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

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ISRO Scores Big With This ASTROSAT Instrument

 

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http://www.isro.gov.in/pslv-c30-astrosat-mission/astrosat-support-cell-asc-has-been-set-iucaa-pune

This Red Star Had Scientists Stumped for Decades. India’s ASTROSAT Solved Puzzle in 7 Months
Public Access will be there from October.

 

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AstroSat Announcement of Opportunity (AO)

AstroSat observes the high energy X-ray variability of a black hole system

India's first dedicated satellite, AstroSat, which was launched by ISRO on Sept 28, 2015, has observed for the very first time rapid variability of high energy (particularly >20keV) X-ray emission from a black hole system.

In black hole systems, mass from a regular star gets stripped off and falls towards the black hole forming a disk around the black hole. The temperature of the disk is more than ten million degrees and hence the system emits X-rays. The total power coming out of these systems is often more than ten thousand times that of the sun. Yet these systems vary rapidly in time-scales much less than a second.

Astronomers have always been puzzled by the enigmatic black hole system called GRS 1915+105. It shows many different kinds of behavior and its X-ray emission sometimes oscillates nearly periodically (hence these oscillations are termed as Quasi-period oscillations) on time-scale of a few hundred milli-seconds. Astronomers believe that these oscillations may occur because the inner part of the disk surrounding the black hole precesses (i.e. wobbles) because the spinning black hole drags the space-time fabric around it as predicted by Einstein's General theory of relativity.

While these oscillations have been known and studied earlier in low energy X-rays using the American satellite Rossi X-ray Timing Experiment, they have now been detected and characterized in high energy X-rays by the Large Area X-ray Proportional Counter (LAXPC) on board the ISRO space mission, AstroSat. Observing the phenomenon in high energy X-rays is critical since the higher energy photons are expected to be emitted closer to the black hole than the low energy ones. The highly sensitive instrument, LAXPC, also measured the arrival time difference between the high and low energy X-rays (which is of the order of tens of milli-seconds) providing direct clues to the geometry and dynamic behavior of the gas swirling round a spinning black hole.

All this was obtained by just nine orbits or a few hours of AstroSat observation of the source and no other observatory at present (or earlier) is capable of achieving these results. After careful performance verification of the instruments on board AstroSat, Indian Scientists are now using AstroSat to unravel the mysteries of the Universe and this finding is just the beginning of a large number of such discoveries that AstroSat is expected to make. This marks a new era for Indian Astronomy with AstroSat being a front-line dedicated astronomy satellite.

The findings have been reported by a team led by Prof. J. S. Yadav and other scientists from the Tata institute of Fundamental Research (TIFR) along with astronomers from the Inter-University Centre for Astronomy and Astrophysics (IUCAA), University of Mumbai and the Raman Research Institute (RRI). Their report will be published in the Astrophysical Journal.

The LAXPC instrument was developed indigenously at the Tata Institute of Fundamental Research (TIFR) Mumbai.

 

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NGC 2336 in one of its best resolved view from UVIT

CZT Imager detects a Gamma-ray Burst

During the first week of CZTI operation, the supernova remnant Crab Nebula and the black hole source Cyg X-1 were monitored. The Crab Nebula can be treated as a standard candle and it was used as a calibrator for timing and imaging, and also to measure the response of the instrument at large off-axis angles. One of the projected objectives of CZTI is wide-angle monitoring of the sky in hard X-ray band to record strange and rare events like Gamma-ray Bursts (GRB).

Luckily, on the first day of operation of CZTI, the Swift satellite reported the detection of a Gamma-ray burst, at 09:55:01 UT, named GRB 151006A. We were eager to know whether CZTI was operational at that time (i.e. outside SAA) and if the GRB was in a favourable condition to be observed. A quick calculation showed that this GRB was 60.7 degrees away from the CZTI pointing direction and, at this angle, CZTI should be sensitive to this GRB at energies greater than about 60 keV. The instrument time is yet to be calibrated precisely as the data analysis pipe-line is yet to be streamlined; still a band of youngsters delved into the voluminous data to extract the precious information about this messenger of a blast from the extremities of the universe: GRB 151006A.

Yes, there it is: the GRB made its presence felt as an increase in the recorded counts, shown in Figure 1. At higher energies (above 100 keV), the shielding material at the side of the CZTI is designed to be more transparent and one can see a significant and sharp jump in the counts above 100 keV during the GRB time.

One of the much anticipated properties of CZTI is its ability to identify X-rays depending on the method by which they interact with the detector. If it is by inelastic scattering (called the Compton scattering), they should obey certain scattering principles; and when all the recorded events were subjected to the Compton scattering criteria, there indeed was a significant jump in the count rate. In Figure 2, the so-called `Compton’ events (that is, double events satisfying all the requirements of the theoretical expectations of Compton scattering) are plotted as a function of time, the reference time (time zero) being the trigger time of the GRB reported by the Swift satellite.

This information was flashed to the scientific community through GCN (the Gamma-ray Coordinates Network maintained by NASA), and the resultant GCN circular is shown in Figure 3.

Gamma-ray bursts - blasts from the past: Gamma-ray bursts are, as the name suggests, bursts of gamma-rays, coming from apparently random directions in the sky. They were discovered serendipitously in the sixties by the American Vela satellites designed to detect possible surreptitious nuclear weapon tests by the then Soviet Union. For long, they remained a mystery, but in the late ninties the Italian-Dutch satellite Beppo-SAX managed to measure longer wavelength lingering radiations from them in soft X-rays (called the after-glows) and identify them with far away galaxies. Currently, there are two dedicated satellites measuring their properties: the Swift and the Fermi satellites. Thousands of GRBs have been detected and some of them are identified to be so far away that they originated when the universe was less than a billion years old (the current age of the universe is 13 billion years).

So, what is the big deal of CZTI detecting one more GRB ?

In spite of the vast amount of data available, GRBs still remain a mystery. One class of GRBs called the long GRBs are associated with newly formed black holes while another class, called the short GRBs, are believed to be the tell-tale signs of the merger of two compact objects. There is also an emerging school of thought which postulates that GRBs originate from neutron stars with extremely high magnetic field, called the magnetars. The current debate about the origin of GRBs is accentuated by the fact that the characteristics of the burst of gamma-rays are ill understood and the radiation mechanisms responsible for the emission is not quantified.

The Swift satellite, as the name suggests, is swift in pointing itself towards new GRBs and locating the afterglows: it has limited response above 150 keV and it is unable to fix the spectral parameter like the peak energy for many GRBs with `hard’ spectrum. A simultaneous observation with the CZTI, which is sensitive upto 250 keV and has the best spectral capability, ever, for GRB studies in the 80 – 250 keV region, will certainly help in measuring the spectral parameters. The Fermi satellite, on the other hand, is very sensitive to higher energy emission and detects a lot of short-hard GRBs, but it has very limited localisation capability. CZTI can pitch in for short-hard GRBs and localise them much better than Fermi. If the spectral and localisation capabilities of CZTI can be demonstrated by a detailed analysis of GRB 151006A, it will enrich the GRB science by providing spectral properties for long GRBs and localisation of short GRBs (it is estimated that 50 to 100 GRBs would be detected by CZTI in a year).

