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

TL;DR

This study analyzes the spectral evolution of the black hole candidate EXO 1846-031 during its 2019 outburst, revealing insights into the inner accretion disk behavior, the high hardening factor, and black hole spin estimation discrepancies.

Contribution

It provides a detailed spectral analysis across states, discusses the high hardening factor's impact, and compares spin measurement methods for the first time in this context.

Findings

Inner disk radius remains constant in soft states, indicating it reaches ISCO.

High hardening factor (2.0-2.7) explains unphysically small inner radii in hard states.

Discrepancies in spin measurements are partly due to different assumptions of the hardening factor.

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, $f_{\rm col}\approx2.0-2.7$, 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 $f_{\rm col}$.