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We report the identification of a bright hard X-ray source dominating the M31bulge above 25 keV from a simultaneous NuSTAR-Swift observation. We find thatthis source is the counterpart to Swift J0042.6+4112, which was previouslydetected in the Swift BAT All-sky Hard X-ray Survey. This Swift BAT source hadbeen suggested to be the combined emission from a number of point sources; ournew observations have identified a single X-ray source from 0.5 to 50 keV asthe counterpart for the first time. In the 0.5-10 keV band, the source had beenclassified as an X-ray binary candidate in various Chandra and XMM studies;however, since it was not clearly associated with Swift J0042.6+4112, theprevious E < 10 keV observations did not generate much attention. This sourcehas a spectrum with a soft X-ray excess (kT~ 0.2 keV) plus a hard spectrum witha power law of Gamma ~ 1 and a cutoff around 15-20 keV, typical of the spectralcharacteristics of accreting pulsars. Unfortunately, any potential pulsationwas undetected in the NuSTAR data, possibly due to insufficient photonstatistics. The existing deep HST images exclude high-mass (>3 Msun) donors atthe location of this source. The best interpretation for the nature of thissource is an X-ray pulsar with an intermediate-mass (<3 Msun) companion or asymbiotic X-ray binary. We discuss other possibilities in more detail.

Identification of the Hard X-ray Source Dominating the E > 25 keV Emission of the Nearby Galaxy M31