards the central region and contribute to its growth in mass and luminosity. This process that is now witnessed in several dwarf galaxies is a very important link in understanding the bigger picture of galaxy growth and evolution.

The discovery was made by an international team of astronomers from India, the USA and France. The study was conceived by Professor Kanak Saha at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, and is published as a research paper on July 20, 2022, by the main journal of Nature [DOI: Click here], which is the world’s leading multidisciplinary science journal.

Mr. Anshuman Borgohain, the lead author of the paper, is a young Ph.D. student at Tezpur University, Assam, mentored by Prof. Rupjyoti Gogoi from Tezpur University, Assam who is a visiting Associate at IUCAA and a contributor to this discovery. Anshuman says that “Capturing the assembly process in dwarf galaxies is considered important because the diversity in their physical properties observed today challenge the current theoretical models of galaxy evolution. AstroSat/UVIT has been a remarkable addition to the list of UV observatories to date and has opened up promising windows to probe the understanding of the galaxy assembly process”. While expressing his excitement he also mentioned that this discovery forms an important part of his Ph.D. thesis for which a lot of support was provided by IUCAA and Tezpur University. This has enabled him to achieve this discovery at such an early stage in his career.

Prof. Saha, who conceived the study, has his primary research focused on how galaxies form in the early universe and how they evolve into present-day ones. He says “We are witnessing the ‘live’ formation of these far-way dwarf galaxies! UVIT's resolving power and deep field imaging techniques have been the key to spotting some very young, large star-forming clumps. These form on the periphery and then spiral into the visible (optical) boundary of their galaxy within a billion years timescale thus adding to the growth of the galaxy. A good part of our research work consists of meticulously calculating the time required for the clumps to migrate inside the galaxy”.

He also emphasised that the key challenge has been to firmly establish the detection of these faint, extremely blue, star-forming clumps which are very far away to see although they have a million solar masses of material within them. At slightly larger distances, the UVIT would not resolve these galaxies and we do not have an example of an extended disk seen in UV in any present-day dwarf galaxies. The redshift (cosmological distance) of these 12 dwarfs has been just optimal to probe these blue clumpy structures in their outskirts.

Prof. Francoise Combes of Observatoire de Paris, France, another coauthor further added that “the discovery teaches us how surprisingly stars can form in metal-poor gas disk. Normally these dwarf galaxies are dominated by dark matter and the gas disk would not be unstable. But our discovery is direct evidence that even such a gas disk fragments.” Prof. Shyam Tandon, who is also a co-author of this study, has wondered whether these clumps could have been sources of Lyman continuum photons.

“It has been a mystery how some small galaxies like these can have such active star formation” says Prof. Bruce Elmegreen of IBM Watson Research Division, USA, who contributed to the study and mentioned that these observations suggest that accreting gas in the far outer parts can be forced to move towards the centre because of the inward torques exerted by giant gas and stellar complexes. This migration builds up the central density over the lifetime of the galaxy.

Prof. Somak Raychaudhury, Director of IUCAA, Pune, points out how this is another major success story for the visiting Associateship programme of the UGC at IUCAA: “There are currently 200 such associates who visit IUCAA with their students from time to time and always interact online, and many collaborate among themselves on national and international projects, with or without IUCAA faculty. Major national facilities thus get connected to the educators and students who form the bulk of the researchers in India. At IUCAA, we train many of them on how to use facilities such as ISRO's AstroSat, and enable access to the resources that are necessary for world-class research”

The team is thankful and privileged to have such a state-of-the-art observational facility, developed and operated by the Indian Space Research Organisation in collaboration with several Indian and foreign research laboratories, that led to this important discovery. Saha mentions that future endeavours to create such facilities would ensure the continuity of scientific excellence in India.

About the lead author:
Anshuman Borgohain is a DST-INSPIRE fellow and a research scholar at Tezpur University who worked under the guidance of Prof. Kanak Saha of the Inter-University Centre for Astronomy and Astrophysics, (IUCAA) Pune who mentored him remotely through IUCAAs Associateship Programme. Under this programme, Prof. Rupjyoti Gogoi from Tezpur University who is an associate at IUCAA jointly along with Prof. Saha mentored Anshuman during his entire research journey. Prof. Gogoi said, “the current work is an inspiration to young researchers of the country as this utilises data from India's indigenous satellite, AstroSat and also showcases the glorious association of IUCAA and a university, which surely will motivate the researchers working in Indian Universities. We look forward to enhancing this type of collaborative endeavour between IUCAA and Tezpur University”.

About the Associateship programme at IUCAA
A programme wherein researchers from all over the country can avail the infrastructure facilities available at IUCAA to pursue their research while they continue to remain affiliated with their parent institution. The associate who is either a faculty member of an Indian university or a post-graduate department in a college carries out the research in their institution with scheduled short and long-duration visits to IUCAA and collaborates with scientists from the institution. It aids research scholars from remote areas of the country to contribute toward cutting-edge research and work with the best in the field.

More about UVIT & AstroSat
AstroSat was launched on Sept. 28, 2015, by the Indian Space Research Organization (ISRO) and has onboard it the indigenously developed UltraViolet Imaging Telescope (UVIT). The 38-cm diameter UVIT is capable of simultaneous imaging in far and near-ultraviolet bands with a wide field of view. It was developed by teams from IIA, IUCAA and TIFR from India, and CSA of Canada under the leadership of Shyam Tandon, Ex Emeritus Professor, IUCAA. The development of all the instruments for AstroSat was strongly supported by ISRO. This work is the second Nature article which uses the AstroSat UV Deep Field South (AUDFs), see the webpage: audf.iucaa.in

Copyright © 2022 The IUCAA. All rights reserved.

 

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From ISRO/IUCAA (Inter-University Centre for Astronomy and Astrophysics) - URL embedded in main text (No Copyvio intended and content has been copied for purpose of archival)
Black hole bonanza: India’s AstroSat witnesses black hole birth for the 500th time

Figure 1: Positions of all 500 Gamma Ray Bursts detected by AstroSat CZTI. Credit: AstroSat CZTI team / Aswin Suresh, Gaurav Waratkar, Varun Bhalerao (IIT Bombay).​

The background is an optical view of the night sky (Background image credit: ESA/Gaia/DPAC).​

​

Black holes are the ultimate cosmic enigmas: objects with a gravitational pull so strong that not even light can escape. They are the subjects of intense scrutiny from astronomers from all over the world. But how are these black holes formed? Indian scientists are making great headway in studying black hole births using our very own space telescope – AstroSat.


