In a major breakthrough in astronomy, gravitational waves from a pair of colliding neutron stars were detected for the first time by the US-based laser Interferometer Gravitational-Wave Observatory (lIGO) and the Europe-based Virgo on August 17 this year, with contributions from 40 scientists from 13 Indian institutions.
The development is significant considering that this happens to be the strongest gravitational-wave signal detected so far, owing to the relatively close location of about 130 million light-years from earth. The detection was also confirmed by a large number of telescopes around the world that studied various forms of radiation from the merger.
“This is a new milestone in the success saga of advanced gravitational wave detectors, which have announced the discoveries of four black hole mergers to date. The first such detection in 2015 led to the awarding of the Nobel prize in physics this year,” a news release put out by IIT-Bombay stated here on Monday.
Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovas. Typical neutron stars are heavier than the sun, but have a diameter of just about 20 kilometers: objects so dense that a teaspoon of neutron star material weighs more than Mount Everest.
According to researchers from IIT-B who contributed to the project, scientists could track these neutron stars, weighing about 1.1 to 1.6 times the mass of the sun, for about 100 seconds as they spiralled towards each other in a final deadly dance and collided. These observations contain important clues about the nature of the dense matter that constitute these stars.
The collision created a flash of gamma rays that was detected by earth-orbiting satellites just two seconds after the gravitational waves. This is the first conclusive evidence that short gamma ray bursts, often seen by orbiting satellites, are indeed created by colliding neutron stars — something that had only been speculated for decades.
The lIGO-Virgo results were published on Monday in the journal Physical Review letters; additional papers from the lIGO and Virgo collaborations and the astronomical community have been either submitted or accepted for publication in various journals.
Indian scientists have made pioneering contributions to the gravitational-wave science over the last three decades. 40 scientists from 13 Indian institutions are part of the lIGO-Virgo discovery paper.
The Indian scientists contributed to the fundamental algorithms crucial to search for inspiraling binaries in noisy data from multiple detectors, in computing waveforms for these signals by solving Einstein’s equations, in separating astrophysical signals from numerous instrumental and environmental artefacts, in interpretation of joint gravitational-wave and gamma-ray observations, tests of Einstein’s theory and many other aspects of the data analysis.
In addition, several Indian telescopes such as AstroSat, Giant Metrewave Radio Telescope (GMRT) and the Himalayan Chandra Telescope (HCT) participated in the search for electromagnetic flashes. The sensitive CZTI instrument on AstroSat helped narrow down the location of the gamma-ray flashes.
While HCT obtained optical images at locations of neutrinos detected by other telescopes at the same time as the burst, and showed that they were unrelated to the gravitational-wave trigger, GMRT played a key role in understanding jet physics and refining models of radio emission from the remnant formed by the merging neutron stars.
Two faculty members from the IIT Bombay Department of Physics – Archana Pai and V Gayathri -- are members of the lIGO Scientific Collaboration with complementary expertise. Archana Pai has expertise on gravitational wave multi-detector algorithms and probing astrophysical parameters in the joint detection scenario.
The group worked on interpretation of joint-gravitational wave and gamma-ray observations along with K. G. Arun, (CMI, Chennai) and a PhD student M. Saleem (IISER Trivandrum) within the collaboration. Varun Bhalerao’s expertise is in the search and study of counterparts to gravitational wave sources. He was involved in the EM follow-up efforts: the X-ray searches with AstroSat along with student Arvind B., a search for optical sources with the Himalayan Chandra Telescope, and a radio search with GMRT.
The Indian team in lIGO includes scientists from CMI Chennai, ICTS-TIFR Bangalore, IISER Kolkata, IISER Trivandrum, IIT Bombay, IIT Gandhinagar, IIT Hyderabad, IIT Madras, IPR Gandhinagar, IUCAA Pune, RRCAT Indore, TIFR Mumbai and UAIR Gandhinagar. Astronomers from IISER Pune, IIT Bombay, IUCAA Pune, TIFR Mumbai, PRl Ahmedabad, IIT Hyderabad, IIA Bangalore, NCRA-TIFR Pune, ARIES Nainital and IIST Trivandrum participated in the electromagnetic follow-up of this event using a variety of telescopes.