But, the biggest deal, however, is the mouth-watering profile shown in Figure 2. CZTI, as designed, is sensitive to detect Compton scattered events and a demostration of this capability in GRB 151006A is extremely significant for the following reason: the Compton scattering process is sensitive to the polarisation of the incident X-rays and if CZTI is sensitive to Compton scattering, then it is surely sensitive to the polarisation characteristics. Hence, for brighter GRBs, a precise value of polarisation amplitude should be measurable (this GRB has about 500 counts detected as Compton scattered events and it is estimated that one needs at least 2000 counts to make a reliable polarisation measurement). Though polarisation has been measured in a few GRBs, this is the first time ever that spectral, timing, and polarisation properties of GRBs in hard X-rays will be measured simultaneously and it will have far reaching implications in the understanding of the radiation mechanisms of GRBs.

Meanwhile, as they say: During the first week of CZTI observations, CZTI measured the pulse period of Crab Pulsar (shown in Figure 4), demonstrating the timing capability of the instrument.

Fig 4: Power spectrum of Crab observations. Crab pulsar frequency with its harmonics are clearly seen at a frequency corresponding to 29.65 Hz and its multiples.

SSM First light of GRS 1915+105 - a Black Hole - on 14th October 2015

Following this, SSM was maneuvered to a field that contains the enigmatic Galactic Black Hole source GRS 1915+105. Even with many challenges on mission operations, AstroSat was oriented to the required field with GRS 1915+105 in the FOV of SSM on October 14th, 2015. THANKS to mission operations team!

The particular field was crowded with few other bright sources (eg. Cyg X-1, Cyg X-2, Ser X-1), while GRS 1915+105 was the strongest source with intensity ~2 Crab. GRS 1915+05 also displays very peculiar, but ‘structured’ X-ray variability known as ‘class’.

Figure 3: AstroSat – SSM First light of the enigmatic Black Hole GRS 1915 + 105

AstroSat-first light from Galactic Black Hole GRS 1915+105 as observed by SSM is shown in figure 3. A quick look of the variability profile of the light curve matches well with one of the earlier observations of the source with NASA's Rossi X-ray Timing Explorer (RXTE) satellite as shown in the figure. More detailed analysis results will follow.

The Astronomer's Telegram: We're Sorry!

www.astronomerstelegram.org

This observation is reported as a “Astronomers' Telegram” ATel #8185. The link for the ATel is:

SSM – Indication of a M - class Solar Flare - on 16th October 2015 During a scheduled flux calibration observation with SSM pointed to Crab, during a specific part of the orbit, at ~6:12 UT, October 16th 2015, SSM (all three detectors) recorded a sudden upsurge in counts – with a rise time of ~2 minutes and a decay time of ~18 minutes as shown in figure 4.

Figure 4: SSM observations with Earth in its FOV on October 16, 2015 during a M-class Solar Flare at 6:10 UT

Figure 5: GOES observations of M-class Solar flare at 6:10 UT on 16th October, 2015.

This occurred when the pointing of the SSM cameras was facing the Earth such that the FOVs of all the three cameras had Earth within. The Sun was almost 180 degrees away. . The upsurge in counts was understood to be X-rays due to a M-class Solar flare, which was confirmed with the time of occurrence, type of flare etc. from the US satellite “GOES” data. The correlation of the time of detection of upsurge of counts in SSM and the time of occurrence of the flare can be observed from both the figures - 4 and 5.

ASTROSAT/LAXPC: Observation of Cygnus X-1 in the Hard State - Tata Institute of Fundamental Research (Uploaded PDF File)

https://www.google.com/url?sa=t&sou...oQFnoECCoQAQ&usg=AOvVaw2tTAAnNWXqJyhkTmhiF1EE

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X-ray Spectrum of Tycho supernova remnant using SXT
Uploaded PDF File 0587

 

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Some data published on a Science Journal called Arvix.

AstroSat CZT Imager observations of GRB 151006A: timing, spectroscopy, and polarisation study

AstroSat/LAXPC reveals the high energy variability of GRS 1915+105 in the chi class

 

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AstroSat Completes One Year in Orbit

 

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Astronomers discover ‘powerful cosmic double whammy’ with help of India’s GMRT
A supermassive black hole and the collision of giant galaxy clusters have combined to create a cosmic acceleration.
By: PTI | Washington |
Updated: January 7, 2017 5:06 PM

The two clusters are both very massive, each weighing about a quadrillion – or a million billion – times the mass of the Sun.

Astronomers, using data from India’s Giant Metrewave Radio Telescope (GMRT), have discovered two of the most powerful phenomena in the universe – a supermassive black hole and the collision of giant galaxy clusters about two billion light years from Earth.

The two phenomenon have combined to create a stupendous cosmic particle accelerator, researchers said.

By combining data from NASA’s Chandra X-ray Observatory, the Giant Metrewave Radio Telescope (GMRT) in Pune and other telescopes, researchers found what happens when matter ejected by a giant black hole is swept up in the merger of two enormous galaxy clusters.

“We have seen each of these spectacular phenomena separately in many places,” said Reinout van Weeren of the Harvard-Smithsonian Centre for Astrophysics (CfA) in the US, who led the study.

“This is the first time, however, that we see them clearly linked together in the same system,” said van Weeren. This cosmic double whammy is found in a pair of colliding galaxy clusters called Abell 3411 and Abell 3412 located about two billion light years from Earth.

The two clusters are both very massive, each weighing about a quadrillion – or a million billion – times the mass of the Sun.

The comet-shaped appearance of the X-rays detected by Chandra is produced by hot gas from one cluster plowing through the hot gas of the other cluster. Optical data from the Keck Observatory and Japan’s Subaru telescope, both on Mauna Kea, Hawaii, detected the galaxies in each cluster.

First, at least one spinning, supermassive black hole in one of the galaxy clusters produced a rotating, tightly-wound magnetic funnel.

The powerful electromagnetic fields associated with this structure have accelerated some of the inflowing gas away from the vicinity of the black hole in the form of an energetic, high-speed jet.

These accelerated particles in the jet were accelerated again when they encountered colossal shock waves – cosmic versions of sonic booms generated by supersonic aircraft -produced by the collision of the massive gas clouds associated with the galaxy clusters.

“It’s almost like launching a rocket into low-Earth orbit and then getting shot out of the Solar System by a second rocket blast,” said Felipe Andrade-Santos, also of the CfA. “These particles are among the most energetic particles observed in the universe, thanks to the double injection of energy,” said Andrade-Santos.

This discovery solves a long-standing mystery in galaxy cluster research about the origin of beautiful swirls of radio emission stretching for millions of light years, detected in Abell 3411 and Abell 3412 with the GMRT.

The team determined that as the shock waves travel across the cluster for hundreds of millions of years, the doubly accelerated particles produce giant swirls of radio emission. The study appears in the journal Nature Astronomy.

 

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Discovery of a hot companion associated with a Blue Straggler in NGC-188 using AstroSat UVIT data

An Open Star Cluster consists of hundreds to thousands of stars, which are loosely bound. They are formed most likely from a single gas cloud, and are therefore roughly of same age. Open clusters and particularly old open clusters therefore are ideal sites to study the stellar evolution for both single and binary stars. Most stars evolve away from the main sequence once their hydrogen burning phase is over. The turn over point of the Hertzsprung – Russell (HR) diagram of an open cluster is indicative of its age.

Blue Straggler Stars (BSS) are members of old clusters that are brighter and bluer than stars on the upper main sequence. They appear to 'extend' the main sequence in a HR diagram of the cluster and appear as if they are 'younger' stars. They are termed stragglers because they do not move away from the main sequence like the other stars in the same cluster.