One way of forging black holes is the deaths of massive stars in “Gamma Ray Bursts ” – explosions so powerful that they have been called “mini big-bangs”. They send intense jets of light and high-energy radiation shooting across the universe. Another way to create Gamma-ray Bursts (GRBs) is the collision of two neutron stars – the kind of events that generate gravitational waves which LIGO-India will start detecting in the near future. Astronomers study the Gamma-rays and X-rays from such bursts to better understand explosion and black hole formation.

AstroSat is one of the most sensitive space telescopes in the world - consisting of five instruments that can simultaneously study the universe in ultraviolet, optical, and X-ray radiation. One of these instruments is the Cadmium Zinc Telluride Imager (CZTI) - which has just witnessed the birth of black holes for the five hundredth time. “This is a landmark achievement”, said Prof. Dipankar Bhattacharya of Ashoka University and IUCAA, the current Principal Investigator of CZTI. “The wealth of data obtained by CZTI on Gamma Ray Bursts is making a big impact worldwide.”

CZTI has been studying GRBs since it first opened its eyes 6.5 years ago. “The very first scientific result from AstroSat was the detection of GRB 151006A: just hours after the instrument was powered on after launch”, said Prof. Varun Bhalerao, who leads the GRB search effort. Numerous GRB studies from CZTI have been published in reputed journals worldwide. The team has worked continuously to improve the search and detection methods, getting better results every year. A key highlight of the searches is the role played by young scientists in the process: large parts of the search procedures have been developed by undergraduate students, Ph.D. students, and trainees. They are also the group responsible for daily analysis of data. Ph.D. student Gaurav Waratkar says, “It is very exciting to work on this. Every time we look at the data, I am tantalised by the possibility of discovering a signal that originated billions of light years away!”

A unique aspect of CZTI is the ability to measure the “polarisation” of X-rays: an ability that is lacking in flagship missions like NASA’s Neil Gehrels Swift Telescope or the US-Europe Fermi Space Telescope. Tanmoy Chattopadhyay (Stanford university) plays a key role in these polarisation studies. Tanmoy says, “Polarisation tells us what is happening just outside the newly formed black hole. It is the most important measurement to distinguish between different theories of Gamma-ray Bursts”.

Animation Video :

​

Animation showing all the positions of all 500 Gamma Ray Bursts detected by AstroSat CZTI.​

Animation credit: AstroSat CZTI team / Aswin Suresh, Gaurav Waratkar, Varun Bhalerao (IIT Bombay).​

The background is an optical view of the night sky. (Background image credit: ESA/Gaia/DPAC)​

Acknowledgements:
CZT–Imager is built by a consortium of Institutes across India. The Tata Institute of Fundamental Research (TIFR), Mumbai, led the effort with instrument design and development. Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram provided the electronic design, assembly and testing. The U R Rao Satellite Centre (URSC), Bengaluru provided the mechanical design, quality consultation and project management. The Inter University Centre for Astronomy and Astrophysics (IUCAA), Pune did the Coded Mask design and instrument calibration, and runs the Payload Operation Centre. Space Application Centre (SAC) at Ahmedabad provided the analysis software. Physical Research Laboratory (PRL), Ahmedabad, provided the polarisation detection algorithm and ground calibration. The Indian Institute of Technology Bombay (IITB), Mumbai leads the search and study of Gamma Ray Bursts, working closely with the Payload Operations Centre at IUCAA. A vast number of industries participated in the fabrication and the University sector pitched in by participating in the test and evaluation of the payload. The Indian Space Research Organisation funded, managed and facilitated the project.

 

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Beginning from this
Amateur astronomers in ISRO capture rare celestial event with minimal equipment

Amateur astronomers in ISRO, Fahd Bin Abdul Hasis, Kiran Mohan, and VishakSasidharan from Liquid Propulsion Systems Centre (LPSC), Valiamala have photographed SN2023ixf using a modest setup consisting of a DSLR camera and basic sky tracking equipment, despite the challenging climatic conditions.

The images below showcase the progression of SN2023ixf over time. Comparing two photographs taken on May 19, 2020 and May 22, 2023.

Renowned amateur astronomer Mr. Koichi Itagaki discovered SN2023ixf on May 19, 2023. This remarkable celestial event, classified as a Type-II supernova, is located in the Pinwheel Galaxy (M101), approximately 21 million light years away from Earth. Supernovae are awe-inspiring phenomena that result from the explosive death of massive stars.

What's truly remarkable about the photograph by the amateur astronomers from ISRO is that the team captured this celestial event using a Nikon Z6 ii camera equipped with a Samyang 135 mm lens at f2.8 and ISO 1000, along with the iOptronSkyGuider pro as their sky tracking device. This simple setup allowed them to record the supernova's evolving appearance in the night sky.

The image processing techniques employed by the team involved stacking multiple frames to enhance the details of SN2023ixf. On May 22, 2023, they stacked 107 light frames of 20 seconds each, totaling 35 minutes of exposure time (shot at 135 mm and cropped). Similarly, May 19, 2020 photo was stacked from 107 light frames of 25 seconds each, totaling 45 minutes of exposure time (shot at 300 mm).

The passion, dedication, and ingenuity of amateur astronomers demonstrating that some rare celestial events can be observed and captured even with basic equipment, given the right skills and determination is praiseworthy. The wonders of the universe are within reach for those who dare to explore and observe.

 

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Scientists Discover an extreme Massive Giant and Most Dense Exoplanet
May 26, 2023

A new Jupiter size exoplanet with highest density of ~14 g/cm3 known till this date, and mass 13 times that of Jupiter has been discovered by an international team of scientists led by Prof. Abhijit Chakraborty at the Exoplanet Research Group of the Physical Research Laboratory (PRL), Ahmedabad. The team includes scientists from India, Germany, Switzerland and the USA. The discovery of this massive exoplanet was made using the indigenously made PRL Advanced Radial-velocity Abu-sky Search spectrograph (PARAS) at the 1.2 m telescope of PRL at its Gurushikhar Observatory in Mt. Abu by measuring the mass of the planet precisely.

