The Unusual Spectrum of the Ultraluminous X-Ray Source M82 X-1

The results of a spectral analysis, using {\it XMM-Newton} and {\it Chandra}data of the brightest ultra luminous X-ray source in the nearby galaxy M82, arepresented. The spectrum of M82 X-1, was found to be unusually hard (photonspectral index $\Gamma \approx 1$) with a sharp cutoff at $\approx 6$ keV. Diskblack body emission model requires a nonphysically high temperature. Instead,the spectrum is better described, with a lower reduced $\chi^2$, as emissiondue to nearly saturated Comptonization of photons in an optically thick ($\tau\approx 10-30$, depending on the geometry) plasma having a temperature $kT\approx 2$ keV. This is in contrast to the high energy spectra of other blackhole systems, which are relatively steeper ($\Gamma > 1.5$) and hence aremodeled as un-saturated thermal and/or non-thermal Comptonization of softphotons, in an optically thin ($\tau \approx 1$) high temperature plasma. AnIron line emission which is marginally resolved ($\sigma \sim 0.2$ keV) isrequired to fit the data. We argue that the standard geometries for the X-rayproducing region, which are an optically thin inner disk or an uniform/patchycorona on top of a cold disk, are not applicable to this source. Alternatively,the geometry of the X-ray producing region could be a large sphere surroundinga cold accretion disk or an optically thick inner disk region which cools bybremsstrahlung self-Comptonization. For the latter scenario, such an inner diskregion, whose effective optical depth to absorption is less than unity, isexpected in the standard accretion disk theory for near Eddington accretionrates.Comment: Accepted for publication in Astrophysical Journal Letter