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

Abstract

The results of a spectral analysis, using XMM-Newton and Chandra data of the brightest ultraluminous X-ray source in the nearby galaxy M82, are presented. The spectrum of M82 X-1 was found to be unusually hard (photon spectral index Γ ≈ 1) with a sharp cutoff at ≈6 keV. The disk blackbody emission model requires a nonphysically high temperature. Instead, the spectrum is better described, with a lower reduced χ2, as emission due to the nearly saturated Comptonization of photons in an optically thick (τ ≈ 10-30, depending on the geometry) plasma having a temperature kT ≈ 2 keV. This is in contrast to the high-energy spectra of other black hole systems, which are relatively steeper (Γ > 1.5) and hence are modeled as the unsaturated thermal and/or nonthermal Comptonization of soft photons, in an optically thin (τ ≈ 1) high-temperature plasma. An iron line emission that is marginally resolved (σ ~ 0.2 keV) is required to fit the data. We argue that the standard geometry for the X-ray-producing region, which consists of an optically thin inner disk or a uniform/patchy corona on top of a cold disk, is not applicable to this source. Alternatively, the geometry of the X-ray-producing region could be a large sphere surrounding a cold accretion disk or an optically thick inner disk region that cools by bremsstrahlung self-Comptonization. For the latter scenario, such an inner disk region, whose effective optical depth to absorption is less than unity, is expected in the standard accretion disk theory for near-Eddington accretion rates.