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

Abstract

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, using 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, ${\nu }_{\mathrm{QPO}}$, evolves from $\sim 0.3$ Hz to $\sim 7$ 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 disk and corona parameters. At ${\nu }_{\mathrm{QPO}}=0.34$ Hz, the QPO phase lags are hard, with 10 keV photons lagging 0.5 keV photons by $\sim 0.5$ rad. As ${\nu }_{\mathrm{QPO}}$ increases, the lags for the same energy bands decrease, reaching near zero at ${\nu }_{\mathrm{QPO}}\sim 1.2$ Hz, and then reverse to soft lags of $\sim -1.1$ rad at ${\nu }_{\mathrm{QPO}}\sim 7$ Hz. Initially, the inner radius of the accretion disk is truncated at $\sim 30-40R_g$ (assuming a 10 solar-mass black hole) and, as the QPO frequency increases, the truncation radius decreases down to $\sim 10R_g$. Initially, two coronas of sizes of $\sim 6.5\times 10^3$ km and $\sim 2\times 10^3$ km, extend over the disk and are illuminated by different regions of the disk. As the QPO frequency increases, both the coronas shrink to $\sim 2\times 10^3$ km at ${\nu }_{\mathrm{QPO}}=2.5$ Hz. Following a data gap, one corona expands again, peaking at a size of $\sim 2\times 10^4$ 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.

arXiv comment18 pages, 11 figures, 6 Tables, Accepted for publication in Astronomy & Astrophysics

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Cited by in corpus (3)

• Black-hole X-ray binary Swift J1727.8$-$1613 shows simultaneous Type-B and Type-C quasi-periodic oscillations across the hard-intermediate and soft-intermediate states
• Detection of a Type-C QPO during the soft-to-hard transition in Swift J1727.8-1613
• Misalignment of the Lense-Thirring precession by an accretion torque