Sunday, 18 June 2017

The Hindu: AstroSat rules out afterglow in black hole merger

18TH-SCIVARUNjpgRecently US-based LIGO group announced having detected gravitational waves emanating from the merger of two massive black holes located nearly 3 billion light years away. Simultaneously, Hawaii-based ATLAS group identified a fading glow from the part of the sky where these black holes were roughly estimated to lie. The group surmised that this was an electromagnetic (light based) afterglow emanating from the merger. ISRO’s space observatory, AstroSat, however, has, with extremely sensitive measurements, ruled out the possibility that this has any connection with the black hole merger.

Image: Eye in the sky Varun Bhalerao (right) and his students Vidushi Sharma (left) and Sujay Mate
stand beside a model of the CZTI.

In collaboration with the GROWTH (Global Relay of Observatories Watching Transients Happen) network of observatories, AstroSat team has concluded that this event is due to a gamma ray burst. A gamma ray burst is light emanating from a bursting star, for example, an exploding supernova, that may lead to the formation of a black hole. This places the afterglow among a class of phenomena detected routinely by the space observatory. The discovery was made with the help of the Cadmium Zinc Telluride Imager (CZTI), an x-ray telescope aboard AstroSat.

The burst of light, dubbed ATLAS17aeu, appeared to Varun Bhalerao of IIT Bombay, who leads the searches for transients and explosive sources, to have come from a burst that took place on January 5 and not January 4, the date on which the signal from the black hole merger was picked up by the LIGO detectors. “I shot off a mail 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,” Dr Bhalerao is quoted as saying in a press release circulated by the AstroSat team. CZTI had indeed recorded a gamma ray outburst on January 5. Named GRB170105A, this outburst had happened about 21 hours after the detection of the gravitational wave signal, thereby dissociated from the signal detected by LIGO.

Later analysis made the team guess that this signal was part of a gamma ray outburst. “There are two types of Gamma Ray Bursts: short, hard and long, soft. We showed that this is a long gamma ray burst. These long gamma ray bursts are associated with a particular type of supernova which creates a black hole,” says Dr Bhalerao when contacted by phone.

Prof. A. R Rao of Tata Institute of Fundamental Research, the principal investigator of the CZTI project, describes the special features of AstroSat that make it the most sensitive detector.

“An important innovation of the CZTI is that it is transparent to x-rays above 100 kilo electron volts in energy. So... the instrument receives signals from all directions in the sky,” he says.

The Hawaii group, in their paper still mentions the existence of a small probability that the two events might be related. However this falls far below even a 5-sigma level which is what one expects for the probability to be significant.

This outburst was also independently discovered by a Chinese-European mission called POLAR.

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