ISRO AstroSat Unveils Unique High-Energy X-ray Modulations in Black Hole Binary Swift J1727.8-1613

AstroSat has discovered unique aperiodic modulation of high-energy X-ray photons in the black hole binary Swift J1727.8-1613, revealing evolving QPO frequencies. This finding enhances our understanding of black hole accretion dynamics and high-energy astrophysics.

A team of astrophysicists from the U R Rao Satellite Centre (URSC)/ISRO, along with collaborators from the Indian Institute of Technology Guwahati (IITG), University of Mumbai, and Tata Institute of Fundamental Research (TIFR), has made a significant discovery using AstroSat. The team observed ‘aperiodic modulation’ of high-energy X-ray photons during an unusual outburst phase of the black hole binary source Swift J1727.8-1613. Their findings have been published in the international journal Monthly Notices of the Royal Astronomical Society (MNRAS).

In black hole X-ray binary (BH-XRB) systems, a black hole and its companion star are gravitationally bound. The black hole draws matter from its companion, forming a hot, luminous accretion disk that emits high-energy X-rays. These X-rays serve as cosmic indicators, allowing scientists to study black holes, which cannot be observed directly.

Swift J1727.8-1613, a black hole X-ray transient (BH-XRT), was discovered on August 24, 2023, by Swift/BAT. Subsequent monitoring by MAXI/GSC identified it as one of the brightest sources, peaking at around 7 Crab in X-ray intensity. AstroSat first observed this source on September 2, 2023, and continued observations until September 14, 2023, with a total exposure time of approximately 207 kiloseconds.

The study revealed that the ‘aperiodic modulation’ of high-energy X-ray photons led to Quasi-periodic Oscillation (QPO) at a fixed frequency. Over seven days, the QPO frequency of these high-energy photons (around 100 kilo-electron Volts, equivalent to 1 billion Kelvin) shifted from 1.4 Hz to 2.6 Hz. This phenomenon, detected for the first time in a BH-XRB, is notable for its rapid evolution.

The research highlighted that high-energy X-rays, or hard X-rays, are produced by the reprocessing of low-energy photons (soft X-rays) from the accretion disk by hot electrons in the inner disk through the Compton scattering process. During AstroSat's observations, Swift J1727.8-1613 was primarily in a Comptonized emission state (~90%) with minimal thermal disk emission (~10%). This indicates that the Comptonized X-ray photons exhibit ‘aperiodic modulation,’ resulting in the observed QPO features.

QPOs are crucial for understanding accreting black hole systems. By examining the periodic variations of X-ray photons, scientists can gain insights into the black hole’s gravity and accretion processes.

The team utilized the Large Area X-ray Proportional Counter (LAXPC) on AstroSat, which offers high time resolution (~10 microseconds) and a large photon collecting area, enabling the detailed study of QPOs at higher energies. They also used data from the NICER instrument on the International Space Station (ISS) to understand the source's spectral energy distribution.

This discovery showcases the advanced capabilities of AstroSat in studying high-energy astrophysical phenomena and contributes significantly to our understanding of black hole dynamics and accretion processes.