NGC-188 is a well-studied old open cluster with an estimated age of 7 Gyr (Billion year, astronomically known as Giga Year- Gyr) and exhibits high metallicity. It is located about 5000 light years away and has about 1050 stars as its members with 20 BSSs confirmed. WOCS-5885, most likely a member of NGC-188 (with a high probability of 53 to 80% quoted in literature), was one of the 3 objects identified with exceptionally blue color. Various classifications -a BSS or a sub-dwarf or a binary with a red giant and a pre-white dwarf to name a few- were attributed to this object, because its spectrum did not match with any single identification. This could only be resolved if the hot (UV, blue) and the cool (red, IR) part of the spectrum of this object could be fitted together with spectral models of stars. This had been done with observations from space (GALEX, UIT, UVOT, SPITZER, WISE) and several ground based observatories, spanning the IR, optical and UV bands.

The UV band observations from the Ultraviolet Imaging Telescope (UVIT) on ASTROSAT have provided additional points in the Spectral Energy Distribution (SED) thus resulting in a much better spectral fit over the wavelength range of 0.15 µm to 7.8 µm. With this data set, WOCS-5885 has been classified as a binary consisting of a BSS and a hot star which is either a post Asymptotic Giant Branch or Horizontal branch (post-AGB/HB) star.

The UVIT contains two 38-cm telescopes; one for the far-ultraviolet (FUV) region, the other for the near-ultraviolet (NUV) and visible (VIS) regions. These are divided using a dichroic mirror for beam splitting. UVIT is primarily an imaging instrument, simultaneously generating images in the FUV, NUV and VIS channels over a 28 arcmin diameter circular field. Each channel can be divided into smaller pass bands using a selectable set of filters.

UVIT observed NGC-188 both as a first light object and for regular calibration. The observations have been done in both NUV and FUV filters in the wavelength band of 0.3 to 0.15 µm. With these observations, it is found that the SED can only be fit with spectra consisting of 2 stars. The cooler star is found to be a BSS with a temperature of 6,000+150 K, and the temperature of the hotter star is 17,000+500 K. The estimated size and luminosity of the hotter star rule out a white-dwarf or a sub-dwarf classification and hence it is proposed that it could be a post AGB/HB star. If the membership of WOCS-5885 to NGC-188 is confirmed, it could be a rare BSS + post AGB/HB binary, the first of its kind to be identified (for which probability is high) in an open cluster. This system therefore provides a great opportunity to constrain theories of BSS formation via mass transfer.

Thus, observations from the UVIT were used to solve the puzzle of a star WOCS-5885 which appeared as a single star but whose spectra did not match with this identity.

For details:

Subramaniam Annapurni et al., A Hot Companion to a Blue Straggler in NGC-188 as Revealed by the Ultra-Violet Imaging Telescope (UVIT) on ASTROSAT-The Astrophysical Journal Letters, Volume 833, No. 2, 19 December 2016.

FUV (left) and NUV (right) images of NGC-188 obtained on 18 February 2016.
WOCS-5885 is marked as red square

Spectral Energy Distribution (SED) for WOCS-5885. Points in light green are from UVIT

Story of the Week - Archive
Jan 09, 2017 : Discovery of a hot companion associated with a Blue Straggler in NGC-188 using AstroSat UVIT data
Jan 02, 2017 : Golden Jubilee of Composite Materials Activities at ISRO
Dec 26, 2016 : Indigenous Development of 4.5 ton Vertical Planetary Mixer
Dec 19, 2016 : First National Finite Element Developers/FEASTSMT Users’ Meet at VSSC
Dec 05, 2016 : National Rollout of GeoMGNREGA held at Vigyan Bhavan, New Delhi
Nov 28, 2016 : Applications of Unmanned Aerial Vehicle (UAV) based Remote Sensing in NE Region
Nov 21, 2016 : ISRO Telemetry, Tracking and Command Network (ISTRAC) Celebrates Ruby Year

• 1 of 12
• next ›

 

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'Vampire' star caught in the act by Indian space observatory

India's first dedicated space observatory, ASTROSAT, has captured the rare phenomenon of a small, 6-billion-year-old "vampire" star "preying" on a bigger celestial body.

Scientists say the smaller star, also called a "blue straggler", feeds off its companion star by sucking out its mass and energy, causing its eventual death.

"The most popular explanation is that these are binary systems in which the smaller star sucks material out of the bigger companion star to become a blue straggler, and hence is called a vampire star.

"The small star becomes bigger, hotter and bluer, which gives it the appearance of being young, while the ageing companion burns out and collapses to a stellar remnant," said Annapurni Subramaniam, a Professor at the Indian Institute of Astrophysics.

Though this phenomenon is not unheard of, the observation of the entire process through the telescope will provide insights that will help scientists in studying the formation of 'blue straggler' stars.

This discovery also highlights the capabilities of the telescopes on ASTROSAT, a dedicated space observatory satellite launched in September 2015.

The study was recently published in Astrophysical Journal Letters by a team of scientists from IIA, Inter-University Centre of Astronomy and Astrophysics (IUCAA), Tata Institute of Fundamental Research (TIFR), Indian Space Research Organisation (ISRO) and the Canadian Space Agency (CSA).

Scientists are now looking to understand the chemical composition of the 'blue straggler' using high resolution spectroscopy, which could reveal more about the evolution of these peculiar celestial objects.

The stars are part of a "cluster" called NGC 188 formed some 6 billion years ago, and are much older than the sun, which is believed to have come into existence nearly 4.5 billion years ago.

"As the sucked up material from the ageing star will be polluted with material processed within the ageing star, this blue straggler will throw light on the kind of nuclear processing that happens in ageing stars", Subramaniam, also the Calibration Scientist of UVIT on board the ASTROSAT, said.

"The data from six filters of UVIT were used to estimate the precise temperature of the companion star, its size and brightness. This has been possible only due to the excellent capability of the UVIT telescope," Subramaniam added.

The UVIT is made of twin telescopes with an effective diameter of 375mm each and records images in the Far UV, Near UV and visible frequencies. UVIT provides very sharp UV images over a field of view as large as the Moon.

"The companion (the bigger star) is still going through ageing and has not yet become a remnant. It is also hot and large. Therefore it appears very bright in ultraviolet image, but not so bright in an image taken with an optical telescope looking at visible light," she said.

This is why previous studies of the 'blue straggler' in the optical range could not detect the companion. "This pair thus becomes a rare sample to study the details of the formation of blue straggler stars," Subramaniam added.

The purpose of ASTROSAT is to understand high energy processes in binary star systems containing neutron stars and black holes, estimate magnetic fields of neutron stars, study star birth regions and high energy processes in star systems lying beyond our galaxy.

It is also tasked to detect new briefly bright X-ray sources in the sky and perform a limited deep field survey of the universe in the Ultraviolet region.

Apart from the UVIT, the ASTROSAT also includes a Large Area X-ray Proportional Counter (LAXPC), a Soft X-ray Telescope (SXT), a Cadmium Zinc Telluride Imager (CZTI) and a Scanning Sky Monitor (SSM).

These instruments observe the sky for electro-magnetic radiations in the visible, ultraviolet and X-ray frequency ranges coming from distant celestial sources.

