Nearly a century after A. Einstein formulated the theory of gravitational waves (GW), the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors detected gravitational waves from a binary black hole (BBH) merger in 2015. The 2017 Nobel Prize in Physics was awarded for the development and ultimate success of LIGO. Later, in the year 2017, the Advanced LIGO and VIRGO detectors detected GW from a binary neutron star merger event (GW170817). This GW detection was followed by EM detections -- where a short GRB (X-rays) and a kilonova (radio, optical, and X-rays) were observed through a tremendous collaborative effort across the EM spectrum.
We search for such Electro-Magnetic counterparts to Gravitational Waves (EMGW) in hard X-rays using the CZTI payload onboard AstroSat, and in optical wavelengths using the GROWTH-India Telescope at Hanle. The LIGO-Virgo-KAGRA Collaboration (LVK) detects gravitational waves, and issues notices to astronomers, like us, around the world. We then start searching for any new astrophysical sources in the regions that the GW detectors tell us the signal is likely to have come from.
During the search for GW170817, which was the first-ever Binary Neutron Star merger event observed, AstroSat CZTI data was used to help localize the position of the source galaxy by non-detection. Since then, we have been using CZTI to search for prompt X-ray bursts around the time of GW candidates, and establish sensitive upper limits to prompt emission from GW sources. The GROWTH-India telescope also scans the sky for new astrophysical sources, taking several images a night, and comparing them to previously observed images of the same parts of the sky, and searching for new transients. During the hunt for an EM counterpart for another GW event detected on 26th April 2019, GIT collaborated with other telescopes around the world, hunting for any signs of a kilonova by observing 18% of the total localization region. Check out more details about this work in our publications section.
The exciting hunt for EMGW is quite literally a race against time; we are in the process of automating all these searches and the day is not far away when we finally detect an EMGW counterpart!