The newly discovered exoplanet is found around the star called TOI4603 or HD 245134. NASA’s The Transiting Exoplanet Survey Satellite (TESS) initially declared TOI4603 as a possible candidate to host a secondary body of unknown nature. Using PARAS, scientists discovered it as a planet by measuring the mass of the secondary body and hence the planet is called TOI 4603b or HD 245134b. It is located 731 light years away. It orbits a sub-giant F-type star TOI4603 every 7.24 days. What sets this discovery apart is that the planet falls into the transition mass range of massive giant planets and low-mass brown dwarfs with masses ranging from 11 to 16 times the mass of Jupiter. Only fewer than five exoplanets are currently known in this mass range so far.

Massive giant exoplanets are those having mass greater than four times that of Jupiter. The newly discovered exoplanet TOI 4603b is one of the most massive and densest giant planets that orbits very close to its host star at a distance less than 1/10th the distance between our Sun and Earth. The exoplanet with a surface temperature of 1670 K is likely undergoing high-eccentricity tidal migration with an eccentricity value of approximately 0.3 The detection of such systems provides valuable insights into the formation, migration, and evolution mechanisms of massive exoplanets.

This discovery marks the third exoplanet discovery by India, and by the PRL scientists using PARAS spectrograph and the PRL 1.2m telescope, following the discoveries in 2018 (K2-236b) and 2021 (TOI-1789b).

The findings of this study is published in the journal Astronomy & Astrophysics Letters. The publication titled "Discovery of a massive giant planet with extreme density around the sub-giant star TOI-4603." provides details of the discovery and a comprehensive account of exoplanet's characteristics.Click here to access the publication.

Figure 1. The figure on the left shows an artistic impression comparing the distances between the TOI-4603 star-planet system and the Sun-Mercury and Sun-Jupiter systems. It is noteworthy that the TOI-4603b planet, which has the same size as Jupiter, is situated more than 50 times closer to its star than Jupiter is to the Sun. On the right is a comparison between the TOI-4603b planet and Jupiter, which is 13 times more massive than Jupiter.

Figure 2. Planetary density as a function of planetary mass for transiting giant planets and Brown Dwarfs (0.25–85 Jupiter Mass, MJ) is plotted. The shaded area represents the overlapping mass region of massive giant planets and Brown Dwarfs based on the deuterium burning limit, and the dotted lines are at Planet Mass = 13 MJ and 85 MJ. The position of discovered exoplanet (TOI-4603 b) is denoted by the red filled circle clearly showing that it is the densest exoplanet seen till today.

 

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Likely by evening, I will replace all broken image links with attached photographs of galaxies and nebulas.

 

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I have edited restored most dead images and links. I will shortly again upload all the pictures of month from APOM archives.

 

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Astrosat Picture of the Month (APOM) archives - Public Outreach & Education Commitee (POEC) - Astronomical Society of India

#001 September 2017

This APOM shows us newly formed star clusters in the dwarf galaxy Wolf-Lundmark-Melotte (WLM), imaged by the Ultra Violet Imaging Telescope on board the ASTROSAT.
(Picture Credits: Annapurni Subramaniam)
To download a high resolution image, see link.

Spoiler: About WLM

WLM: a dwarf galaxy, efficiently forming stars in grand isolation
Located in the constellation Cetus, 3 million light years away, is a faint dwarf galaxy,Wolf-Lundmark-Melotte, or WLM for short. It is relatively isolated, lying in the outskirts of our Local Group of Galaxies. It has a mass that is thousands of times less than the Milky Way and a metallicity that is only 13% solar. Lower metallicity implies less heavier elements, which in turn hinders forming new stars. So why did AstroSat even look at this galaxy?
WLM is a dwarf irregular galaxy with a low mass and metallicity and exists in solitude. Nevertheless, it manages to form new stars extremely efficiently. Adjusted for their respective masses, WLM forms stars at a rate that is 12 times higher than our own Milky Way! Astronomers are still not sure as to how WLM does this.
Annapurni Subramaniam and her student Chayan Mondal at the Indian Institute of Astrophysics in Bengaluru wanted to understand how this diminutive galaxy is such an efficient star factory. They decided to use the Ultra Violet Imaging Telescope on board the AstroSat to image the younger star clusters in WLM. In this image, the blue dots are the star clusters imaged in Far Ultra-Violet (130-180 nm) and the yellow dots are those imaged in Near Ultra-Violet (180-300 nm). They are currently analysing this data and will soon be able to fit one more piece into the puzzle that is WLM.

#002 October 2017

The AstroSat Picture of the Month for October 2017 is the Near-UV image of the beautiful barred spiral galaxy NGC 2336, taken by UVIT on board the ASTROSAT. The bright spots along the spiral arms are regions of active star formation.
(Picture Credits: UVIT Team)
To download a high resolution image, see link.

Spoiler: About NGC 2336

NGC 2336: A classic spiral galaxy captured in fine detail by the UVIT
NGC 2336 is a magnificent barred spiral galaxy located in the northern constellation of Camelopardalis, or the giraffe. At a distance of 105 million light years away from us, it can even be seen through medium-sized amateur telescopes under dark skies. This galaxy was discovered by the German astronomer Ernst Tempel in 1877. NGC 2336 has a highly developed and splendid spiral arm structure that emanates from a ring of stars surrounding a central bar. The spiral arms contain a number of star forming regions, or nebulae. These nebulae shine because of hot young stars that are bright in the ultraviolet.
This was one of the first objects chosen to be imaged by the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat, in order to test its ability to resolve complex structure. The Near-UV (200-300 nm) and Far-UV (130-180 nm) images obtained were spectacular, showing details finer than in the image from the GALEX ultraviolet telescope. Astronomers found that the resolution of UVIT was 1.2 arc-seconds in the Near-UV and 1.5 arc-seconds in the Far-UV, which was much better than the initial goal of 1.8 arc-seconds. This superior resolving power, along with its large field of view, make UVIT an excellent instrument for investigating star formation in large galaxies like NGC 2336.

#003 November 2017

The AstroSat Picture of the Month for November 2017 are the Near-UV (left) and Far-UV (right) images of the Globular Cluster NGC 1851, taken by UVIT on board the ASTROSAT. The FUV image shows only the hottest stars in the cluster. All colours are artificial.

