Disk evolution and black hole mass constraints on EXO 1846−031 based on NICER/XTI, Insight-HXMT and NuSTAR observations

¹ Department of Astronomy, School of Physics and Astronomy, Yunnan University, Kunming, Yunnan 650500, China
² RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
³ Department of Physics, Tokyo Institute of Technology, 2-12-1, Ookayama, Tokyo 152-8551, Japan
⁴ Department of Physics, Ehime University, 2-5, Bunkyocho, Matsuyama 790-8577, Japan
⁵ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
⁶ Dongguan Neutron Science Center, Zhongziyuan Road, Dongguan 523808, People’s Republic of China
⁷ University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China

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Received: 15 October 2024
Accepted: 28 September 2025

Abstract

We studied the spectral properties and the accretion disk evolution of the black hole candidate EXO 1846−031, based on NICER/XTI observations. The combined models, consisting of a multicolor disk and a Comptonized component, revealed a truncated disk at the beginning of its 2019 outburst. To obtain better constraints on system parameters (i.e., inclination and mass), we further investigated the reflection features using simultaneous NICER and NuSTAR observations. In combination with the analysis of HXMT reflection spectra, a lower inclination (i.e., i < 55°) is preferred based on the reflection model fittings. We also studied the MAXI and Swift/XRT data during the transition toward the low-hard state in the end of the outburst as a complement to NICER data. Using the Swift/XRT transition flux and the latest empirical Eddington ratio at the index transition toward the low-hard state, the model-independent black hole mass was estimated to be M < 8.2 M_⊙ given a distance of 2.4 − 7.5 kpc. In addition, we discuss the mass constraints assuming different models and different inclinations and spins. Assuming an extreme spin (a = 0.998) and adopting a complex model consisting of a relativistic disk and a reflection component to the HXMT spectra in the high-soft state, a black hole mass of $10.{49}_{-1.12}^{+1.49}{\mathrm{M}}_{\odot }$ at a source distance of 7 kpc is suggested.