Scientists found the cause behind the variations in brightness around a stellar-mass black hole in outer space. Observations over almost an entire year revealed an unexpected warp in the accretion disk.
Ground-based and space telescopes have helped astronomers determine that the accretion disk around the black hole in the MAXI J1820 + 070 binary X-ray system is curved, which causes strong fluctuations in brightness in the optical range in the system. Further studies of this system could improve our understanding of the behavior of such systems during the phase of activity of black holes.
Astronomers discovered a curved accretion disk around a black hole
Accretion processes
Accretion processes are widespread in the Universe – thanks to them, stars and planets are formed. It is the same reason why type Ia supernova explosions or black holes gain mass. In the latter case, observations of such compact objects surrounded by accretion disks can tell a lot about the black holes of different masses and the mechanism of generation of relativistic plasma ejections – jets.
MAXI J1820 + 070
MAXI J1820 + 070 (or ASASSN-18ey) was detected in the optical range on March 6, 2018, as part of the ASAS-SN survey, and 5 days later it was spotted by the MAXI (Monitor of All-sky X-ray Image) X-ray telescope installed on the ISS. This X-ray binary system is 9,600 light-years from Earth and consists of a black hole about eight times the mass of the Sun and a star about half the mass of the Sun. It is a source of bursts of radiation in the X-ray and radio wave ranges. Earlier observations of the system helped to reveal the movement of plasma in the jets.
Light curves of the binary system
A group of astronomers led by Jessymol K. Thomas of the South African Astronomical Observatory has published the results of the analysis of the light curves of MAXI J1820 + 070 in the optical, X-ray, and radio bands, obtained with ground-based and space telescopes and covering several months of observations in 2018-2019.
First 86 days
Scientists have identified several phases in the behavior of the system. The first 86 days after the onset of the flare, astronomers observed the classical light curve of an X-ray / optical flare of a low-mass X-ray binary with a gradual decrease in the emission level.
Days 87 to 112
From 87 to 112 days, there was a gradual increase in brightness along with the sudden appearance of huge modulation of radiation in the optical range, during which the peak brightness of the source doubled in almost 17 hours.
Days 112 to 253
From 112 to 253 days, a change in the properties of X-ray radiation began, which passed from a hard range to a soft one.
Conclusion
The researchers concluded that this strange behavior of the system cannot be explained by the heating of the donor star’s surface by X-rays, but is the result of the curvature of the accretion disk. In this case, the source of hard X-ray radiation, which may be the base of the jet, is located above the deformed outer regions of the accretion disk. This makes it possible to increase the area of the disk that can be illuminated, thereby significantly increasing the radiation flux at certain points of observation.
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Sources:
• Chandra X-ray Observatory. (n.d.). Maxi J1820+070.
• Phys.org. (2021, October 27). Blackhole with warped accretion disc discovered.
• Sci News. (n.d.). Warped Accretion Disk Spotted around Stellar-Mass Black Hole.
• Thomas, J. K., Charles, P. A., Buckley, D. A. H., Kotze, M. M., Lasota, J.-P., Potter, S. B., Steiner, J. F., & Paice, J. A. (2021, October 26). Large optical modulations during 2018 outburst of maxi J1820+070 reveal evolution of warped accretion disc through X-ray state change. OUP Academic.
