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

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