Identification of the Hard X-ray Source Dominating the E > 25 keV
  Emission of the Nearby Galaxy M31
  We report the identification of a bright hard X-ray source dominating the M31
bulge above 25 keV from a simultaneous NuSTAR-Swift observation. We find that
this source is the counterpart to Swift J0042.6+4112, which was previously
detected in the Swift BAT All-sky Hard X-ray Survey. This Swift BAT source had
been suggested to be the combined emission from a number of point sources; our
new observations have identified a single X-ray source from 0.5 to 50 keV as
the counterpart for the first time. In the 0.5-10 keV band, the source had been
classified as an X-ray binary candidate in various Chandra and XMM studies;
however, since it was not clearly associated with Swift J0042.6+4112, the
previous E < 10 keV observations did not generate much attention. This source
has a spectrum with a soft X-ray excess (kT~ 0.2 keV) plus a hard spectrum with
a power law of Gamma ~ 1 and a cutoff around 15-20 keV, typical of the spectral
characteristics of accreting pulsars. Unfortunately, any potential pulsation
was undetected in the NuSTAR data, possibly due to insufficient photon
statistics. The existing deep HST images exclude high-mass (>3 Msun) donors at
the location of this source. The best interpretation for the nature of this
source is an X-ray pulsar with an intermediate-mass (<3 Msun) companion or a
symbiotic X-ray binary. We discuss other possibilities in more detail.
