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

Understand

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).