Evidence for High Frequency QPOs in the Black Hole Candidate EXO 1846-031

We present evidence for a pair of 3:2 ratio high frequency quasi-periodic oscillations (HFQPO) at 500 and 750 Hz in the X-ray flux from the black hole candidate EXO 1846$-$031 in observations made with the {\it Neutron Star Interior Composition Explorer} ({\it NICER}). The source went into outburst in late July, 2019 after 34 years in quiescence, with {\it NICER} monitoring beginning on 31 July. An average power spectrum accumulated in the 1 - 6 keV band over the initial 9 day intensity rise shows evidence at the $4\sigma$ significance level for a HFQPO at 500 Hz, with a fractional amplitude (rms) of $3.3 \%$, and coherence $Q = \nu / \Delta\nu \approx 20$. A second, weaker feature is also evident at 750 Hz, consistent with a 3:2 relationship with the 500 Hz QPO. Assessing the significance of both features together yields an overall significance at the $4.4\sigma$ level. The source spectral state evolves from a hard intermediate (or steep power-law) state to a soft-intermediate state during the outburst rise, states known to be associated with HFQPO in other black hole systems. Based on the observed inverse mass scaling of 3:2 ratio black hole HFQPOs the detection of 500 and 750 Hz signals in EXO 1846$-$031 would suggest a mass of $3.24 \pm 0.14$ $M_{\odot}$.