Guwahati: A multi-institutional research team including the Indian Institute of Technology Guwahati (IITG), Bengaluru-based U R Rao Satellite Centre (URSC), Indian Space Research Organisation (ISRO), University of Mumbai and Mumbai-based Tata Institute of Fundamental Research (TIFR), has studied a newly discovered black hole binary system called ‘Swift J1727.8-1613,’ using data obtained from ‘AstroSat.’
The research team has discovered intriguing X-ray characteristics that can potentially provide insights into the nature of black holes. Studying black holes directly is challenging as nothing escapes from black holes to be detected or measured. However, black hole binaries, where a black hole is paired with another object, such as a normal star, provide a unique opportunity for investigation.
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In these binary systems, the black hole’s gravity pulls material from its companion star, forming an accretion disk of gas and dust spiraling into the black hole. As the material in the accretion disk is pulled closer to the black hole, it heats up to extremely high temperatures, often millions of degrees, and emits X-rays. These X-rays can be detected using space-based telescopes, providing valuable information about the black hole itself.
The research team recently studied the black hole binary system Swift J1727.8-1613 using AstroSat, India’s first dedicated space astronomy observatory, which is in orbit around the earth. AstroSat is equipped with instruments capable of observing the universe in multi-wavelengths, including X-rays, making it ideal for studying high-energy phenomena such as black hole binaries.
The researchers detected Quasi-periodic Oscillations (QPOs) in the X-ray light emitted by the accretion disk of Swift J1727.8-1613. QPOs are the flickering of X-ray light from an astronomical object around specific frequencies.
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Speaking about his research, Prof Santabrata Das from the Department of Physics of the IITG, said,“QPOs are indispensable for investigating mysterious black hole systems. By examining the periodic variations of X-ray photons at high energies, QPOs help decode the footprints of a black hole’s strong gravity. This aids in understanding their fundamental properties and the dynamics of how the black hole attracts matter from the neighboring environment.”
Remarkably, these QPOs changed their frequency over just seven days, shifting from 1.4 to 2.6 times per second. This change of frequency is observed in extremely high-energy X-rays, which are incredibly hot, around a billion degrees. The implications of this discovery are profound. QPOs can help astronomers study the inner regions of accretion disks and determine the masses, and spin periods of black holes.
They can also test Einstein’s theory of general relativity, which describes gravity as a geometric property of space and time. According to this theory, massive objects like black holes and neutron stars warp the fabric of space-time around them, and this curvature dictates the paths that accreting matter will follow, which we perceive as gravitational attraction.
Highlighting the impact of this research finding, Anuj Nandi of the URSC, ISRO, said,“Unique capabilities of AstroSat, namely the high time resolution and large X-ray photon collecting area, made the discovery of evolving QPO frequency in high energy X-rays possible.”
“These high energy X-rays are generated when low energy photons interact with hot material from the inner disk around black holes via the Compton scattering process. AstroSat observations distinctly confirm that Swift J1727.8?1613 was in an accretion state dominated by Comptonized emissions that manifest aperiodic modulation, resulting in observed QPO features,” he added.