(Picture Credits: Annapurni Subramaniam and collaborators )

To download a high resolution image, see link.

Spoiler: About NGC 1851

NGC 1851: One cluster, two populations
NGC 1851 is a Globular Cluster which is almost 40000 light years away from us, in the southern constellation of Columba, near Canis Major. A Globular Cluster is a group of stars tightly bound together by their own gravity. All the stars in these spherical clusters orbit around the centre of our galaxy together. NGC 1851, or Caldwell 73, is one such cluster, visible in a moderate telescope at a magnitude of 7.3, with a size that is a third of the full moon. It was discovered by James Dunlop from Australia in 1826.
The stars in a Globular Cluster are usually born together, and hence share similar properties. However, NGC 1851 is one of the few clusters where two distinct types of stars with different properties seem to co-exist! Many individual stars in this object have been studied before with the HST, but good ultraviolet images were needed to understand this mystery better. This prompted a group of 18 astronomers, including 12 from India, to use the UVIT on board the AstroSat. They imaged this cluster in the Near and Far ultraviolet wavebands far better than earlier attempts with other telescopes.
The superior resolution of AstroSat allowed them, for the first time, to measure the ultraviolet properties of individual stars in the inner crowded region of the cluster. Using this data, they could show that NGC 1851 does indeed have two distinct families of stars within it, which still retain their separate histories. This tells us that NGC 1851 was probably formed when two smaller clusters merged together some time in the past!
The paper describing these results can be downloaded from here.

#004 December 2017

The AstroSat Picture of the Month for December 2017 are the the ultra-violet images of NGC 40 using ASTROSAT. The pink-red part is the gas cloud NGC 40, also known as the Bow-Tie nebula, which is being illuminated by its central hot star. The new discovery is that of the halo region surrounding the nebula. It is observed to be glowing in diffuse, ultra-violet light and shown as golden here in this false-colour image.

(Picture Credits: Kameswara Rao and collaborators )

To download a high resolution image, see link.

Spoiler: NGC 40

NGC 40: A Planetary Nebula with an Ultra-Violet Halo
NGC 40, or the Bow-Tie Nebula, is a Planetary Nebula about 3500 light years away from us, in the northern constellation of Cepheus. Discovered by William Herschel in 1788, it can be seen in a moderate sized telescope by amateur astronomers. Earlier optical images of NGC 40 show a central star as hot as about 70000 K surrounded by expanding gas that gives it its characteristic shape. The central hot star is blowing a fast hot wind into this surrounding gas at 1700 km/s, and heating it up.
Kameswara Rao and his colleagues used the AstroSat to image this object in many regions of the ultra-violet. First, however, let us look at what a planetary nebula is. Some old Red Giant stars throw out their outer layers of gas, which expands away from the star. This exposes the hot inner part of the star, whose radiation makes the outer gas layers shine brightly as a planetary nebula. Our Sun too, will meet this same fate. With time, the inner star will evolve to become a White Dwarf, a very strange object indeed. And the heavier elements cooked inside the stars, thrown out into space, go back into forming newer stars and planets like ours.
The astronomers who looked at the ultra-violet images of NGC 40 using AstroSat, were not only able to study the central hot star and the surrounding gas, but also made a new discovery. As the image shows, they discovered, for the first time, a large halo of ultra-violet radiation surrounding the entire nebula. This halo, they figure out, is due to molecules energized due to the light from the central star.
The paper describing these results can be downloaded from here.

#005 February 2018

The AstroSat Picture of the Month for February 2018 is the ultra-violet image of NGC 6960 or the Witch's Broom, using ASTROSAT. The beautiful filaments are due to gas heated up because of the shocks from the Supernova explosion that happened thousands of years ago.

(Picture Credits: Firoza K. Sutaria, K.P. Singh, P. T. Rahna, J. Murthy, A.K. Ray, N.K. Rao & A. Kumar )

To download a high resolution image, see link.

Spoiler: About 6960 aka Witch's Broom

The Witch’s Broom
The Witch's Broom or the Western Veil, is a part of a large Supernova Remnant called the Cygnus Loop or the Veil Nebula. Extending over 3 degrees in the sky (compared to the full moon which is 0.5 degrees), and located in the northern constellation of Cygnus, the entire Cygnus Loop is 75 light years in diameter, and around 1470 light years away. Though the nebula is one of the most beautiful and colorful objects in the sky, it is quite faint due to its large angular size and a big telescope in a dark sky is needed to fully appreciate it in all its glory.
Different parts of this object were discovered separately and given different names. The Witch's Broom, or NGC 6960 is a part of this gigantic Supernova Remnant. This remnant is the result of a very massive star exploding sometime between 3000 and 6000 B.C. The shock waves of this explosion, as they blast through the surrounding gas, produce emission in all bands of light, including radio, visible, ultra-violet and X-rays. Since the expanding shells are extremely thin and is almost transparent to background optical light, only the edges are bright enough to see. This is why we see fine filaments or ropes that resemble a broom.
The Near Ultra-Violet and Far Ultra-Violet images of the Witch's Broom captured by AstroSat's UVIT show emission from these delicate glowing filaments, primarily from ionized Silicon, Carbon, Iron and Helium. Astronomers are using this data to study the chemicals in this gas, and how they are heated by the shock of the explosion.

#006 March 2018

The AstroSat Picture of the Month for March 2018 is the ultra-violet image of NGC 7252 or the 'Atoms for Peace' galaxy, using ASTROSAT. The two tails of gas and stars ripped out due to the merger of two galaxies can be seen in the image on the left (going upwards and also downwards and right). The central part, which hosts filaments and loops, is magnified and shown on the right.

(Picture Credits: Koshy George )

To download a high resolution image, see link.

Spoiler: About NGC 7252 aka Atoms for Peace

The Ultraviolet Tails of the “Atoms for Peace” Galaxy
A billion years ago, two massive galaxies collided together, ripping apart tails of gas and dust from each other. The two galaxies then merged to form a single galaxy, NGC 7252. Located 220 million light years away in the southern constellation of Aquarius, the optical images show stars and gas twirling around in intricate filaments around a bright central core. These loops resemble a picture of electrons going around the nucleus, and hence NGC 7252 was named 'Atoms for peace' galaxy, in honour of the influential speech by the US President Eisenhower in 1961, where he advocated the use of nuclear energy for peaceful purposes.
The effects of the violent collision are evident from the furious star forming activity going on in the two tails of the galaxies. These tails of gas and stars were pulled out from the galaxies in their dance around each other before their merger. Since ultraviolet light traces young hot stars, Koshy George and his collaborators used the UVIT of AstroSat to trace the locations of ongoing star formation along the tidal tails. Using their data, they were able to measure the rate at which new stars are being formed, and study how this rate changes as we move along this tail.
These studies have been published in this paper.

