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) ∝ r−p) is significantly flatter (p = 0.55 −0.04) 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 (MBH=26 +9 −6 M ) when assuming zero spin, or an IMBH (MBH=125 +45 −30 M ) when assuming maximal spin. Subject headings: black hole physics – X-rays: binaries – X-rays: individual (M82 X-1)