Dr. Divya Rawat

Evidence for a strong connection between the X-ray corona and the radio jet in black hole X-ray binaries has grown significantly. During state transitions, the inferred corona size correlates with both the jet’s flux and its synchrotron spectrum properties, offering insights into jet acceleration mechanisms. We investigate the properties of the Comptonizing medium in Swift J1727.8-1613 using the time-dependent Comptonization model vkompth, with NICER observations of type-C QPOs in the hard and hard-intermediate states. By simultaneously fitting the time-averaged spectrum of the source and the rms and lag spectra of the QPO (which evolves from 0.3 Hz to 7 Hz), we derive the evolution of the disk and corona parameters. Initially, the inner radius of the accretion disk is truncated at ~30-40 Rg (assuming a 10 solar-mass black hole) and as the QPO frequency increases, the truncation radius decreases down to 10 Rg. Our results suggest that the observed rms and lag spectra are consistent with the presence of two coronas, varying in size from ~2 × 104 km to 10³ km, extending over the disk and illuminated by distinct disk regions. We interpret the evolution of the coronal size in the context of accompanying radio observations with VLITE and RATAN-600, discussing its implications for the interplay between the corona and the jet.