Peculiar disk behaviors of the black hole candidate MAXI J1348-630 in the hard state observed by Insight-HXMT and Swift

Abstract

We present a spectral study of the black hole candidate MAXI J1348-630 during its 2019 outburst, based on monitoring observations with Insight-HXMT and Swift. Throughout the outburst, the spectra are well fitted with power-law plus disk-blackbody components. In the soft-intermediate and soft states, we observed the canonical relation L ~ T_in^4 between disk luminosity L and peak colour temperature T_in, with a constant inner radius R_in (traditionally identified with the innermost stable circular orbit). At other stages of the outburst cycle, the behaviour is more unusual, inconsistent with the canonical outburst evolution of black hole transients. In particular, during the hard rise, the apparent inner radius is smaller than in the soft state (and increasing), and the peak colour temperature is higher (and decreasing). This anomalous behaviour is found even when we model the spectra with self-consistent Comptonization models, which take into account the up-scattering of photons from the disk component into the power-law component. To explain both those anomalous trends at the same time, we suggest that the hardening factor for the inner disk emission was larger than the canonical value of ~1.7 at the beginning of the outburst. A more physical trend of radii and temperature evolution requires a hardening factor evolving from ~3.5 at the beginning of the hard state to ~1.7 in the hard intermediate state. This could be evidence that the inner disk was in the process of condensing from the hot, optically thin medium and had not yet reached a sufficiently high optical depth for its emission spectrum to be described by the standard optically-thick disk solution.

arXiv comment20 pages, 10 figures, 4 tables, accepted by ApJ

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Cited by in corpus (11)

• Transitions and Origin of the Type-B Quasi-Periodic Oscillation in the Black Hole X-ray Binary MAXI~ J1348--630
• Discovery of optical and infrared accretion disc wind signatures in the black hole candidate MAXI J1348-630
• Insight-HXMT Study of the Inner Accretion Disk in the Black Hole Candidate EXO 1846--031
• Fast transitions of X-ray variability in the black hole transient GX 339--4: comparison with MAXI J1820+070 and MAXI J1348-630
• MAXI~J1348--630: Estimating the black hole mass and binary inclination using a scaling technique
• A moderate spin for the black hole in X-ray binary MAXI J1348-630 revealed by Insight-HXMT
• Reflare in MAXI J1348-630]{Evolution of disc and corona in MAXI J1348-630 during the 2019 reflare: NICER and Insight-HXMT view
• Broadband Spectral properties of MAXI J1348--630 using AstroSat Observations
• The spin measurement of MAXI J1348-630 using the Insight-HXMT data
• An Insight-HXMT View of the Evolution of the Type-C Quasiperiodic Oscillation during the Flaring State of Swift J1727.8-1613
• A search for optical counterparts in quiescent black hole X-ray transients