#007 April 2018

The AstroSat Picture of the Month for April 2018 is the ultra-violet image of the Jellyfish Galaxy JO201, using ASTROSAT. This image shows hot young stars formed in the tentacles, visible to the left of the parent galaxy. These have been formed from the gas stripped from JO201 as it falls through the galaxy cluster.

(Picture Credits: This image is from the work done as part of an international collaboration (GASP))

To download a high resolution image, see link.

Spoiler: About JO201 aka Jellyfish Galaxy

A Jellyfish in the Sky
Jellyfish are gelatinous creatures of the sea, but did you know that there are giant jellyfish in the sky as well? Here we present an ultraviolet image of one such 'Jellyfish Galaxy', JO201. We know that galaxies usually cluster together because of their mutual gravity. These Galaxy Clusters can have 100s to 1000s of galaxies in them! In these clusters, the space between these galaxies is filled with very hot gas. When an external galaxy is attracted by the gravitational force of a cluster, it moves through this hot gas. When it does so, it feels this gas as a wind that is blowing against it, much like a bicyclist or a runner feels the air blowing back past them.
If the galaxy is falling fast enough through this gas, then this wind can even dislodge the gas from this galaxy's gravitational pull. This stripped-off gas forms tails behind the infalling galaxy, and new stars can form out of this gas. Hence, these galaxies resemble a jellyfish with tentacles. We know a number of such jellyfish galaxies in the sky, and this image shows JO201, one such galaxy falling into the galaxy cluster Abell 85. The ultraviolet image taken by AstroSat shows the hot young stars formed from the gas that have been pushed out of this galaxy. The galaxy JO201 itself is moving in a direction towards us with an inclination towards the right.
Koshy and his collaborators have used the superior resolution of UVIT to study how stars are being formed in individual parts of the tentacles of the jellyfish galaxy JO201. The paper describing their work can be found here.

#008 May 2018

The AstroSat Picture of the Month for May 2018 is the X-ray data from the Crab Nebula, using the ASTROSAT. The left panel shows the new result. One rotation period of the Crab Pulsar, of 33 milliseconds, is represented as the phase going from 0.0 to 1.0 and is repeated once more, from 1.0 to 2.0, for clarity. The grey line is the X-ray brightness of the Crab lighthouse, observed by CZTI. The data in colour, obtained by aggregating a large number of measurements, shows what fraction of the X-ray light is polarised, i.e., can be given a specific direction. It can be seen that when the X-ray emission (grey line) is low, the polarisation fraction is high, which is unexpected. The right panel is an artist's impression of how the supernova explosion would have looked like, in the past (credit: ESA/Hubble).

Spoiler: About Crab Nebula

X-raying the celestial lighthouse in the Crab
This month's APOM is a bit different from the rest. Instead of ultraviolet images from UVIT, we bring you an exciting plot from the X-ray telescope, CZTI, onboard the AstroSat. This new result, covered by the press last year (here, here & here), announced the discovery of X-ray polarisation from the Crab Nebula that seemed to vary in an unexpected way over the period of the pulsar within. This discovery also heralded the beginning of the field of X-ray polarisation in astronomy. Let us look at what this means.
The Crab Nebula which is about 6500 light years away in the constellation Taurus, is a result of a supernova explosion of a massive star, that was seen in 1054 AD. Powering this supernova remnant is a very strange object, known as the Crab Pulsar. The end products of massive stars are neutron stars, which weigh as much as the Sun but are only as big as a city. These rapidly spinning objects are made of exotic matter and have very strong magnetic fields. Under the right circumstances, we see them as a pulsar, which resembles a celestial lighthouse whose beam sweeps past the Earth once every pulsar rotation. Our Crab Pulsar rotates as fast as 30 times a second, and its lighthouse-like pulses have been studied extensively all the way from radio to X-rays.
We usually describe light, which is electromagnetic radiation, by its strength and its frequency (or wavelength). In addition, we can also measure the direction in which the electric and magnetic fields of the light wave oscillates. This feature, called polarisation, is even used to make some kinds of 3-D glasses and anti-glare sunglasses. Light from celestial sources too shows polarisation but measuring this in the X-rays in incredibly difficult. So difficult, in fact, that this was seen in 1976 from the same Crab Nebula, but not from any other source since then. What Santosh Vadawale and his collaborators have done, is to use a clever trick that has been talked about for a while, and apply it to the Cadmium-Zinc-Telluride Imager (CZTI) on AstroSat which is a X-ray telescope. They perfected this technique and were able to measure X-ray polarisation accurately from the Crab Nebula. In fact, they even managed to measure it fast enough to derive how this polarisation changed during one full rotation of the Crab Pulsar, which is 33 milliseconds long. The result, which is our APOM, surprised everybody. For one, they found that the amount of polarisation was higher than expected. Second, it was high during those times when the pulsar lighthouse was pointing away from us! Astronomers are now revisiting their theories of pulsar emission to understand this. In any case, AstroSat has now opened up this new and exciting field of X-ray polsarisation. Who knows what they will discover next!
For more information, see the original press release as well as a description by the ASI POEC.

 

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Continued...
#009 June 2018

The AstroSat Picture of the Month for June 2018 is the ultraviolet image of the merging galaxy cluster Abell 2256, more than 800 million light years away. This cluster contains galaxies spread over a large area, and we have zoomed in on six of these galaxies to show you their ultraviolet images. The brightest objects in the full image are actually foreground stars in our galaxy which happen to lie in the same direction as Abell 2256.

Picture Credits: UVIT team/ISRO/CSA.

