NuSTAR and Chandra Insight into the Nature of the 3-40 keV Nuclear Emission in NGC 253

We present results from three nearly simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR) and Chandra monitoring observations between 2012 September 2 and 2012 November 16 of the local star-forming galaxy NGC 253. The 3-40 keV intensity of the inner ~20 arcsec (~400 pc) nuclear region, as measured by NuSTAR, 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 (L 2-10 keV ~ few × 1039 erg s-1) point source located ~1 arcsec from the dynamical center of the galaxy (within the 3σ positional uncertainty of the dynamical center); this source drives the overall variability of the nuclear region at energies gsim3 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 (N H ≈ 1.6 × 1023 cm-2) broken power-law model with spectral slopes and break energies th