| Authors Affiliation: | Dr Akash Garg (Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, India),
Dr Somasri Sen (Jamia Millia Islamia, Delhi, India),
Prof. Ranjeev Misra (Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, India)
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| Abstract : | In recent decades, advanced multi-wavelength telescopes have opened a window to the most luminous objects in the cosmos, allowing scientists to delve into their physical mechanisms. Among these celestial phenomena are Galactic X-ray binaries, where a dense object like a black hole due to its intense gravitational pull, draws in material from a nearby star, emitting across the electromagnetic spectrum, particularly in X-rays. Previous studies have revealed that black hole binaries exhibit rapid X-ray fluctuations on millisecond to second timescales during outbursts. Analyzing their light curves in Fourier space through the power density spectrum has specific peaks known as Quasi-periodic oscillations (QPOs).
However, despite these observations, there exist fewer models that can interpret this variability. To address this gap, we have devised a comprehensive approach. This method involves modelling the time-varying energy spectra using known radiative processes. By converting spectral parameters into physical counterparts, we can explore how variations in these parameters, such as accretion rate, and coronal heating rate correspond to the observed energy-dependent fractional root mean square (rms) and time-lags.
Applying this technique to the black hole system GRS 1915+105 observed by AstroSat, we uncovered a relationship between accretion rate, inner disc radius, coronal heating rate, and optical depth, explaining the QPOs and their harmonics. Encouraged by these findings, we expanded our investigation to MAXI J1535-571. We found that variations in accretion rate and inner disc radius occurred coherently with no time lag, while the heating rate exhibited a time lag compared to other the two parameters.
We discuss how the employed technique is a potential step towards unravelling the radiative process responsible for variability using high-quality spectral and temporal data from AstroSat and other X-ray missions. |
