AstroSat Spots Rare X-Ray Signals From Spinning Black Hole, Reveals New Cosmic Behaviour

By Surabhi Gupta

New Delhi: India's space science is once again at the forefront, with the Indian Space Research Organisation (ISRO) confirming a substantial new finding from its AstroSat multiwavelength space telescope, bringing clarity on the perplexing black hole binary GRS 1915+105. This discovery is another step in understanding how matter behaves in the extreme gravitational field of a rotating black hole, and thus marks another momentous finding by Indian astronomy.

The Black Hole Who Never Lets Anyone Rest Easily: GRS 1915+105

Discovered over twenty years ago, GRS 1915+105 is about 8,600 parsecs or 28,000 light years from Earth and has been studied more than any other stellar mass black hole system, largely due to its extreme X-ray variability. The source of its emissions, which sometimes vary on timescales of less than a second, will often show quasi‐periodic oscillations (QPOs), which are variations in the accretion flow of matter very near the event horizon.

Previous studies from observatories like NASA's RXTE had observed QPOs with characteristic frequencies near ~67 Hz, with the newly published AstroSat studies building on this in multiple dimensions of timing, spectral and state-dependent behaviour, including at higher X-ray energies closer to the black hole.

Discovery of a New Flaring Class: the ‘n-class’

In April 2025, research published on arXiv by teams from Tokyo, Pune, Brookhaven, and institutions in India describes the discovery of a previously unknown flaring state in GRS 1915+105, dubbed the η‑class. Identified using AstroSat’s LAXPC and SXT instruments in data from mid-2017, this class is characterised by:

• The source toggles quasi-regularly between two high-soft (HS) spectral states, each with a steep power-law index less than 4
• Flares recur about every 50 seconds
• A strong correlation between X-ray flux and hardness ratio, counter to other variability classes
• High-Frequency Quasi-Periodic Oscillations (HFQPOs) detected around 70 Hz, persistent during this new η‑class
• Accretion rate oscillating up to threefold during the cycle

This finding marks a rare addition to the taxonomy of flaring behaviours in GRS 1915+105, which already hosts nearly a dozen known classes named after Greek letters.

Timing Signature: Soft‑Lag and HFQPO Dynamics

One of the most exciting discoveries is the detection of soft‑lag in HFQPOs, meaning hard X-ray photons arrive before the softer ones, a reversal of earlier RXTE observations that showed hard-lag. AstroSat observed soft-lag values up to ~ 3 ms in the δ and ω classes, and around 2 ms in κ and γ classes, with lag amplitude rising with photon energy.

AstroSat at a glance (Image Credits: ISRO)

A deeper spectral‑timing analysis in late 2025 further reveals that HFQPOs align strongly with non‑dip, high count-rate intervals, where a compact Comptonizing corona of size ≈ 2.8–16 gravitational radii modulates emission near 70 Hz. Stronger QPOs correlate with spectral hardness and increased Compton normalisation, while the photon index shows anti‑correlation, indicating harder spectra when QPOs are strongest.

Atmospheric lag behaviour shifting from hard‑lag in RXTE to soft‑lag in AstroSat data is interpreted as the signature of reflection off a cooler accretion disc and evolving optical depth in the Compton medium.

What it means: Increasing clarity about black hole environments

Compact Corona & Radiative Mechanisms

Detection of HFQPOs in hard X‑ray bands implies that the photons originate nearer to the black hole. This supports models where compact coronae oscillate, producing timed modulations tied to disc–corona dynamics. AstroSat’s broad-band spectral modelling yields electron temperatures of ~2–2.5 keV and photon indices ~1.7–3.3 in thermal Comptonization fits.

"This new flaring pattern, what we now call the n-class, gives us rare insight into how matter behaves under extreme gravity just outside a black hole's event horizon. AstroSat has proven its global relevance once again by capturing X-ray oscillations that even earlier missions struggled to resolve with this clarity, a senior astrophysicist at the Indian Institute of Astrophysics, told ETV Bharat.

"The detection of soft-lag, where hard X-rays precede the softer ones, is particularly intriguing and suggests a compact, oscillating corona close to the black hole. It challenges some of our older assumptions. Findings like these help constrain theoretical models about how accretion discs and relativistic jets interact with the black hole's spin and magnetic field,“ said space scientist and X-ray timing specialist.

Accretion Physics & Spin Estimates

Spectral modelling using tools like Kerrbb suggests super-Eddington accretion rates (~1.2 × ); mass estimates for the black hole range between 12.4 to 13.1 . M and spin between 0.99–0.997, indicating near-maximal rotation.

Disc–Corona Geometry & Time-Lag Evolution

Soft-lag detection signifies a scenario where hard X-ray photons are emitted first in the corona, followed by soft reflected photons from the inner disc, a geometry-sensitive insight that wasn't available before AstroSat. The lag trend increasing with photon energy and optical depth provides stringent constraints on corona size and degree of disk wrapping.

AstroSat: India’s Multi‑Wavelength Marvel

Launched on 28 September 2015, AstroSat is India's first dedicated multi‑wavelength space telescope, combining imaging and timing capabilities in far UV, near UV, optical, soft X-ray, and hard X-ray bands. Its instrumentation includes UVIT, SXT, LAXPC, CZTI, SSM, and CPM, allowing unique simultaneous coverage across the spectrum.

Although designed for a nominal five‑year mission, AstroSat continues to operate robustly into its tenth year. Data from its instruments have delivered more than 100 research publications, including numerous collaborations with international observatories like NASA’s NICER (Neutron star Interior Composition Explorer), Chandra, LAS Cumbres Observatory, and others.

Indian Science Teams and Collaborations

Several Indian institutions have played pivotal roles in these studies:

• URSC / ISRO, IIT Guwahati, TIFR, IUCAA Pune, University of Mumbai, and others contributed to a detailed analysis of Swift J1727.8‑1613 and GRS 1915+105.
• Technical teams from URSC ISRO coordinated data operations; model development and interpretation came from collaborators across Indian academic centres.
• The collaborative work is a strong testament to the growing maturity of India's astrophysics community in producing high-quality, globally competitive research.

Implications and Future Plans

• New phenomenology: The n-class adds to the variety of known variability types in GRS 1915+105 and implies that accretion-disc dynamics are much richer and varied than previously anticipated.
• Timing constraints: Soft-lag detection provides new tools to identify corona size, disc geometry and accretion time-scales in strong gravitational scenarios.
• Model Refinement: These data inform theoretical models of accretion physics, corona–disc coupling, and jet-launching processes around spinning black holes.
• AstroSat’s Legacy: These groundbreaking observations reinforce the mission’s ongoing value well beyond its conceived life-span, cementing India’s role in high-energy astrophysics.
• Toward Next Missions: With the success of AstroSat, ISRO has proposed a successor, AstroSat‑2, to continue wide-band astrophysical studies, not to mention XPoSat, launched January 1, 2024, carrying polarimeter and spectroscopy payloads for studying X‑ray polarisation in black hole systems.

Key Findings:

From capturing the fastest oscillations around a stellar-mass black hole to detecting novel flaring states, AstroSat continues to demonstrate instrumental excellence and scientific depth. These latest findings on GRS 1915+105 affirm India’s growing leadership in high-energy astrophysics and open fresh windows into the physics of extreme gravity.

As ISRO charts its future path, from AstroSat‑2 proposals to polarimetry missions like XPoSat, and long-term goals such as the NISAR mission launching July 30, 2025, India’s astrophysical vision is expanding rapidly. With synergy between observatories at home and abroad, AstroSat’s revelations mark just the beginning of a new era of cosmic discovery, and India is very much at the helm.