Spoiler: About Abell 2256 Galactic Cluster

Entire clusters of galaxies merging together – an ultraviolet view
Previously, we had brought you AstroSat images of individual galaxies, two galaxies merging with each other, and even a lone galaxy falling into a cluster of other galaxies. This month, we give you Abell 2256, an extremely well studied and special galaxy cluster. Abell 2256 is actually made of three separate clusters of galaxies that are all merging with each other, and will form a single massive cluster in the future. This object is at a distance of more than 800 million light years from us, and is the most distant APOM so far. The three merging clusters in Abell 2256 contains more than 500 galaxies, and the cluster is almost 100 times larger and more than 1500 times as massive as our own galaxy! This merger has produced a rich diversity of structures that have been imaged in radio wavelengths by every radio telescope in the world.
We know that galaxy clusters are places where many spiral galaxies transform slowly into lenticular and elliptical galaxies. Spiral galaxies, like our own Milky Way, are bluer in colour and are forming stars constantly. Elliptical and lenticular galaxies however, are redder and have mainly old stars in them. Abell 2256 is one such galaxy cluster where we believe many galaxies are going through this metamorphosis. Astronomers stared at Abell 2256 for 5 hours using the UVIT on board AstroSat to image these star forming spiral galaxies, using the ultraviolet light emitted by their hot young stars. The fine detail with which the entire galaxy cluster could be imaged out to its edges by UVIT is keeping astronomers busy over the last few months. They are investigating the nature of individual galaxies in Abell 2256. They also hope to understand how these galaxies will transform into lenticular and elliptical galaxies in the future.

#010 July 2018

The AstroSat Picture of the Month for July 2018 is an image of the central part of the merging galaxy NGC 7252. This image has been obtained by dividing the NUV by the FUV image, pixel by pixel, indicating 'ultraviolet colour'. The outer redder parts are where the stars older than 400 million years are, the blue ring is the location of younger stars which are 250 million years or younger.

Picture Credits: Koshy George et al.

Spoiler: About Central Portion of NGC 7252

Seeing a galaxy in ‘colour’
We have met NGC 7252 before, where we had shown you an ultraviolet image of the loops and tails of gas and dust ripped apart from two galaxies as they merged to form NGC 7252. This month, we zoom in to look at the central part of the merging galaxy itself, but in colour!
UVIT onboard ASTROSAT imaged this galaxy in the Near UV (around 242 nanometres) and the Far UV (around 148 nanometres). When we divide these two images pixel by pixel, we get an 'ultraviolet colour' image. Imagine a light bulb emitting all visible colours. We will perceive it as a particular colour depending on the fraction of red versus green versus blue light it emits. Similarly, dividing the FUV and the NUV images will tell us what the 'ultraviolet colour' of NGC 7252 is, and how this colour varies across the galaxy. These 'ultraviolet colours' are represented as red to blue in the image here.
In the figure, we see a central 'red' region surrounded by a 'blue' ring, with an outer 'red' region. The size of the image is marked in arcseconds (3600 arcseconds make a degree) and equivalently in kpc (1 kpc is 3260 light years). Koshy and his collaborators have modelled this colour to calculate the corresponding ages of the stars that emit in these regions. They have shown that there is a bunch of stars right in the middle, which are around 320 million years old. The surrounding blue ring has stars that are around 250 million years old. The even bluer clumps within the ring are only about 150 million years old. The surrounding larger region of red has stars that are more than 400 million years old.
Remember, NGC 7252 is born out of two galaxies merging together. This merger is a violent event whose effects on the gas in these galaxies is complex. The authors of this study explain how this merger would naturally lead to this scenario where different parts of the galaxy hosts stars of different ages. These have been discussed in detail in their paper.

#011 August 2018

The AstroSat Picture of the Month for August 2018 is an X-ray image of the Tycho Supernova Remnant in the 0.8-2.0 nanometres (0.6-1.6 keV) range, made by the Soft X-ray Telescope on board AstroSat. The supernova remnant is roughly 8 arcminutes big (3.7 times smaller than the full moon in the sky) and the emission is brighter near the edge of the expanding supernova remnant.

Pic Credit: Kulinder Pal Singh (IISER Mohali) and the entire SXT Instrument and POC teams at TIFR, University of Leicester, and IUCAA

http://www.iucaa.in/

Spoiler: About SN1572 aka Tycho Supernova

X-raying a Supernova Remnant
This month, for the first time, we bring you an X-ray image from AstroSat. We feature the image of the Tycho Supernova remnant or SN 1572, imaged by the Soft X-ray Telescope (SXT). Located in the constellation Cassiopeia, at a distance of about 10000 light years, SN 1572 is a historic object. It is one of the 8 supernova explosions that were seen with the naked eye. This new star appeared in the sky during early November in 1572, and was observed by many astronomers across Europe and China. It is named after Tycho Brahe since he was the one who studied it in great detail till it faded away in 1574. He published his observations in his work 'Concerning the Star, new and never before seen in the life or memory of anyone', which included a star chart too. At its peak, it rivalled Venus at its brightest, confounded astronomers at that time, and changed their perspective of an unchanging sky.
We now know, from historic data, that this was a Type 1a supernova explosion. Sometimes, a normal star and a white dwarf (which is a very compact object that is the end stage of stars like our Sun) orbit each other. Material from the normal star is pulled on to the white dwarf due to gravity, making it heavier. When the mass of the white dwarf exceeds the famous Chandrasekhar Limit, it explodes, leading to a Type 1a supernova, like our SN 1572. What we see today is what is left of this explosion. The debris is expanding outwards like a sphere, with an edge which is the shock front. This supernova remnant, discovered first in radio wavelengths, and then in optical and X-rays and infrared, is a beautiful object indeed.
X-rays can penetrate metal easily. Hence, the cleverly designed Soft X-ray Telescope uses 320 concentric gold coated mirrors and a very cold CCD to form images in the X-ray. The image of the Tycho Supernova remnant shown here is made from photons with wavelengths between 0.8 to 2.0 nanometres (0.6-1.6 keV). Most of this emission, coming from the limb of the expanding shell, is due to emission from Iron atoms where electrons jump from higher levels to the 2nd level.
The paper can be downloaded here.

#012 September 2018

The AstroSat Picture of the Month for September 2018 is a photograph of AstroSat itself. The top panel has two photographs of the fully assembled AstroSat. The bottom panel is an artists's conception of the observatory in space. Can you identify each of the five telescopes in the top panel photographs?