 

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India's AstroSat catches astronomical imposter
India’s AstroSat catches astronomical imposter Download PDF

Indian scientists leading an international team of astronomers showed that a new object discovered in the sky, believed to be related to the latest gravitational wave discovery, was in fact an unrelated gamma ray burst.

The LIGO scientific collaboration’s discovery of GW170104 led to a frenzy of activity among partner astronomers around the world, each trying to find any associated explosions in the sky. The Hawaii-based ATLAS group found a source that was in the right place in the sky, and was fading fast – causing excitement all around. But was it really associated with GW170104? Was it the first discovery of an optical source related to a gravitational wave detection? No, according to a study by the AstroSat CZTI team and the international GROWTH collaboration.

While studying observations of the source – named ATLAS17aeu – the team noticed something odd about how fast it was fading. “Analysing the data, I concluded that ATLAS17aeu must be related to some explosion on 5th January, not the 4th”, says Varun Bhalerao (IIT Bombay), the lead author in this study. The team had already used CZTI to look for X-rays coming from GW170104, and not seen any. CZTI (Cadmium Zinc Telluride Imager), a gamma ray telescope on ISRO’s maiden space observatory AstroSat, proved to be the most sensitive instrument in the world to find transient sources with sub-second durations. So if there was another burst in the sky, they were sure they would find it in CZTI data. Varun continues, “I shot off an email to my student Sujay, asking him to search for a burst in CZTI data in the calculated time window. And then I noticed an email from Vidushi (another student) in my inbox: she had found the burst I was looking for!”

The culprit seemed to be a gamma ray burst GRB 170105A – that exploded in the same part of the sky 21 hours later. But to be sure of this, astronomers needed more data. Enter GROWTH: Global Relay of Observatories Watching Transients Happen. This multi-national team had already swung into action, observing ATLAS17aeu with optical, X-ray and radio telescopes. Team member Dipankar Bhattacharya (IUCAA) said, “The team studied the source with radio, optical and X-ray telescopes for a few days, till it faded away into oblivion. Based on its behaviour we concluded that this event signalled the birth of a new black hole when a massive star imploded in a galaxy several billion light years away.”

This gamma ray burst was missed by several other international satellites which were pointing at other parts of the sky at that instant. It was detected only by the Cadmium Zinc Telluride Imager (CZTI) on AstroSat, and by the Chinese-European POLAR instrument. “This is the result of insightful instrument design, imaginative onboard software, and collaborative data analysis from a nationwide team”, says A.R. Rao (TIFR). But the team has its eyes on the prize: finding the first electromagnetic counterpart to a gravitational wave source. And the highly sensitive CZTI might not be enough. Rao adds, “We need wide angle detectors scattered over interplanetary space to discover X-rays from LIGO sources. The CZTI team has proposed a small sized instrument called MOTIVE to ISRO as a likely payload for a future interplanetary mission. Together, CZTI and MOTIVE can revolutionize the field!”

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CZT-Imager is built by a consortium of Institutes across India. The Tata Institute of Fundamental Research, Mumbai, led the effort with instrument design and development. Vikram Sarabhai Space Centre, Thiruvananthapuram provided the electronic design, assembly and testing. ISRO Satellite Centre (ISAC), 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, instrument calibration, and 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. 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.

This work was supported by the GROWTH project funded by the National Science Foundation under Grant No 1545949. GROWTH is a collaborative project between California Institute of Technology (USA), Pomona College (USA), San Diego State University (USA), Los Alamos National Laboratory (USA), University of Maryland College Park (USA), University of Wisconsin Milwaukee (USA), Tokyo Institute of Technology (Japan), National Central University (Taiwan), Indian Institute of Astrophysics (India), Inter-University Center for Astronomy and Astrophysics (India), Weizmann Institute of Science (Israel), The Oskar Klein Centre at Stockholm University (Sweden), Humboldt University (Germany). GROWTH is supported by the Science and Engineering Research Board, Department of Science and Technology, India.

More information about the discovery of the gravitational wave source GW170104 can be obtained at https://www.ligo.caltech.edu/page/press-release-gw170104, and in the IndIGO press release.

 

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Jun 01, 2017
AstroSat CZTI and the search for X-ray counterparts to gravitational wave sources

The Ligo scientific collaboration has discovered the third confirmed gravitational wave event, GW170104. Several Indian and international astronomy groups searched for corresponding electromagnetic signals in the sky. The Cadmium Zinc Telluride Imager (CZTI) on AstroSat conducted the most sensitive search for short duration X-ray flashes associated with this event, but did not find anything. The Hawaii-based ATLAS group found a transient optical source, which was thought to be related to GW170104.
The CZTI team, in collaboration with the international GROWTH collaboration, studied this transient extensively, and proved that it was not the counterpart of GW170104. Instead, it was a Gamma ray burst event caused by the explosive death of a massive star in a galaxy several billion light years away giving birth to a new black hole.

 

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AstroSat Picture of the Month - Sep 2017

Stellar Porn!

 

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AstroSat contributes to the saga of Gravitational Wave Astronomy
On August 17, 2017, scientists seeking the holy grail of gravitational wave (GW) astronomy struck gold. The elusive and long sought after GW signals from merging binary neutron stars were found and multi-messenger observations provided tell-tale signs of this merger to clinch the issue without any qualms. Two of the GW detectors in the US picked up the signal and a third, working in Europe, confirmed it. Several of the satellites in the sky detected signals from this event across various bands of the electromagnetic spectrum, and a vast array of optical and radio telescopes worldwidetrained their vision into this new phenomenon, finding a variety of corroborating signals.

The AstroSat scientists, who pitched in with their efforts, today stand shoulder to shoulder with a few thousand scientists across the globe (including three Nobel Prize winners and a few scores of other Indian scientists) to announce this momentous discovery and an `open sesame’ moment of staring at the huge cache of scientific discovery that this new era of`multi-messenger, time-domain astronomy’ opens up.

Gravitational Wave Astronomy: the last frontier

Any accelerated electronic charge emits electromgnetic radiation: scientists routinely use this to generate and send electromagnetic waves like radio waves, optical light, and X-rays. Any moving mass disturbs the space time and a `quadropole’ moment in the moving mass should generate gravitational waves: theorised Albert Einstein a hundred years ago. Einstein’s words are treated as Veda Vakya or Gospel Truth, and astronomers routinely use this to understand the dynamics of compact large masses in the cosmos. Russell A. Hulse and Joseph H. Taylor, Jr discovered two radio pulsars going around each other, slowly hurtling towards each other, and they invoked Einstein’s gravitational wave theory to understand their behavour: they were duly awarded a Nobel Prize for this work. This opens up an interesting question-shouldn’t astronomers, who use every branch of electromagnetic radiation from radio to gamma-rays to prise open the secrets of the Universe, use gravitational waves to understand exotic features of the cosmos- like the ripples of the Big Bang or merging of black holes when galaxies collide ?

Well, they should,but the catch lies in the fact that the gravitational force is extremely weaker than the electromagnetic force, and common sense deems that even the most sensitive detectors that humans can build cannot detect the most exotic gravitaional wave sources that we can imagine. However, during the past few decades, a huge number of dedicated scientists have built the most sophisticated detectors capable of measuring infinitesimal movement of mass corresponding to a tiny fraction of a nanometer in kilmoter sized objects so that they would be sensitive to the gravitational waves from outer space. Year after year, they kept looking for signs of merging neutron stars, but the quest was in vain !

