X-ray variability in black hole systems
Scientists in Pune and Kanpur have identified how X-rays emitted by the accretion disk around black holes vary in frequency, a statement issued on 23 January 2020 said. A team of scientists from the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune and the IIT-Kanpur studied X-ray emissions from a black hole binary (a black hole and a star orbiting each other) GRS 1915+105 using the Indian Space Observatory AstroSat launched by ISRO.
- With this research, we have been able to identify 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, Divya Rawat, a PhD student at IIT-Kanpur, and part of the investigation team, said. Accretion disk is a disk of slowly spiralling gas formed by matter from a star falling into the black hole.
- The scientists analysed data from the Large Area X-ray Proportional Counter (LAXPC) and the Soft X-ray Telescope (SXT) instruments, which were both developed in TIFR Mumbai.
- Using the unique capabilities 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.
- Professor Ranjeev Misra (IUCAA) said, For many years, scientists have observed that X-ray emissions 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.
- The inner regions of the disk produce X-rays, which sometimes vary in time as oscillations, whose frequencies must correspond to a characteristic timescale of the system. Due to proximity of the black hole, these timescales should be modified by Albert Einstein’s General Theory of Relativity. These oscillations may be associated with different physical phenomena and the theory predicts how their frequency would change with the inner disk radius.
- The scientists found that the frequency varies with the inner radius and accretion rate in the 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 systems laboratories to test General Relativity.
- ASTROSAT is India’s first dedicated multi wavelength space observatory. This scientific satellite mission endeavours for a more detailed understanding of our universe.
- One of the unique features of ASTROSAT mission is that enables the simultaneous multi-wavelength observations of various astronomical objects with a single satellite.
- ASTROSAT observes universe in the optical, Ultraviolet, low and high energy X-ray regions of the electromagnetic spectrum, whereas most other scientific satellites are capable of observing a narrow range of wavelength band.
- Multi-wavelength observations of ASTROSAT can be further extended with co-ordinated observations using other spacecraft and ground based observations. All major astronomy Institutions and some Universities in India are participating in these observations.
- ASTROSAT with a lift-off mass of about 1513 kg was launched into a 650 km orbit inclined at an angle of 6 deg to the equator by PSLV-C30.
- After injection into Orbit, the two solar panels of ASTROSAT were automatically deployed in quick succession. The spacecraft control centre at Mission Operations Complex (MOX) of ISRO Telemetry, Tracking and Command Network (ISTRAC) at Bangalore manages the satellite during its mission life.
- The science data gathered by five payloads of ASTROSAT are telemetered to the ground station at MOX. The data is then processed, archived and distributed by Indian Space Science Data Centre (ISSDC) located at Byalalu, near Bangalore.