Mumbai The Indian Space Research Organisation’s (Isro) AstroSat, India’s first dedicated astronomy satellite, in coordination with The National Aeronautics and Space Administration’s (Nasa) Chandra X-ray Observatory has found that a black hole in the binary stellar system 4U 1630−47 spins at a rate that is close to the maximum possible rate.Researchers said studying black holes with high spin rates is important as it paves the way to test the fundamental laws and theories of the universe, one of which is Albert Einstein’s theory of gravity – general relativity. The significance? Black holes generate the strongest gravitational force in the universe.A black hole, which is an exotic but a very simple astronomical object, is created when a massive star dies and the matter gets squeezed into a tiny space under a heavy force of gravity, trapping in the light. Using the X-ray data from AstroSat and Chandra, an international team of astronomers from multiple institutions led by the Tata Institute of Fundamental Research (TIFR) found that the spin rate of the black hole was about 0.9. The spin rate is a dimensionless unit ranging between 0 and 1. The study comprising an international team of astronomers from multiple institutions and led by TIFR has been accepted for publication in The Astrophysical Journal, a peer-reviewed scientific journal of astrophysics and astronomy.The findings are significant, said researchers, because so far, high spin rates of only about five black holes have been quantified accurately. Measurement by AstroSat-Chandra is one of them. This is one of the 20 black holes that have been discovered in our galaxy. Each of them about 10 times the mass of the Sun. “As black holes get created, mass and spin rate are two properties that characterise them. Mass can be measured more easily as it has a long-range effect because of gravity as the black hole gets created,” said Sudip Bhattacharyya of TIFR, and principal investigator of Astrosat’s Soft X-ray Telescope (SXT).“However, measurements especially of the spin rate are very difficult to make, and can be done only by high-quality X-ray observations in the correct state of the binary stellar system, in which the black hole is a gobbling matter from its companion star”, said the lead author Mayukh Pahari. Pahari started this work at TIFR, before joining a Royal Society-SERB Newton International Postdoctoral Fellow position at the University of Southampton in UK.It was the indigenously built SXT and Large Area X-ray Proportional Counter (LAXPC) instruments on-board the Astrosat that first observed the black hole in a favourable state so as to be able to interpret the X-ray at the end of 2016. Following this, the Astrosat team requested director of Chandra X-ray Observatory to observe the same black hole. Having independently measured the spin rates of the black hole, the results from Astrosat were consistent with data from Chandra satellite, making the findings robust. Bhattacharyya said, “This is the first cooperation between India and the US using AstroSat and Chandra for black hole studies. Since it was very successful, other collaborations will also now be possible.”FactsSudip Bhattacharyya and AR Rao from TIFR (India), Mayukh Pahari, IM McHardy, Poshak Gandhi and Diego Altamirano from the University of Southampton (UK), Dipankar Bhattacharya and Gulab Dewangan from the Inter-University Centre for Astronomy and Astrophysics (India), Santosh Vadawale from Physical Research Laboratory (India), Stephane Corbel from University Paris Diderot and CEA Saclay and Observatoire de Paris (France) and Norbert Schulz from Massachusetts Institute of Technology (USA) contributed to this research.