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We present nearly simultaneous Chandra and NuSTAR observations of twoactively star-forming galaxies within 50 Mpc: NGC 3256 and NGC 3310. Bothgalaxies are detected by both Chandra and NuSTAR, which together provide thefirst-ever spectra of these two galaxies spanning 0.3-30 keV. The X-rayemission from both galaxies is spatially resolved by Chandra; we find that hotgas dominates the E < 1-3 keV emission while ultraluminous X-ray sources (ULXs)dominate at E > 1-3 keV. The NuSTAR galaxy-wide spectra of both galaxies followsteep power-law distributions with Gamma ~ 2.6 at E > 5-7 keV, similar to thespectra of bright individual ULXs and other galaxies that have been studied byNuSTAR. We find that both NGC 3256 and NGC 3310 have X-ray detected sourcescoincident with nuclear regions; however, the steep NuSTAR spectra of bothgalaxies restricts these sources to be either low luminosity AGN or non-AGN innature (e.g., ULXs or crowded X-ray sources that reach L2-10 keV ~ 10^40 erg/scannot be ruled out). Combining our constraints on the 0.3-30 keV spectra ofNGC 3256 and NGC 3310 with equivalent measurements for nearby star-forminggalaxies M83 and NGC 253, we analyze the SFR-normalized spectra of thesestarburst galaxies. The spectra of all four galaxies show sharply decliningpower-law slopes above 3-6 keV due to ULX populations. Our observationstherefore constrain the average spectra of luminous accreting binaries (i.e.,ULXs). This result is similar to the super-Eddington accreting ULXs that havebeen studied individually in a targeted NuSTAR ULX program. We also find thatNGC 3310 exhibits a factor of ~3-10 elevation of X-ray emission over the otherstar-forming galaxies. We argue that the excess is most likely explained by therelatively low metallicity of the young stellar population in NGC 3310.

The 0.3-30 keV Spectra of Powerful Starburst Galaxies: NuSTAR and Chandra Observations of NGC 3256 and NGC 3310