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The black hole candidate EXO 1846-031 underwent an outburst in 2019, after atleast 25 years in quiescence. We observed the system using \textit{NuSTAR} onAugust 3rd, 2019. The 3--79 keV spectrum shows strong relativistic reflectionfeatures. 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 valuenominally excludes the possibility of the central engine harboring a neutronstar. Using several models, we test the robustness of our measurement toassumptions about the density of the accretion disk, the nature of the corona,the choice of disk continuum model, and addition of reflection from the outerregions of the accretion disk. All tested models agree on a very high blackhole spin value and a high value for the inclination of the inner accretiondisk of $\theta\approx73^\circ$. We discuss the implications of this spinmeasurement in the population of stellar mass black holes with known spins,including LIGO events.

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