Indian astronomers play Sherlock Holmes

By T V Venkateswaran

New Delhi: Abit of astronomical sleuthing by a team of Indian astronomers led by Varun Bhalerao of IIT Bombay has hit pay dirt. They have unravelled the true identity of a sudden gamma ray burst that has been puzzling astronomers across the world since early this year.

The Asteroid Terrestrial-impact Last Alert System (ATLAS) a Hawaii based observatory specializing in detecting transient celestial events lasting only for a short time detected a strong burst of light, named ATLAS17aeu in January this year.

Such sudden brightening of a stellar object, emitting more than the combined energy output of millions of stars, are usual. However location of the transient event appeared to coincide with the location of the third gravitational wave source detected on January 4, 2017 and since named GW170104.

The optical burst was seen 23.1 hours after GW170104 and rapidly faded over the next three nights. The transient was also detected as a fading x-ray source by Swift space telescope and astronomers suspected that transient ATLAS17aeu, was the afterglow of the cataclysmic cosmic event that generated gravitational wave source GW170104

"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 "

Massive amount of gravitational waves, ripples in the fabric of space-time, is caused when massive bodies such as black holes merge. In all three gravitational wave events emerging from merger of massive black holes have been detected so far by the Laser Interferometer Gravitational-wave Observatory (LIGO). The latest, GW170104, observed by the LIGO on January 4, 2017, is by far the farthest located about 3 billion light-years away.

As the Hawaii-based ATLAS group reported that they have found associated explosions in the sky, in the right place, fading fast, astronomers were in frenzy as this would be the first time an optical associate is found for the invisible gravitational wave source. Astronomers would learn a lot if we can observe these sources through telescope. Certain gravitational wave events are expected to emit large dose of x rays and gamma rays and therefore as and when the gravitational wave detection was suspected various space telescopes were trained in the direction of the source.

But, it was not to be. Data from AstroSat, Indian space telescope mission has thrown water on the assumption. Varun Bhalerao of IIT Bomay, who was studying the data from the satellite, established that a gamma ray burst had separately occurred on January 5, 2017 and that the transient cosmic event ATLAS17aeu detected by the Hawaii based group was because of that and not gravitational wave source GW170104.

The Indian team led by Dr. Bhalerao, had lurched into action when the Hawaiian team reported on ATLAS17aeu. They immediately started looking into the data collected by CZTI (Cadmium Zinc Telluride Imager), a gamma ray telescope aboard AstroSat. CZTI is the most sensitive instrument in the world to find transient sources even if they flicker less than a fraction of a second.

Even as Dr. Bhalerao, after a detailed analysis of the data, was concluding that ATLAS17aeu must be related to a different event than GW170104, he got a mail from a student identifying a gamma ray burst named GRB 170105A,in the same part of the sky, 21 hours later than the gravitational wave burst.

To check on his suspicion a multi-national team under the international `Global Relay of Observatories Watching Transients Happen (GROWTH)' programme swung into action observing ATLAS17aeu with optical, X-ray and radio telescopes.

Dipankar Bhattacharya, from Inter University Centre for Astronomy and Astrophysics, Pune a member of GROWTH team says, "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." It was clear that ATLAS17aeu was an afterglow of the birth of a new blackhole as a star imploded and not signal from the merger of two black holes generating massive gravitational waves.

Incidentally, many international space telescopes missed this gamma ray burst. Only India's CZTI on AstroSat and the Chinese-European POLAR instrument captured it.

Based on the success of the CZTI instrument aboard ISRO's AstroSat, Indian astronomers have proposed a small sized instrument called MOTIVE to ISRO as a likely payload for a future interplanetary mission. A.R. Rao, from TIFR says "Together, CZTI and MOTIVE can revolutionize the field and one day find the first electromagnetic counterpart to a gravitational wave source".

The work led by V. Bhalerao included M. M. Kasliwal, D. Bhattacharya, A. Corsi, E. Aarthy, S. M. Adams, N. Blagorodnova, T. Cantwell, S. B. Cenko, R. Fender, D. Frail, R. Itoh, J. Jencson, N. Kawai, A. K. H. Kong, T. Kupfer, A. Kutyrev, J. Mao, S. Mate, N. P. S. Mithun, K. Mooley, D. A. Perley, Y. C. Perrott, R. M. Quimby, A. R. Rao, L. P. Singer, V. Sharma, D. J. Titterington, E. Troja, S. V. Vadawale, A. Vibhute, H. Vedantham, S. Veilleux. (India Science Wire)

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