Black holes are among the most mysterious objects in the universe. We can’t see them directly, but their immense gravity reveals their presence.
- Formation: They form when massive stars run out of fuel and collapse.
- Invisibility: Not even light can escape their pull, making them truly invisible.
- Detection: In binary systems (with a companion star), they pull in material. This “accretion” heats the matter to over 10 million degrees, emitting intense X-rays. Space telescopes capture these X-rays, offering a rare glimpse into their lives.
GRS 1915+105: A Cosmic Enigma
Located nearly 28,000 light-years away, GRS 1915+105 is a particularly intriguing black hole.
- System: It’s an X-ray binary system with a rapidly rotating black hole (about 12 times the Sun’s mass) and its companion star.
- Unusual Behavior: This system is known for its dynamic and often unpredictable behavior, making it a key focus for scientists.
AstroSat: India’s Eye on the Black Hole
AstroSat, India’s first dedicated multi-wavelength space observatory, has been vital in studying GRS 1915+105.
- Monitoring: Launched in September 2015, AstroSat continuously monitors this enigmatic black hole.
- Key Instruments: It uses two main instruments: the Large Area X-ray Proportional Counter (LAXPC) and the Soft X-ray Telescope (SXT).
Dramatic X-ray Fluctuations
Indian scientists, from institutions like the University of Haifa, IIT Guwahati, and ISRO, observed fascinating X-ray patterns from GRS 1915+105.
- Brightness Changes: The X-ray brightness fluctuates dramatically over time.
- Unique Pattern: It shows alternating phases:
- Low-brightness (‘dips’): Periods where X-ray emissions are low.
- High-brightness (‘non-dips’): Periods where X-ray emissions are strong.
- Duration: Each phase typically lasts a few hundred seconds.
Unveiling Fast Flickers: Quasi-periodic Oscillations (QPOs)
During the high-brightness phases, the team made a remarkable discovery.
- Rapid Flickers: They observed rapid X-ray flickers, repeating about 70 times per second (νQPO∼70 Hz).
- Name: These are known as Quasi-periodic Oscillations (QPOs).
- Vanishing Act: Interestingly, these ‘fast’ flickers completely disappear during the low-brightness ‘dip’ phases.
The Corona’s Role: The Source of QPOs
The research team investigated the cause of these mysterious fast flickers.
- Super-heated Cloud: They found these QPOs are intimately connected to the corona, a ‘super-heated’ cloud of energetic plasma surrounding the black hole.
- Bright Phase Connection:
- When QPOs are strongest (during high-brightness phases), the corona becomes more compact (smaller in size, Rin).
- It also becomes significantly hotter (higher luminosity, L).
- Dim Phase Connection:
- In the dimmer ‘dip’ phases, the corona expands (larger Rin).
- It also cools (smaller L), causing the flickers to vanish.
- Conclusion: This pattern suggests that a compact, oscillating corona is the origin of these fast QPO signals.
GRS 1915+105: A Cosmic Laboratory
These findings are crucial for understanding the extreme environment around black holes.
- Extreme Conditions: They shed light on what happens where gravity is incredibly strong and conditions are harsh.
- Cosmic Lab: GRS 1915+105 serves as a unique cosmic laboratory.
- AstroSat’s Contribution: With AstroSat’s remarkable data, Indian scientists are effectively “decoding the whispers” of this fascinating black hole.