Mother Nature usually likes to keep surprises up her sleeve! When the GW detectors with highly improved sensitivity were switched ON in 2015, they found something: not a neutron star-neutron star merger, but a totally unexpected event of two massive black holes merging and spewing out energy equivalent to the complete burning out of mass corresponding to two Suns. This is indeed a momentous discovery, and the architects of this humongous human effort, Kip Thorne, Rainer Weiss, and Barry Barish, duly got this year’s Nobel Prize.

What about the elusive case of the merger of two neutron stars anticipated from the discovery of Messers Hulse and Taylor?During the past two years, four GW events were discovered, however, they were all due to mergers of black holes. The problem with merging black holes is that they are, as apparent from the name, `black’;i.e., apart from the GW events,there are no tell tale signs of the merger in any other branch of electromagnetic radiation. So, we cannot determinewhere they are coming from, or what are their progenitors. This is not the case for neutron star mergers. It was firmly believed that when GW events are discovered from neutron star mergers, they would be accompanied by huge amounts of electromagnetic radiation, which will help us pin down the sources of these events.

The whole scientific community was eagerly waiting for this much anticipated event.

CZT Imager of AstroSat pitches in

AstroSat was launched on September 28, 2015 and the CZT Imager (CZTI) instrument of AstroSat was the first instrument to be made operational. On October 6, 2015, the first day of operation, CZTI detected a gamma-ray burst (GRB) and proved to be an efficient GRB detector. The scientists working with the CZTI data realised that it would be a wonderful instrument to detectany gamma-ray events accompanying the GW sources.

The problem with detecting such gamma-ray events is that they are rare, unpredictable, and can come from any direction in the sky. Hence, the detectors need to have all sky sensitivity, and generally, there is a trade off in their observing capabilities. Currently, there are three sensitive operating GRB monitors, along with a few more less sensitive detectors, each having their own capabilities and limitations. The most sensitive GRB monitor currently operating is the Swift satellite, however, it can observe only one tenth of the sky at any given time. CZTI and the Fermi satellite, on the other hand, are sensitive to much larger regions in the sky, but have very limited capability to localise these events. The anti-coincidence shield of the INTEGRAL satellite, too, can act as a GRB monitor.

Each of these instruments played their part in the race to detect gamma-ray signals accompanying the GW events. During the very first GW event on September 14, 2015 (before the launch of AstroSat), Fermi claimed that it haddetected a GRB like event within 0.4 s of the GW event. Observations from theINTEGRAL satellite, however, disagreed: the consensus was that this could be some unrelated spike in the background. During another GW event detected in January 2017, optical astronomers saw, the very next day of the event, some source gradually diminishing in brightness. Could this be the tell-tale signs of something happening in the GW source? CZTI chipped in with a firm No! It had detected a GRB, 21 hours after the GW event. The fading optical source was shown to be this GRB, unrelated to the GW event.

Aug 17, 2017: a red letter day

On August 17, 2017, the much anticipated event occurred.

The GW detectors in US registered a very long series of signals, or `chirps’, closely resembling what the scientists have simulated for decades to be coming from neutron star coalescence. Even before they could announce this discovery, the Fermi satellite had detected a GRB at the same time: in fact within a couple of seconds of the GW event. Could this also be an unrelated background fluctuation event? Very unlikely, because, at exactly the same time, the anti-coincidence shield in the INTEGRAL satellite had also detected this GRB. What about Swift and CZTI? They didn’t detect any! The event should probably be outside the narrow field of view of Swift. What about CZTI? It was active and operating and the GRB should have been detected. The only way to reconcile was to assume that the source was blocked by the Earth: this helped to narrow down the possible source regions of the GW event.

Soon, the GW detectors from Europe too pitched in, and the region of the sky responsible for the GW event and GRB was narrowed down to a small region. Optical telescopes around the world scanned each and every galaxy in this region and, lo and behold, there indeed was a bright optical object, not seen before, near a galaxy called NGC 4993.

The rest, as they say, is history. Soon, infra-red and ultraviolet emissions were seen from this source. Nine days later, an X-ray source was detected, and fifteen days later, radio emission was also observed. From such vast multi-wavelength data, the physics of colliding and merging neutron stars were studied in depth. An exciting find is that the material ejected in the event is rich in heavy elements, so much so that, colliding neutron stars can account for the entire supply of precious metals, like gold, platinum and silver, in the universe. Production of these elements have been difficult to understand, and now the source has been found!

The story of GW170817 bears testimony to the amazing outcome possible when all the world’s best instruments are combined for a single purpose. The collaborative efforts of a number of teams worldwide lends an added credibility to this exciting and substantial discovery and ushers in a new era in multi-messenger, time-domain astronomy!

Scientific curiosity: never satiated

The GW detectors are taking a year off to return with an improved sensitivity. Neutron star merger events and the accompanying `kilonova’ should be fairly common observations during the next run. There should be more of black hole merger events as well. Scientists are already dreaming about the rich future harvests:

Can we get any tell-tale signatures of black hole mergers to identify where they are coming from? Perhaps more sensitive all sky detectors would help with an answer.

Can these events be used as a tool for distance measurement to refine cosmology? A massive collaboration between GW theorists and kilonova observers should be able to do it.

Can we learn anything about the regions close to black hole? Possible.

Are there some strange stars among the neutron stars? Certainly more such objects will tell us. Finally, has Mother Nature more surprises up her sleeve? Only the future will tell us!

What next? Significant next steps will involve making detectors more sensitive, improving localisation capability and most importantly, continued collaboration of observatories worldwide spanning all electromagnetic bands, neutrinos and gravitational waves.

In the Indian space science context, the capability of CZTI would certainly be improved through better algorithms and simulations: it should be possible to independently confirm and localise gamma-ray events for future GW associations. Perhaps, even a much improved CZTI like all sky monitor could be designed and flown!

Multi-messenger studies of GW170817 incorporating the contribution of AstroSat CZTI are published in the journals Science and Astrophysical Journal Letters.

Papers

• Illuminating gravitational waves: A concordant picture of photons from a neutron star merger - Science 10.1126/science.aap9455 (2017).
• Multi-messenger Observations of a Binary Neutron Star Merger - The Astrophysical Journal Letters, 848:L12 (59pp), 2017 October 20

https://www.isro.gov.in/update/17-o...butes-to-saga-of-gravitational-wave-astronomy

 

[Indx TechStyle]

Story of the Week
CZT Imager of AstroSat measures first phase resolved X-ray polarisation of Crab pulsar
X-ray polarisation: All types of electromagnetic radiation, like X-rays and optical light, are bundles of energy called photons defined by an electric vector, and an orthogonal magnetic vector. The electric vectors are mostly random in orientation, but quite often, they are aligned to a particular direction depending on the conditions in the source of these photons. For example, optical light scattered in the sky are aligned, or polarised, in the plane of the scattering and you can take a simple optical polariser to look at the sky and determine the direction of the incident photons (prior to scattering) from the source – in this case the Sun. The polarisation properties of electromagnetic radiations have been regularly used by astronomers to study the conditions of the cosmic sources emitting these radiations. For example, the direction of the magnetic field in our galaxy, Milky Way, can be precisely mapped using polarisation measurements.

