Researchers look for high-energy light from gravitational-wave candidates in LIGO-Virgo-KAGRA Observation Runs with India’s AstroSat-CZTI

We can “see” our universe because our eyes can detect light in the form of energy quanta or packets of energy called photons or electromagnetic (EM) waves. However, besides electromagnetic (EM) waves, our universe also uses other messengers to carry information such as the recently discovered gravitational waves (GW). These waves are produced when pairs of massive objects, such as extremely magnetic neutron stars and black holes, rapidly accelerate towards each other and eventually merge into a single entity. Today, the global GW Network, which includes the American LIGO, the European Virgo Gravitational Wave interferometer (Virgo), and the Japanese Kamioka Gravitational Wave detector (KAGRA), is geared towards capturing the GW signals from such extreme gravitational interactions.

Prof Varun Bhalerao and his research team from the Department of Physics at the Indian Institute of Technology Bombay are investigating the detection of an electromagnetic (EM) signal in conjunction with the gravitational waves (GW) produced by merging pairs of neutron stars, black holes, or a neutron star-black hole pair. Their research seeks to address a significant question: can a merger of two black holes generate a ‘light’ signal (EM signal), or is it essential for one of the objects in the pair to be a neutron star?

Determining the answer to this question will greatly enhance our capacity to monitor these gravitational wave events through existing observational facilities as detecting electromagnetic waves is easier as compared to detecting gravitational waves. The answer will also help model the characteristics of gravitational wave sources, and eventually grasp the emission mechanisms of high-energy light resulting from these merging incidents. Notably, among the 90 detections to date the global GW network witnessed an EM signal associated with a GW only once in 2017, specifically for a binary neutron star merger, which varied from gamma-ray bursts to optical light observed days after the merging event. So far we have not detected EM signals from merging of two black holes or a neutron star-black hole pair.

Gaurav Waratkar, who was involved in this work, explains, “If binary black hole mergers emit X-rays, it would challenge our current understanding of the fundamental physics governing these extreme, highly energetic collisions, shedding new light on their dynamics and energy release mechanisms. These exotic events are thought to produce X-ray bursts, and detecting them would provide critical insights into their physical processes”.

The Cadmium Zinc Telluride Imager (CZTI) on India’s AstroSat space observatory is ideal for detecting such high-energy interactions. It is one of the four X-ray instruments onboard AstroSat and can detect X-rays across a broad energy spectrum ranging from 20 - 200 keV. Gaurav and team used AstroSat-CZTI to search for high energy X-ray emissions coinciding with GW events recorded by LIGO-Virgo-KAGRA within 100 seconds following the GW emission trigger. “AstroSat-CZTI is crucial for this search, having proven to be a highly capable detector with numerous burst discoveries since its launch,” remarks Gaurav.

The research team utilised data from the three initial observing runs of the GW detector to conduct a comprehensive all-sky search. The searches encompassed a luminosity distance of up to 457 million light-years for binary neutron stars and 32 billion light-years for binary black hole mergers. With AstroSat observing around ~70 of these GW detections, they methodically examined each neutron star-black hole pair and binary black hole pairs.

Due to the complexities of space observations, the search faced many challenges. These could include instances where our Earth gets in the way of the probable target and AstroSat. Additionally, certain events might fall outside of Astrosat-CZTI’s sensitivity range or simply be too distant to capture the EM radiation.

AstroSat-CZTI did not detect any of these events in X-rays. This information is extremely useful since it allows us to constrain the characteristics of such events. With such a large sample, this study has been able to confidently state that the maximum brightness these events could have had is lower than the lowest brightness that AstroSat can detect. The limits from AstroSat are as good as those from any other X-ray telescope in the world. These limits are essential to decide on the EM radiation models complementing the GW emission from these binary black hole mergers.

Elaborating on the future scope of this work, Prof. Varun Bhalerao explains, “The universe is the most extreme laboratory of physics and has always surprised us with new, unexplained phenomena. Looking for X-ray bursts from black hole mergers helps us validate humanity’s understanding of physics and look for newer, deeper theories. When LIGO India becomes operational, we will find more distant black hole mergers. For studying those, the proposed Indian Daksha mission would prove to be revolutionary.”

The GW detectors are expected to end their fourth observing run in October 2025. This will further extend the luminosity search distance to at least 554 million light years for binary neutron star mergers and give us a wider sample of GW sources from our surrounding universe. India’s future space mission, Daksha, is being built specifically to detect and understand EM radiation from such GW sources, thanks to the planned extremely sensitive Cadmium Zinc Telluride (CZT) detectors and all-sky coverage.

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