Insight-HXMT Study of the Inner Accretion Disk in the Black Hole Candidate EXO 1846--031

Abstract

We study the spectral evolution of the black hole candidate EXO 1846$-$031 during its 2019 outburst, in the 1--150 keV band,with the {\it {Hard X-ray Modulation Telescope}}. The continuum spectrum is well modelled with an absorbed disk-blackbody plus cutoff power-law, in the hard, intermediate and soft states. In addition, we detect an $\approx$6.6 keV Fe emission line in the hard intermediate state. Throughout the soft intermediate and soft states, the fitted inner disk radius remains almost constant; we suggest that it has settled at the innermost stable circular orbit (ISCO). However, in the hard and hard intermediate states, the apparent inner radius was unphysically small (smaller than ISCO), even after accounting for the Compton scattering of some of the disk photons by the corona in the fit. We argue that this is the result of a high hardening factor, $f_{\mathrm{col}}\approx 2.0-2.7$, in the early phases of outburst evolution, well above the canonical value of 1.7 suitable to a steady disk. We suggest that the inner disk radius was close to ISCO already in the low/hard state. Furthermore, we propose that this high value of hardening factor in the relatively hard state is probably caused by the additional illuminating of the coronal irradiation onto the disk. Additionally, we estimate the spin parameter with the continuum-fitting method, over a range of plausible black hole masses and distances. We compare our results with the spin measured with the reflection-fitting method and find that the inconsistency of the two results is partly caused by the different choices of $f_{\mathrm{col}}$.

21 pages, 6 figures

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• Fast transitions of X-ray variability in the black hole transient GX 339--4: comparison with MAXI J1820+070 and MAXI J1348-630
• Accretion Flow Properties of EXO 1846-031 During its Multi-Peaked Outburst After Long Quiescence
• Impact of reflection Comptonization on X-ray reflection spectroscopy: the case of EXO 1846-031
• Accretion flows in the hard state of black hole X-ray binaries: the effect of hot gas condensation
• Likely detection of magnetic field related LFQPO in the soft X-ray re-brightening of GRS~1915+105
• Accretion Geometry of Black Hole X-ray Binaries: Insights from X-ray Observations
• The peculiar disk evolution of 4U 1630--472 observed by Insight-HXMT during the 2022 and 2023 outbursts