NGC 2403: A hard X-ray look of a nearby low metallicity bulgeless spiral galaxy

We present a NuSTAR investigation of the hard X-ray source population of the spiral galaxy NGC 2403, the most isolated of the 3 nearby (≤3 Mpc) morphologically analogous galaxies. NGC 2403 extends the sample of galaxies used to derive the first scaling relations in the hard X-rays to very vigorously star-forming galaxies with specific star-formation rates (sSFR) of log(sSFR)= -10.4 yr^-1. The supporting XMM-Newton observations provided the point source lists for the localization and determination of the source properties in the NuSTAR observations. We perform a detailed spectroscopic analysis of NGC 2403 X-1, a well-known ULX with a black-hole mass of 10-15 Mo. Reaching down to L_X,limit ~10³⁸ ergs s^-1 (4–25 keV), we also studied a transient accreting X-ray pulsar discovered in 2007 with a spin period of 18s, and two additional NuSTAR sources. We also discuss the luminosity distribution of the remaining X-ray binaries detected in the XMM-Newton and NuSTAR observations of NGC 2403 and constraints on their nature based on the NuSTAR X-ray colors. This work is supported by the National Aeronautics and Space Administration under Grant No. 80NSSC21K0018.