A New Spin on an Old Black Hole: NuSTAR Spectroscopy of EXO 1846-031

TL;DR

This study uses NuSTAR X-ray spectroscopy to measure a nearly maximal spin of the black hole candidate EXO 1846-031 during its 2019 outburst, confirming its high spin robustness across models.

Contribution

First detailed NuSTAR spectral analysis of EXO 1846-031 revealing a high black hole spin, with robustness checks against various modeling assumptions.

Findings

Black hole spin measured as a=0.997 with small errors

Inner disk inclination estimated at approximately 73 degrees

High spin measurement remains consistent across models

Abstract

The black hole candidate EXO 1846-031 underwent an outburst in 2019, after at least 25 years in quiescence. We observed the system using \textit{NuSTAR} on August 3rd, 2019. The 3--79 keV spectrum shows strong relativistic reflection features. Our baseline model gives a nearly maximal black hole spin value of $a=0.997_{-0.002}^{+0.001}$ ($1\sigma$ statistical errors). This high value nominally excludes the possibility of the central engine harboring a neutron star. Using several models, we test the robustness of our measurement to assumptions about the density of the accretion disk, the nature of the corona, the choice of disk continuum model, and addition of reflection from the outer regions of the accretion disk. All tested models agree on a very high black hole spin value and a high value for the inclination of the inner accretion disk of $\theta\approx73^\circ$. We discuss the implications of this spin measurement in the population of stellar mass black holes with known spins, including LIGO events.