Starburst Galaxy NGC 253 in a Hard (X-ray) Light: Resolving its Emission with NuSTAR

We present the latest results from simultaneous NuSTAR, Chandra, and VLBA monitoring of the X-ray binary population and diffuse emission in the local starburst galaxy NGC 253. Focusing on the combined NuSTAR dataset, comprised of three ~165 ks observational periods, we spatially characterize the sources of hard X-ray (E > 10 keV) emission in this galaxy for the first time. We detect >10 distinct sources in energy bands up to 25 keV, which consist mostly of intermediate state high-mass X-ray binaries (HMXBs), although the hardest (E > 12 keV) emission is dominated by two ultraluminous X-ray Sources (ULXs). The global X-ray emission falls steeply (photon index > 3), consistent with that of ULX spectra, and no significant excess above backgrounds is detected at E > 30 keV. A prominent diffuse inverse Compton component, as predicted in leptonic models meant to describe Fermi and HESS gamma-ray emission from NGC 253, is thus disfavored. However, even with NuSTAR's unprecedented spatial resolution and low background, we are unable to fully exclude leptonic models at this time, although we do constrain their acceptable parameters.