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Evolution of the Comptonizing medium of the black-hole candidate Swift J1727.8-1613 along the hard to hard-intermediate state transition using NICER
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DOI：10.1051/0004-6361/202453538.png)
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Abstract
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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, nu(QPO), evolves from similar to 0.3 Hz to similar 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 nu(QPO) = 0.34 Hz, the QPO phase lags are hard, with 10 keV photons lagging 0.5 keV photons by similar to 0.5 rad. As nu(QPO) increases, the lags for the same energy bands decrease, reaching near zero at nu(QPO)similar to 1.2 Hz, and then reverse to soft lags of similar to-1.1 rad at nu(QPO)similar to 7 Hz. Initially, the inner radius of the accretion disc is truncated at similar to 30-40R(g) (assuming a 10 solar-mass black hole) and, as the QPO frequency increases, the truncation radius decreases down to similar to 10R(g). Initially, two coronas of sizes of similar to 6.5x10(3) km and similar to 2x10(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 similar to 2x10(3) km at nu(QPO) = 2.5 Hz. Following a data gap, one corona expands again, peaking at a size of similar to 2x10(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.
Keywords:
accretion, accretion disks
black hole physics
methods: data analysis
stars: black holes
X-rays: binaries
X-rays: individuals: Swift J1727.8-1613
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5.8
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5.0W
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18.3W
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