Strong X-ray emission from astrophysical sources often signifies the presence of exotic compact objects in the universe: neutron stars and black holes. The X-ray emission from these objects traces the regions of particle acceleration and understanding the conditions of particle acceleration can tell us about several exotic phenomena: for instance, whether the black hole is spinning, does the strong gravity near black holes obey Einstein’s equation, or if neutron stars are made up of ordinary matter or strange matter and so on. It has long been thought that X-ray polarisation properties will tell us more about the mysteries of these strange objects.

Immediately after the birth of X-ray astronomy in 1962, due to the serendipitous discovery of an extra-solar X-ray source in a rocket flight, there was a flurry of activity in all aspects of X-ray astronomy, including polarisation measurement. Scientists at Columbia University flew a simple scattering based X-ray polarimeter in a rocket flight in 1969 to look at the pulsar in the Crab Nebula: they could not detect any polarisation – less than 36% of X-ray photons had their electric vectors aligned. They sent another rocket in 1971 with an improved version with graphite crystals to look at X-rays of specific energy at 2.6 keV and discovered X-ray polarisation in Crab: about 20% of the photons at this energy show aligned electric vectors. Buoyed by this success, they sent a polarimeter in the 8th Orbiting Solar Observatory (OSO-8) in 1975: it confirmed the X-ray polarisation in Crab and put quite stringent (5 – 10%) upper limits on the polarisation of several bright X-ray sources.

Exploring X-ray Polarisation using AstroSat
AstroSat, being India’s first dedicated astronomy mission, was a massive effort involving the participation of many ISRO centres, academic institutes, and universities. Tata Institute of Fundamental Research (TIFR), Mumbai led the scientific effort by shouldering the responsibility to deliver three of the five major payloads of AstroSat.

Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, founded for the explicit purpose of promoting “the nucleation and growth of active groups in astronomy and astrophysics at Indian universities’’, was an indirect participant in the AstroSat instrument development: Dipankar Bhattacharya (IUCAA) designed the specialised `coded aperture masks’ for two instruments of AstroSat. Since AstroSat is supposed to be an observatory class satellite catering to all users, including those from the universities, it was thought that a formal participation of IUCAA in several allied tasks like software development, student training, etc. would enhance the utility of AstroSat data.

Arthur Holly Compton had discovered, in 1923, that X-rays can deposit part of their energy in a material and undergo `Compton’ scattering. The CZT detectors are pixellated (and hence, have position information) and at certain energies, X-rays will interact primarily by Compton scattering. If the instrument can be made sensitive to measure the incident X-rays as well as the scattered X-rays, then the distribution of the scattered X-rays in the neighbouring pixels of the CZT detector will show some tell-tale signs of the polarisation properties of the incoming X-rays. Such pixellated X-ray detectors are thought to work as X-ray polarimeters, but experimentally no one, so far, had demonstrated the polarisation properties of pixellated X-ray detectors. Since X-ray polarisation measurement is a long sought after experimental technique in X-ray astronomy, examining the utility of CZTI as an X-ray polarimeter looked attractive.

Polarisation Experiments
There followed a flurry of activity to demonstrate that CZTI can be used as an X-ray polarisation instrument. Rao ascertained from the supplier of the CZT detectors that the properties of incident and scattered X-rays are measured and retained, and this was quickly demonstrated by some simple experiments using radio-active sources of known energies. Santosh used a sophisticated code to simulate this behaviour and satisfied himself that polarisation measurements can indeed be made. Then followed a series of Varkari between Ahmedabad and Mumbai: Santosh and his student Tanmoy Chattopadhyay (currently working as a post-PhD researcher at Pennsylvania State University) made controlled experiments on the flight models of CZTI detectors. Several improvisations were part of the testing: like the use of simple thermocol pieces as experimental set-up, use of very strong radio-active sources of X-rays and making them scatter from blocks of aluminium to polarise them - but using sophisticated simulations to precisely calculate the energy of scattered X-rays.

All these activities were done while the fabrication and testing of the flight models were going on ! First a single X-ray detector, called a module, was bombarded with X-rays of known energies and its properties were measured. A rigorous modelling, based on these experimental results, showed that CZT Imager as a whole, with 64 detector modules, will have enough sensitivity to measure X-ray polarisation from bright cosmic X-ray sources. Then started a series of controlled experiments for the proof of concept: beams of X-rays were generated, with or without polarisation, and the polarisation signal in the detector was measured and compared with the results from the simulations. All the tests were repeated for X-rays of a single energy, as well as continuum of X-rays (as expected from Cosmic X-ray sources). Finally, some precious time was readjusted from the very tight delivery schedule of the flight model to repeat these tests in a fully flight-like environment.

This is only the beginning of the story. Because, all the tests in the lab are made with small beams of X-rays and in flight, while observing X-ray source situated at thousands of light years away, one expects parallel beam of polarised X-rays. To test with similar large area polarised beam of X-rays in the lab, one needed to build a humongous facility – perhaps taking several years of effort just to fabricate it. The whole detector geometry was simulated and sophisticated simulations were carried out to predict the behaviour of the detector. These were compared with the controlled lab experiments of small beam, at different experimental configurations, to satisfy that the simulation results are indeed as observed in the lab. This gave a confidence for the prediction of the parallel beam of large area and the results indeed showed that CZT Imager is a very sensitive X-ray polarimeter.

Finally, on 2015 September 28, a fully ground calibrated X-ray polarimeter was flown into space.

The special source ‘Crab’ and its polarisation
When a new era of astronomy was ushered in by telescopes in the 17thand 18th century Europe, it was realised that the sky consists of several fuzzy objects, apart from point-like stars, and they were named ‘Nebula’. The Crab Nebula, in the constellation of Taurus, is the first object identified by Messier, a French astronomer, who meticulously catalogued such objects. Crab Nebula cannot be seen by the naked eye, but can be seen as a fuzzy crab-like feature (hence the name) by a simple binocular. Among such nebulae, supernova remnants form a large fraction. Soon, it was realised that the Crab Nebula is none other than the `Guest Star’, or the supernova recorded by the Chinese astronomers in year 1054.

Within a year of the discovery of pulsars (rapidly rotating highly magnetised neutron stars) in 1967, a pulsar with a period of 33 milliseconds was discovered in the Crab Nebula. It was, however, surmised that the explosion of the supernova that occurred about 1000 years ago has long lost its energy and currently, the nebula is powered by the monster sitting at its centre: the fast spinning neutron star. Crab Nebula, along with its pulsar, became the darling of all observers: it is the only pulsar observed to be pulsating in all branches of the electromagnetic radiation, from radio to even ultra high energy gamma-rays. Since the pulsations are due to simple light-house effect of the rotating neutron star, the period should be, and indeed is, the same in all wavelengths - hence the Crab pulsar became a calibrating source to test the timing accuracy of any new window of observation. The emission mechanism is understood as due to the movement of particles in a magnetic field, the particles being accelerated to humongous energies in the polar cap of the highly rotating magnetic neutron star. Since the magnetic field and the rotation period are reasonably stable for several years, the emitted flux, in hard X-rays, is expected to be a constant : hence it also became a good flux calibrator for any new observations. The realisation that particles are accelerated to very high energies has made a strong case for such young supernova remnants as sources of the mysterious particles bombarding the Earth: Cosmic Rays.

