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In a recent study, AstroSat has enabled the discovery of the ‘aperiodic modulation’ of high-energy X-ray photons during an ‘unusual’ outburst phase of the black hole binary source Swift J1727.8-1613. This significant discovery, carried out utilizing the unique capabilities of AstroSat, is reported by a team of astrophysicists led by U R Rao Satellite Centre (URSC)/ISRO in collaboration with the Indian Institute of Technology Guwahati (IITG), University of Mumbai, and Tata Institute of Fundamental Research (TIFR) in the international journal Monthly Notices of the Royal Astronomical Society (MNRAS).

In an astrophysical black hole X-ray binary (BH-XRB) system, both the black hole and its companion are locked in a gravitational embrace. A black hole pulls matter from its tidally deformed companion, leading to the formation of a luminous accretion disk. As matter accretes through the disk, it becomes extremely hot, reaching temperatures in the millions of degrees and emitting high-energy X-rays.

These electromagnetic radiations act as cosmic messengers, indirectly detecting the black holes shrouded in darkness. In a way, BH-XRBs serve as cosmic laboratories, offering insights into the intricate processes governing accretion dynamics around compact objects and providing invaluable opportunities to study fundamental physical mechanisms in extreme environments.

Swift J1727.8-1613 is a black hole X-ray transient (BH-XRT) recently discovered by Swift/BAT on August 24, 2023. Immediate monitoring by MAXI/GSC identified the source as one of the brightest, with an X-ray peak value of approximately 7 Crab (Crab is a standard candle and 1 Crab = 3.8 photons per per s). AstroSat first observed this source on September 2, 2023 (MJD 60189), followed by subsequent observations spanning from September 8, 2023 (MJD 60195) to September 14, 2023 (MJD 60201), with a total exposure time of approximately 207 ks.

This study reveals that the ‘aperiodic modulation’ of high-energy X-ray photons resulted in Quasi-periodic Oscillation (QPO) at a fixed frequency in power-frequency space. Remarkably, over a mere 7 days, the QPO frequency of high-energy photons (?100 kilo-electron Volt, equivalently 1 billion Kelvin) evolved from 1.4 Hz to 2.6 Hz, a unique phenomenon detected for the first time in a BH-XRB.

Moreover, this study emphasizes that high-energy X-rays (hard X-rays) are generated from the reprocessing of the low-energy photons (soft X-rays) from the ‘standard’ accretion disk by the ‘hot’ electrons from the inner disk via the Compton scattering process. During AstroSat observations, Swift J1727.8-1613 was in an accretion state dominated by Comptonized emission (~90%) over thermal disk emission (~10%). These findings ascertain that Comptonized X-ray photons exhibit ‘aperiodic modulation’ resulting in the QPO features.

QPOs are indispensable for investigating accreting black hole systems. By examining the periodic variations of X-ray photons, QPOs decode the footprints of a black hole’s strong gravity, aiding in understanding their fundamental properties and accretion dynamics.

In this work, the team utilized the unique capabilities (high time resolution ~10 µs and large photon collecting area) of the Large Area X-ray Proportional Counter (LAXPC) onboard AstroSat, enabling the variability study of QPO at higher energies. The team also used observational data from the NICER instrument onboard the International Space Station (ISS) to understand the spectral energy distribution of the source.