Indian scientists decode rhythms of a Black Hole: A discovery with AstroSat
AstroSat, has been continuously monitoring the enigmatic black hole--the universe's most enigmatic
powerhouses--since its launch in September 2015 and was providing invaluable insights into source's behaviour.
A group of Indian scientists from University of Haifa, IIT Guwahati, Indian Space Research Organization (ISRO) observed that the X-ray brightness from GRS 1915+105 fluctuates dramatically over time", it said.
This research deepens our understanding of black holes also also highlights India's growing role in
space-based astronomy.
Black holes, the universe's most enigmatic powerhouses, cannot be seen directly, but their immense
gravity reveals their presence.
Born from the collapse of massive stars that exhaust their fuel, these cosmic voids are invisible
because not even light can escape their grasp.
However, a black hole in a binary system with a companion star, triggers a dramatic process known
as accretion, pulling in stellar material that spirals inward and heats up to over 10 million degrees
(far hotter than the Sun's surface temperature of 6000 degrees).
This 'super-heated' matter emits intense X-rays, which are captured by space telescopes, offering
scientists a rare window into the otherwise hidden lives of black holes.
In a distant corner of our galaxy (nearly 28000 light-years away) lies one of the most fascinating
and mysterious black hole, GRS 1915+105.
This intriguing black hole X-ray binary system, consisting of a rapidly rotating black hole with a
mass about 12 times that of a Sun and its companion star, has captured the attention of scientists
due to its unusual and dynamic behaviour.
ISRO also depicted a schematic diagram of the complex accretion process around GRS 1915+105,
forming a swirling disc (1-10 million degree) and corona (∼100 million degree) structure.
"India's first dedicated multi-wavelength space observatory, AstroSat, has been continuously monitoring the enigmatic black hole GRS 1915+105 since its launch (September 2015) and provides invaluable insights into source's behaviour. Using two of its onboard instruments, namely Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT), a group of Indian scientists from University of Haifa, IIT Guwahati, Indian Space Research Organization observed that the X-ray brightness from GRS 1915+105 fluctuates dramatically over time", it said.
It exhibits a unique pattern of alternating low-brightness ('dips') and high-brightness ('non-dips') phases, each lasting a few hundred seconds. During the high-brightness phase, the team discovered something remarkable: rapid flickers in X-rays repeating about 70 times per second, known as Quasi-periodic Oscillations (QPOs). Interestingly, such 'fast' flickers vanish during the low-brightness phases.
Explaining on what causes these mysterious 'fast' flickers', the research team uncovered that these rapid QPOs are intimately connected to a 'super-heated' cloud of energetic plasma surrounding the black hole, known as corona.
During the bright high-energy phases when QPOs are strongest, the corona becomes more compact (smaller in size, R_(in) ) and significantly hotter (higher luminosity, L). Conversely, in the dimmer 'dip' phases, the corona expands (larger R_(in) ) and cools (smaller L), causing the flickers to vanish. This pattern suggests that the compact oscillating corona seems to be the origin of these fast QPO signals.
These findings help scientists understand what happens in the vicinity of a black hole, where gravity is incredibly strong and conditions are extreme.
Indeed, GRS 1915+105 acts as cosmic laboratory, and with AstroSat's remarkable contributions, Indian scientists are decoding the 'whispers' of this black hole.
This work have been published in the prestigious journal, Monthly Notices of Royal Astronomical Society co-authored by Anuj Nandi (ISRO), Santabrata Das (IIT Guwahati), Sreehari H. (Haifa University) and Seshadri Majumder (IIT Guwahati).
Overall, this research not only deepens our understanding of black holes but also highlights India's growing role in space-based astronomy.
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