In recent years, Crab Nebula and pulsar are the subjects of diverse observations and theorising to understand the particle acceleration mechanism in fast spinning neutron stars. Measurement of polarisation as a function of pulse phase in optical and radio wavelengths and detailed theoretical modelling are hinting that particle acceleration happens outside the `light-cylinder’ of this source and scientists across the globe are pondering over the nature of such acceleration

Crab polarisation using AstroSat
Crab Nebula was the first source AstroSat stared at post switch ON and verification, and, it kept looking at this source quite often for a periodic calibration of the X-ray instruments, as well as to measure its polarisation. There followed a very meticulous analysis. Every recorded photon was examined, stringent criteria were imposed on them to be validated as `Compton-scattered’ events, their distribution among their neighbours were examined, for consistency, among different detectors in the instrument. Regions in the sky were identified and the position of the satellite was manoeuvred such that the instrument looks at a `source-free’ region in the sky at similar satellite orientation with respect to Earth. The behaviour of the detector was studied in these `background’ regions and the resultant profile was subtracted from the source observations. To establish that these `Compton-scattered’ photons were really from Crab, their arrival times were investigated and found to be beating at the standard Crab clock of 33 milli-seconds.

Yes, X-ray polarisation was clearly detected even in individual observations lasting about half a day and when all the eleven observations were added together, the measurement is clearly the most precise hard X-ray polarisation measurements till date The results were submitted to the British journal Nature.

It is true that measuring polarisation with the highest accuracy possible is a technical challenge: but what is the new thing we are learning ? After a few weeks of intense debate among the researchers, it was decided to collect more data and attempt a very precise measurement of the variation of the polarisation properties when the pulsar beam sweeps across us, the observers. For this, data from different observations spanning across 18 months had to be added. Do we know the pulsar period accurately for this ? The Crab pulsar was observed in radio wavelengths from the Ooty radio telescope every day and also many times during these months from the Giant Meter-wave Radio Telescope (GMRT) at Khodad, near Pune. The pulse of pulsar was accurately measured, the X-ray photons were assigned a pulse phase and the change in the polarisation property as a function of the pulsed beam emission was studied.

When we think that the pulsar beam is shining elsewhere, the remainder of the beam has sufficient X-ray emission, with the polarisation increasing and also showing a sharp change. What it means is that the light house is leaking and emitting high energy X-rays all the time. Most magnetospheric theories predict that the polarisation of X-ray radiation will show changes only during the emission of a pulse, but not at other times. The new observations thus support the view that the particle acceleration is happening outside the conventional boundary of the magnetosphere, in a region where the charged particles generated by the pulsar are spiralling out in the form of a wind. The surprising observation by CZTI of a sharp change of polarisation in the “off pulse” region is clearly a big challenge to theorists.

This result is published in Nature Astronomy on November 6, 2017, publication doi: 10.1038/s41550-017-0293-z

Left panel: The grey line shows the brightness of the Crab pulsar as observed by AstroSat CZTI. The horizontal axis (phase) represents time expressed in units of the pulsar’s spin period. Phase 0.0 to 1.0 stands for the full rotation cycle of the pulsar. The same result is shown repeated between phase 1.0 and 2.0, for a clear demonstration of the periodic pattern. Colored bars indicate how strongly polarized the observed radiation is. Sharp variation of polarization when the brightness is low is the surprising discovery by AstroSat.

Right panel: The angle of X-ray polarization measured by AstroSat CZTI shown superposed on a composite optical and X-ray image of the Crab nebula, taken by NASA’s Hubble and Chandra telescopes respectively. The white arrow represents the projected spin axis of the pulsar located at the center of the nebula. Other arrows display the orientation of the observed polarization. The color of an arrow indicates the range of phase it belongs to, being equal to that spanned by bars of the corresponding shade in the left panel.
Story of the Week - Archive
Nov 06, 2017 : CZT Imager of AstroSat measures first phase resolved X-ray polarisation of Crab pulsar
Oct 30, 2017 : 38th Asian Conference on Remote Sensing
Oct 23, 2017 : ETS - A System for Transportation of Small Satellite and Flight Hardware
Oct 17, 2017 : ISRO Celebrates World Space Week-2017
Oct 09, 2017 : Successful completion of One Year of Service by SCATSAT-1 Scatterometer
Oct 03, 2017 : Many Satellites Celebrated Birth Anniversary during last week of September
Sep 25, 2017 : Operationalisation of Thunderstorm Nowcasting Services over NE Region using DWR data

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[Indx TechStyle]

Feb 27, 2018
AstroSat Picture of the Month (January 2018)
DETECTION OF MILLI-SECOND TIMING PHENOMENA BY LAXPC
X-ray binaries are a special class of binary stars that are very bright in X-rays. The X-rays are produced when matter is accreted from the donor star (usually a relatively normal star) onto the accretor, which is very compact – a Neutron Star or Black Hole.

4U 1728-34 is a Neutron Star low mass X-ray binary (NS-LMXB) which is known to exhibit regular thermo-nuclear bursts (Type-I) of accreted matter onto the Neutron Star surface. The burst oscillations (BO) observed during the initial phase of the Type-1 burst is one of the important diagnostic tool to measure the spin period of the NS-LMXB.

In addition to BO, which originates at the surface of NS, quasi-periodic oscillations (QPO) of X-ray radiation from the accreting gas is also a common phenomenon observed in X-ray binaries. QPOs in milliseconds timescale are very important tool to understand the dynamics of accretion flow at the close vicinity of the compact objects.

The source 4U 1728-34 was observed by LAXPC onboard AstroSat on 8th March 2016 for ~ 3 ksec duration. Dynamical power density spectrum in the 3-20 keV band during the observation, reveals the presence of a high frequency QPO (HFQPO) whose frequency drifted from ~ 815 Hz at the beginning of the observation to about 850 Hz (fig.1). The QPO is also detected, for the first time in the 10-20 keV band by LAXPC (fig.2).

Fig.1: Dynamic power spectra of HFQPO in the energy range 3-20 keV. X-axis shows time evolution of frequency of QPO. Y-axis shows frequency in Hz. Colour coding indicates the power.

Fig.2: Power spectrum of HFQPO in the energy range 10 – 20 keV
During the end part of LAXPC observation of the source, a typical Type-1 burst was detected, and the count rate in the detector reached ∼10000 c/s. The burst profile is typical with a fast rise and slow decay lasting for ~20 seconds (fig.3).

Fig.3: Time profile of the Type-I X-ray burst observed with LAXPC
In the early phase of the burst, a coherent burst oscillation was observed at around 363 Hz. The frequency varies from ~361.5 to ~363.5 Hz which has been reported by Rossi X-ray Timing Explorer (RXTE) during earlier burst observations.

Thus LAXPC demonstrated the capability of detecting millisecond timing phenomenon even from short observations.

Reference:Jai Verdhan Chauhan et.al., The Astrophysical Journal, 841:41 (5pp), 2017 May 20
Ctsy: isro.gov.in

 

[Indx TechStyle]

Feb 28, 2018
AstroSat Picture of the Month (Science Day-February 2018)
The Witch’s Broom in the sky

Image Credits: F. K. Sutaria, K.P. Singh, P. T. Rahna, J. Murthy, A.K. Ray and N.K. Rao

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.

 

[Indx TechStyle]

Sep 28, 2018

Three years of AstroSat​

AstroSat, India’s first space observatory class satellite dedicated to Astronomy, was launched onboard PSLV from Satish Dhawan Space Centre SHAR, Sriharikota on September 28, 2015 into a low earth orbit. After the first six months of calibration and verification phase, the observatory started observing cosmos in multi-wavelength spanning a wide range from near Ultraviolet (UV) to High Energy X- rays.

