A BROADBAND X-RAY SPECTRAL STUDY OF THE INTERMEDIATE-MASS BLACK HOLE CANDIDATE M82 X-1 WITH NuSTAR, CHANDRA, AND SWIFT

M82 X-1 is one of the brightest ultraluminous X-ray sources (ULXs) known, which, assuming Eddington-limited accretion and other considerations, makes it one of the best intermediate-mass black-hole (IMBH) candidates. However, the ULX may still be explained by super-Eddington accretion onto a stellar remnant black hole. We present simultaneous NuSTAR, Chandra, and Swift/XRT observations during the peak of a flaring episode with the aim of modeling the emission of M82 X-1 and yielding insights into its nature. We find that thin accretion disk models all require accretion rates at or above the Eddington limit in order to reproduce the spectral shape, given a range of black-hole masses and spins. Since at these high Eddington ratios the thin-disk model breaks down due to radial advection in the disk, we discard the results of the thin-disk models as unphysical. We find that the temperature profile as a function of disk radius (T(r) proportional to r(-p)) is significantly flatter (p = 0.55(-0.04)(+0.07)) than expected for a standard thin disk (p = 0.75). A flatter profile is instead characteristic of a slim disk, which is highly suggestive of super-Eddington accretion. Furthermore, radiation hydrodynamical simulations of super-Eddington accretion have shown that the predicted spectra of these systems are very similar to what we observe for M82 X-1. We therefore conclude that M82 X-1 is a super-Eddington accretor. Our mass estimates inferred from the inner disk radius imply a stellar remnant black hole (M-BH = 26(-6)(+9) M-circle dot) when assuming zero spin and face-on inclination, or an IMBH for maximal spin and a highly inclined disk.