Evolution of the Comptonizing medium of the black-hole candidate Swift J1727.8–1613 along the hard to hard-intermediate state transition using NICER

We analyse the properties of the Comptonizing medium in the black-hole X-ray binary Swift J1727.8−1613 using the time-dependent Comptonization model vkompth, applied to NICER observations of type-C QPOs in the hard and hard-intermediate states. During the 2023 outburst of the source, we measure the RMS and phase lags of the QPO across 45 observations as the QPO frequency, ν QPO , evolves from ∼0.3 Hz to ∼7 Hz. By simultaneously fitting the time-averaged spectrum of the source and the RMS and lag spectra of the QPO, we derive the evolution of the disc and corona parameters. At ν QPO = 0.34 Hz, the QPO phase lags are hard, with 10 keV photons lagging 0.5 keV photons by ∼0.5 rad. As ν QPO increases, the lags for the same energy bands decrease, reaching near zero at ν QPO ∼1.2 Hz, and then reverse to soft lags of ∼−1.1 rad at ν QPO ∼7 Hz. Initially, the inner radius of the accretion disc is truncated at ∼30−40 R g (assuming a 10 solar-mass black hole) and, as the QPO frequency increases, the truncation radius decreases down to ∼10 R g . Initially, two coronas of sizes of ∼6.5×10 3 km and ∼2×10 3 km, extend over the disc and are illuminated by different regions of the disk. As the QPO frequency increases, both the coronas shrink to ∼2×10 3 km at ν QPO = 2.5 Hz. Following a data gap, one corona expands again, peaking at a size of ∼2×10 4 km. We interpret the evolution of the coronal size in the context of accompanying radio observations, discussing its implications for the interplay between the corona and the jet.