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

The black hole candidate EXO 1846-031 underwent an outburst in 2019, after at least 25 yr in quiescence. We observed the system using NuSTAR on 2019 August 3. 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 the 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 approximate to 73 degrees. We discuss the implications of this spin measurement in the population of stellar mass black holes with known spins, including LIGO and Virgo events.