Kanpur, Jan 23: A team of scientists from the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune and the Indian Institute of Technology, Kanpur studied the X-ray emission from a well known black hole binary GRS 1915+105 using the Indian space Observatory AstroSat launched by ISRO. They analysed data from the Large Area Xray Proportional Counter (LAXPC) and the Soft X-ray Telescope (SXT) instruments which were both developed in TIFR Mumbai.
Using the unique capability of these instruments, they were able to estimate simultaneously the frequency of the oscillation, the inner accretion disk radius and the accretion rate which is the amount of matter going into the black hole per second. They found that the frequency varies with inner radius and accretion rate in the very same manner as the dynamic frequency, i e inverse of the time taken by sound waves to travel from the inner radius to the black hole, as predicted four decades ago by the General Relativistic theory of accretion disks.
This identification paves the way to make such system laboratories to test General Relativity. Professor Ranjeev Misra (IUCAA) said here on Wednesday that, "For many years scientists have observed that the X-ray emission from black hole systems vary rapidly and sometimes nearly periodically, which implied that they must be modified by General Relativistic effects due to the strong gravity of the black hole. It has been frustrating that we have not been able to concretely match the predictions of the theory and identify the nature of the variability. I am happy that AstroSat has been able to make this important breakthrough by identifying this variability with the General Relativistically modified sound crossing time. We look forward to more such results from AstroSat and other observatories, which will now allow theoretical frameworks to be more rigorously developed."
Ms Divya Rawat (Ph D student, IIT-Kanpur) said "With this research, we have identified the origin of widely known X-ray variability in black hole systems as the time taken by pressure waves to travel through the accretion disk. In our future research projects, we will work on the theoretical models which could explain the physical process involved here. I hope we made our small contribution towards the understanding of complicated Black hole systems.” Prof Pankaj Jain (IIT-Kanpur) said "Using AstroSat data we have been able to probe strong gravity effects predicted by the General Theory of Relativity at a distance as close as three times the distance to the event horizon, i e the surface of no return from the center of a black hole. At such distances Newtonian gravity fails. It is very exciting to find that the data agrees well with the prediction of General Relativity. I am also fascinated by the fact that the black hole in this source is spinning at nearly its maximum allowed value."
Prof JS Yadav (IIT-Kanpur) who retired from TIFR Mumbai in 2018, was the Principal Investigator of the LAXPC instrument during its space qualification tests, calibration and launch. He said “It is indeed very challenging and time-consuming to develop a world-class space instrument in India. However, if the instrument performs with high precision as planned, it is very satisfying and well worth all the risks undertaken. Earlier, I have extensively worked on this black hole system, GRS 1915+105 using data from space observatories launched by other countries. I am very happy that now we have been able to demonstrate this important result Identification of X-ray variability in Black hole binary systems using our own satellite which was not done earlier by other space observatories. This result underlines the importance of future Indian science space missions.” (UNI)