AstroSat carries a total of five scientific payloads, namely, Ultra-Violet Imaging Telescope (UVIT), Soft X-ray Telescope (SXT), Large Area X-ray Proportional Counter (LAXPC), Cadmium Zinc Telluride Imager (CZTi) and Scanning Sky Monitor (SSM). AstroSat has provided good spatial resolution images in UV over half degree field of view and has a large collecting area at High Energy X-rays (LAXPC). Except for SSM, other four payloads onboard AstroSat are co-aligned and capable of performing simultaneous observations of astronomical sources. The observations were carried out based on the proposals received from users in India and abroad. AstroSat has observed more than 750 sources till September 2018. For the proposal cycle starting from October 2018, around 150 of them are approved and scheduled for observations.

From the beginning, AstroSat is providing good results. Data from AstroSat has resulted in close-to 100 publications in refereed journals, and this number is expected to increase with the data now made open to public on September 26, 2018. (Archival Data of AstroSat released).

AstroSat has provided several new and exciting results like

• Solving the decade old puzzle of a cool red star but bright in UV, by identifying it as a binary

• X-ray polarisation from Crab nebula

• Detection of a coronal explosion on the nearest planet-hosting star (simultaneously observed by NASA’s Chandra X-ray observatory and Hubble Space Telescope)

The Ultraviolet Tails of the “Atoms for Peace” galaxy (NGC 7252). Image Credits: Koshy George et al., 2018.

UVIT image of Witch's Broom, part of Veil Nebula. Image Credits: F. K. Sutaria et al.

 

[Indx TechStyle]

ISRO, NASA just spotted a black hole spinning so fast that it could be making space itself rotate

• India’s AstroSat and NASA’s Chandra X-Ray Observatory have confirmed the discovery of a black hole spinning close to the maximum possible speed.
• A black hole spinning that fast can make space itself rotate according to Einstein’s theory of relativity.
• This black hole is only one of five to have an accurately measured high spin rate.
• Researchers hypothesise that this could be the key to understanding how galaxies are formed.

Black holes, while fascinating, aren’t a new discovery. But, a black hole spinning at one of the highest speeds ever is a whole other story. Especially when there have only ever been four others like it.

India’s first dedicated astronomy satellite, the AstroSat spotted a black hole in the binary star system called 4U 1630-47 that’s spinning close to the maximum speed possible. NASA’s Chandra X-Ray Observatory confirmed the high spin rate.

This particular ‘monster black hole’ is spinning very close to the limit set by Albert Einstein’s theory of relativity according to Rodrigo Nemmen, the lead author on the research paper. That means anything that’s being pulled into the black hole is being pulled in at the speed of light.

Currently, scientists only have two ways of measuring black holes – either by their mass or by their spin rate. And, a spin rate can be anywhere between 0 and 1. This black hole was spinning at the rate of 0.9.

Einstein’s theory further implies that if a black hole spinning that fast, then it is capable of making space itself rotate.

In fact, if the conditions around black holes are hypothesised to be correct, then the high spin rate couple with the gaseous elements entering the black hole and high temperatures, could be the key to understanding how galaxies are formed.

Including the black hole discovered by the AstroSat, there are only five black holes have accurately measured high spin rates. Even if you’re not taking spin rates into account, this black hole of one of only 20 others that have been spotted in the Milky Way Galaxy.

The Indian Space Research Organisation’s (ISRO) AstroSat along with the National Aeronautics and Space Administration’s (NASA) Chandra X-Ray Observatory have confirmed the speed of the spinning black hole.

The study was conducted by researchers from multiple institutions led by the Tata Institute of Fundamental Research (TIFR) and has been accepted for publication in The Astrophysical Journal.

 

[Chinmoy]

ISRO's AstroSat Discovers Ultraviolet Wings on the Butterfly Nebula

https://thewire.in/the-sciences/isros-astrosat-discovers-ultraviolet-wings-on-the-butterfly-nebula

Pune

: Indian astrophysicists have discovered large ultraviolet lobes and jets, hurled out from a dying star, using data from AstroSat, the space observatory launched by the Indian Space Research Organisation (ISRO) in 2015. The discovery has been featured as the AstroSat Picture of the Month (APOM) for October.

Kameswara Rao of the Indian Institute of Astrophysics and his collaborators used the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat to study a planetary nebula called NGC 6302, popularly called the Butterfly Nebula. A planetary nebula is formed when a star like our Sun – or a few times heavier – is in its dying days. The term, a misnomer now, was coined by astronomers in the 19th century since the nebula looked like planets through their telescopes.

“When hydrogen and helium fuel that kept the star shining gets exhausted, the star expands in size and becomes a red giant star,” Rao explained. “Such stars shed most of their outer layers which expands outwards, and the inner core, made of carbon and oxygen, shrinks further and becomes hotter. This hot core shines brightly in the ultraviolet, and ionises the expanding gas. This glowing ionised gas is what is seen as a planetary nebula.”

Sriram Krishna, a student of Rao, spent many hours analysing the data from the Butterfly Nebula. “Its central star is one of the hottest that we know, at 220,000 degrees celsius. The name itself comes from the shape of the two lobes of expanding gas that look like the wings of a butterfly,” he said.

One might expect a planetary nebula to be spherical, but it actually exhibits a range of complicated structures. “We used the UVIT on AstroSat to make four images of the nebula, each in different ultraviolet ‘colours’, or filters. The image made with the filter centred at 160.8 nm, called F169M, had a surprise in store for us,” said Sriram.

Astronomers have studied the two lobes of the nebula for many years through visible light images. They expect that the more energetic ultraviolet light would be emitted closer to the central star, where the hot stellar wind hits the slowly expanding gas. “However, we discovered that the lobes imaged with the F169M filter in ultraviolet were about three times larger than the size of the lobes imaged in visible light,” according to Sriram. After careful analysis, their study concluded that this ultraviolet emission must be due to cold molecular hydrogen gas outside the visible lobes, which had gone undetected so far.

“Our discovery points to an unseen companion star in an orbit with the central star,” said Firoza Sutaria, one of the coauthors. In addition, researchers also discovered two faint jets blasting out from the centre at almost right angles to the new ultraviolet lobes.

The team led by Rao recently discovered a large ultraviolet halo in yet another planetary nebula using AstroSat, and will be looking at more such objects in the future. They hope that such discoveries may provide the answer to the age-old puzzle of the missing mass problem in planetary nebulae.

This discovery was made possible because of the uniqueness of UVIT. “Of all the ultraviolet telescopes in space, UVIT is special in its ability to image a large field of view with a very high resolution, or detail”, said V. Girish of ISRO.

“This ability, coupled with a novel image analysis software that we had developed, led us to this discovery”, explained Jayant Murthy, a coauthor of the paper and director of the Indian Institute of Astrophysics.

These results were accepted for publication in the journal Astronomy and Astrophysics on October 3, 2018.

The AstroSat Picture of the Month series, or APOM, is a year-old initiative of the Public Outreach and Education Committee of the Astronomical Society and the AstroSat Training and Outreach Team. The aim of APOM is to share the excitement of AstroSat science as well as the beauty of the universe with everyone. All APOMs are archived here.

 

[Indx TechStyle]

Oct 31, 2018
AstroSat Picture of the month - Oct 2018
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 3,800 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 beyond 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.