NuSTAR and Chandra Insight into the Nature of the 3-40 keV Nuclear
  Emission in NGC 253
  We present results from three nearly simultaneous NuSTAR and Chandra
monitoring observations between 2012 Sep 2 and 2012 Nov 16 of local
star-forming galaxy NGC 253. The 3-40 keV NuSTAR intensity of the inner 20
arcsec (~400 pc) nuclear region varied by a factor of ~2 across the three
monitoring observations. The Chandra data reveal that the nuclear region
contains three bright X-ray sources, including a luminous (L2-10 keV ~ few x
10^39 erg/s) point source ~1 arcsec from the dynamical center of the galaxy
(within the 3sigma positional uncertainty of the dynamical center); this source
drives the overall variability of the nuclear region at energies >3 keV. We
make use of the variability to measure the spectra of this single hard X-ray
source when it was in bright states. The spectra are well described by an
absorbed (NH ~ 1.6 x 10^23 cm^-2) broken power-law model with spectral slopes
and break energies that are typical of ultraluminous X-ray sources (ULXs), but
not AGN. A previous Chandra observation in 2003 showed a hard X-ray point
source of similar luminosity to the 2012 source that was also near the
dynamical center (~0.4 arcsec); however, this source was offset from the 2012
source position by ~1 arcsec. We show that the probability of the 2003 and 2012
hard X-ray sources being unrelated is >>99.99% based on the Chandra spatial
localizations. Interestingly, the Chandra spectrum of the 2003 source (3-8 keV)
is shallower in slope than that of the 2012 hard X-ray source. Its proximity to
the dynamical center and harder Chandra spectrum indicate that the 2003 source
is a better AGN candidate than any of the sources detected in our 2012
campaign; however, we were unable to rule out a ULX nature for this source.
Future NuSTAR and Chandra monitoring would be well equipped to break the
degeneracy between the AGN and ULX nature of the 2003 source, if again caught
in a high state.