Pic Credit: Top panel - ISRO; Bottom panel - ASI POEC, ISRO and Adrita Das

Click here for a full resolution image.

Spoiler: About AstroSat

AstroSat: A 5-in-1 Observatory
AstroSat is India's first dedicated multi-wavelength space observatory. It was launched into space by ISRO exactly 3 years ago, on 28 September 2015, on board the PSLV-C30. This unique observatory has five instruments on board, four of which can look at the same piece of sky simultaneously. These telescopes give AstroSat the capability of observing in the ultraviolet, X-rays as well as gamma rays. Thus, the range of wavelengths that AstroSat can observe spans a factor of about 16000, from the lowest (200 nm in the Near UV) to the highest (0.012 nm or 100 keV in the gamma rays). These five instruments are the Ultra Violet Imaging Telescope (UVIT), the Soft X-ray Telescope (SXT), the Large Area X-ray Proportional Counter (LAXPC), the Cadmium-Zinc-Telluride Imager (CZTI) and the Scanning Sky Monitor (SSM).
Over the last one year, we have brought you 12 images from AstroSat. These APOMs, or AstroSat Pictures of the Month, have mostly been from the UVIT, since it is best suited for producing images. The strength of the X-ray and gamma ray telescopes lies in their incredibly precise timing and spectral capabilities. We will be bringing you spectra and light curves of interesting objects from time to time too.
Today is the 3rd anniversary of the launch of AstroSat and the 1st anniversary of the APOM project. Hence, this month's APOM is of AstroSat itself! The top two panels are photographs of the fully assembled AstroSat from two different angles. All five telescopes, along with many sensors can be seen. The golden colour is due to the layer wrapping the satellite that thermally insulates it in space. Compare these two photographs with the artists conception of Astrosat in the panel below. Here, each of the five telescopes are labelled, along with the solar panels which were unfurled in space after launch. Can you identify each of these five telescopes in the two photographs in the top panel? Note that the LAXPC is made of three similar units, and the UVIT consists of two telescopes next to each other.
Click here for the booklet published during the launch of AstroSat.

#013 October 2018

The AstroSat Picture of the Month for October 2018 presents the false-color ultraviolet images of NGC6302 (Butterfly Nebula) using ASTROSAT. On the left is a cartoon showing the full extent of the nebula with far-ultraviolet represented in blue. The picture on the right shows the zoomed-in view of the brightest part of the butterfly shaped planetary nebula in far-ultraviolet.

Pic Credit: Kameshwara Rao and Sriram Krishna

Spoiler: About NGC 6302 aka Butterfly Nebula

Ultraviolet wings of the Butterfly Nebula
This month, APOM brings to you the ultraviolet view of one of the most spectacular objects in the sky, NGC 6302. Located nearly 3800 light years away in the constellation Scorpius, NGC 6302 is a planetary nebula, whose shape is strikingly similar to the wings of a butterfly, hence aptly named as the Butterfly Nebula. This is the second planetary nebula that we bring forth to you, the first being NGC 40, covered in the APOM issue of December 2017.
Planetary nebulae are beautiful structures formed during the last few stages of the lives of stars like the Sun or a few times heavier. As the stars burn up all the hydrogen or helium fuel, they increase in size and become redder in colour, and are known as giant stars. As the giant star passes through few more stages, it continually sheds its outer layers revealing an inner hot core called the white dwarf. The white dwarf heats up the spewed-out gas which shines in the form of planetary nebula. Many of these planetary nebulae have strikingly symmetric shapes that need not be spherical and it has been suggested that this could be due to the various physical processes occurring in and around the star when it hurls out the gas from the outer layers. These nebulae are named planetary because when astronomers first observed them, they thought that these resembled planets. We now know that this is not the case, although the name has lingered.
Prof Kameshwar Rao, from the Indian Institute of Astrophysics (IIA), and his team have been investigating planetary nebulae in the ultraviolet light. They have imaged the Butterfly Nebula through the far and near-ultraviolet filters of the Ultraviolet Imaging Telescope (UVIT) of AstroSat. Using these images, they have discovered that gas which is bright in the far-ultraviolet extends the known wings of the butterfly out to 5.5 light years from the centre, nearly three times of what is seen in the optical. The reddish coloured figure on the right is the far ultra-violet image of the Butterfly Nebula. The blue image is a cartoon that represents the full extent of the far-ultraviolet emission. These researchers argue that the extended far-ultraviolet light is due to cold hydrogen molecules in the gas present in the outer parts of the nebula which are excited by the central star. They suspect that these far-ultraviolet structures of the planetary nebula point to the possible presence of two central stars in a binary system that are gravitationally bound. The results have been published in the journal Astronomy & Astrophysics and the paper can be read here.

#014 November 2018

The AstroSat Picture of the Month for November 2018 presents false colour image of the Globular Cluster NGC 288 in the ultraviolet taken by UVIT. The near-UV emission the stars are in yellow and the far-UV emission is in white. It is clear that the UVIT image shows only the hot stars as the cooler stars become undetectable.

Picture credit: Snehalata Sahu)

Spoiler: About NGC 288 Globular Cluster

Hot Ultraviolet stars in the Globular Cluster NGC 288
This month, APOM presents ultraviolet images of the globular cluster NGC 288, located at a distance of around 30,000 light years in the constellation Sculptor. This cluster was first described by John Dreyer in 1888. This is the second globular cluster in the APOM series, the first one being NGC 1851, presented a year ago. A globular cluster is a very large group of stars formed from the same cloud of gas and dust at nearly the same time. They are called globular because of the spherical distribution of stars, and each cluster is held together as a single entity by the gravity of the star members. Globular clusters are few of the oldest known objects in our galaxy. The stars in NGC 288 are believed to be nearly 12.6 billion years old.
The Ultraviolet Imaging Telescope (UVIT) on AstroSat has been used to image the stars in NGC 288 in the ultraviolet light, by a group of researchers from the Indian Institute of Astrophysics, Bangalore, and the National Research Council of Canada. The number of stars seen in the far-ultraviolet light are fewer than those seen in the near-ultraviolet light, and the reason for this is that only the hottest stars are seen in the far-ultraviolet.
Since globular clusters have very old stars, many of the heavier ones have already evolved to later phases of stellar lives (eg. red giant stars, horizontal branch stars). About 115 hot horizontal branch stars having surface temperature nearly twice that of the surface of Sun have been detected in NGC 288 using the near and far-ultraviolet filters of UVIT. A couple of very hot stars (extreme horizontal branch stars) whose whose surface temperatures are nearly five times that of the Solar surface have also been found. Using a combination of ultraviolet and optical light, these researchers have also identified 68 blue stragglers in this cluster. A blue straggler is a star formed when two stars either merge or transfer mass between them. Such stars have been mostly found in globular clusters where the star density is high. The capabilities of UVIT have made it easy for the researchers to see and inspect hot stars towards the cluster individually, allowing them to study the properties of each star, such as the mass and temperature.
The paper describing their results can be downloaded here.

