Discovery of a new source of gravitational waves: Collisions between Neutron Star and Black Hole

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Until now, the LIGO-Virgo collaboration (LVC) of gravitational waves detectors has only been able to observe collisions between pairs of black holes or neutron stars. For the first time, in January 2020, the network of detectors made the discovery of gravitational waves from a pair of NS-BH mergers.

Rainbow Swirl Refracted (Source: Carl Knox, OzGrav/Swinburne University)

Raychaudhury said that there is a lot of interesting science that can be learnt from this. “For instance, a neutron star has a surface and black hole does not. A neutron star is about 1.4-2 times the mass of the sun while the other black hole is much more massive. Widely unequal mergers have very interesting effects that can be detected,” he said.

“Inferring from data as to how often they merge will also give us clues about their origin and how they were formed,” said Dr Shasvath Kapadia from the International Centre for Theoretical Sciences (ICTS) in Bengaluru. Dr Kapadia helped with the estimation of the NS-BH merger rate, using a method he co-developed.

“Among the authors of this paper is Bhushan Gadre, who was a PhD student at IUCAA until recently and took part in a lot of our Marathi outreach programmes. He is now with Max Planck Institute in Germany,” Raychoudhury said.

According to the IUCAA director, the technique used here to detect the signal is called matched filtering. “This was also used for the first discovery of gravitational waves. It may be recalled that this was developed at IUCAA in the 1990s by Sanjeev Dhurandhar and others,” he said.

“Basically, we have been detecting binary black hole mergers and binary neutron star mergers (until now). This is a hybrid collision,” according to scientists at the Laser Interferometer Gravitational-wave Observatory, India (LIGO-India).

When contacted, Dr Tarun Souradeep, spokesperson of LIGO-India, said that researchers from LIGO-India have contributed to this major discovery. “This is an ongoing effort and the reason to make the detector network stronger is to discover a newer kind of phenomenon,” said Souradeep.

This is a clear indication of neutron star and black hole merger, Prof Rajesh Nayak from the Center of Excellence in Space Sciences, IISER, Kolkata said.

Prof Sanjith Mitra, scientist with LIGO-India, said that with the development, LIGO-India will significantly improve the sky localisation of these events. “This increases the chance of observation of these distant sources using electromagnetic telescopes, which will, in turn, give us a more precise measurement of how fast the universe is expanding,” Mitra said.

“These observations help us understand the formation and relative abundance of such binaries. Neutron stars are the densest objects in the Universe, so these findings can also help us understand the behaviour of matter at extreme densities. Neutron stars are also the most precise ‘clocks’ in the Universe, if they emit extremely periodic pulses. The discovery of pulsars going around Black Holes could help scientists probe effects under extreme gravity,” scientists said.

How the detections were made

As the two compact and massive bodies orbit around each other, they come closer, and finally merge, due to the energy lost in the form of gravitational waves. The Gravitational Waves signals are buried deep inside a lot of background noise. To search for the signals, scientists use a method called matched filtering. In matched filtering, various expected gravitational waveforms predicted by Einstein’s theory of relativity, are compared with the different chunks of data to produce a quantity that signifies how well the signal in the data (if any) matches with any one of the waveforms. Whenever this match (in technical terms “signal-to-noise ratio” or SNR) is significant (larger than 8), an event is said to be detected. Observing an event in multiple detectors separated by thousands of kilometers almost simultaneously gives scientists increased confidence that the signal is of astrophysical origin, which is the case for both events.

How sure are we that they are NS-BH mergers

Using Parameter Estimation tools, scientists find the probable masses, spins, distances, locations of these mergers from the data. Both of these events occurred 1 billion light years away. As the gravitational waves also travel with the speed of light, this means that we observed mergers that happened ~1 billion years ago — well before life appeared on earth!