#015 December 2018

The AstroSat Picture of the Month for December 2018 presents the Ultra Violet Imaging Telescope itself. A schematic of the design of the UVIT is shown on the top left, and a photograph of the two UVIT telescopes is in the top right. An image of the fully assembled UVIT, containing both telescopes sitting snugly together, wrapped in insulating foil, waiting to be integrated into AstroSat, is shown at the bottom. The Indian Institute of Astrophysics, the Inter-University Centre for Astronomy and Astrophysics, and the Canadian Space Agency developed the UVIT.

Picture credit: ISRO, UVIT Team

Spoiler: About UVIT

UVIT – the bright star in the galaxy of UV telescopes in the sky
Over the past few months, we have brought you many images taken by the Ultra Violet Imaging Telescope (UVIT) on the AstroSat. Don't you think it is time you met the famous UVIT itself? UVIT is a pair of telescopes that has a unique place in the ensemble of ultraviolet telescopes. Let us tell you why.
Though the optics of an ultraviolet telescope is similar to the commonly found optical telescopes, the shorter wavelength of UV demands certain special modifications. UVIT is made of a pair of very similar telescopes, each of which has a 37.5 cm diameter mirror, specially coated to make it highly reflective in the UV. In one of them, the light is split into two beams of different wavelengths, one of which goes into a detector for visible light, and the other into a detector for the Near UltraViolet (NUV, 200-300 nm). The other telescope is solely for the Far UltraViolet (FUV, 130-180 nm). Astronomers can also isolate just a narrow range of wavelengths within the NUV and FUV light by choosing specific filters before the light hits the detector. There is a diffraction grating as well, that can disperse the UV light into a spectrum.
The detectors are places where things get more interesting. The NUV and FUV detectors are photon counting devices and can respond to each individual UV photon. These are capable of measuring the time of arrival of every single photon to an accuracy of 2 milliseconds! The signal from these photons are then amplified before they fall on the 0.25 megapixel CCD cameras. This design makes the UVIT very sensitive, and objects very bright in the UV can also permanently damage the detectors. Because of this, the UVIT is only operated when the Earth hides AstroSat from the Sun. In addition, UV light reflected from the Earth is quite strong and hence the UVIT is never pointed towards us. There are a number of additional built-in safeguards that protect this instrument from any ultraviolet glare that may strike it.
How does the UVIT, with its special design, compare with other UV telescopes in space? Its field of view is about 0.5 degree (i.e., the size of the Moon). This is 80 times larger than that of the Hubble Space Telescope, and about half as much as that of the Galaxy Evolution Explorer (GALEX) of NASA. Although the UVIT's resolution of 1.4-1.7 arcseconds is poorer than the HST, it is much better than the 5 arcseconds resolution of GALEX. Hence, UVIT's uniqueness lies in its ability to image a large field of view combined with its superior resolution. This makes it a remarkable telescope to study large galaxies and galaxy clusters in fine detail, as well as look into the crowded centres of star clusters in our own Galaxy. Since the energy of UV photons is more than that of visible light photons, UVIT has been used extensively to study hot stars, both young and old, as well as final stages of these stars, allowing astronomers to peek into the ultraviolet sky as never before.

#016 January 2019

The AstroSat Picture of the Month for January 2019 shows the 97 minute long orbit of AstroSat around the Earth. This orbit is roughly equatorial (top right), inclined at around 6 degrees to it (top left). This results in each orbit being slightly displaced from the previous one (bottom).

Picture credit: Leo Jackson John, Operation Director - AstroSat, ISTRAC, ISRO

Spoiler: Orbital path of Astrosat

Following the path of our cosmic eye in the sky
When seeing images from AstroSat, have you ever wondered where exactly is the satellite, how does it move, and how do astronomers get their hands on the data? This month's APOM is here to answer those questions for you.
AstroSat was launched by ISRO on 28 September 2015 from Sriharikota, on board the PSLV-C30 into its current orbit. This is a low-earth equatorial orbit, at a height of 650 km above the Earth. This orbit is not exactly over the equator, but is inclined at an angle of about 6 degree to it. In the top left image, the green line marks the equator and the yellow line marks the orbit of AstroSat. The top right image is a view from over the north pole. But why was this orbit chosen?
Our Earth has a magnetic field, which behaves overall like a bar magnet, with its poles a few degrees away from the poles defined by our rotation. These magnetic fields trap charged particles within them, which form the Van Allen belts. These belts are much closer to the Earth over the southern Atlantic Ocean. An equatorial orbit reduces the effect of this South Atlantic Anomaly on AstroSat which carries very sensitive instruments. Making the inclination exactly zero requires more resources and hence a 6 degree inclination was chosen.
AstroSat takes about 97 minutes to orbit the Earth once. Hence, it will not pass directly overhead at the same point in successive orbits. Each orbit, therefore, will be slightly shifted with respect to the previous one. You can track the movement of AstroSat live on this page. The individual orbits shown in red in the bottom image represent orbits that are one week apart, clearly showing this drift. Data is beamed down from an antenna on the satellite once every orbit, when it passes near India. This is received by ISRO's antenna in Byalalu near Bengaluru (marked as BLR). All orbits of AstroSat fall within the visibility of this antenna, which is marked by a circle centred at Byalalu. ISRO can also use an antenna in Indonesia, marked BIK, to monitor the satellite when needed. All the command, control and tracking of AstroSat is done by ISTRAC in Bengaluru.
As of 30 January 2019, AstroSat has completed 18000 orbits around the Earth, acting as our high energy eye, uncovering the nature of neutron stars, black holes, hot stars, and many strange celestial objects. May it continue to do so for many more orbits!