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.
- Publication:
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The Astrophysical Journal
- Pub Date:
- February 2006
- DOI:
- 10.1086/501163
- arXiv:
- arXiv:astro-ph/0601325
- Bibcode:
- 2006ApJ...638L..83A
- Keywords:
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- Galaxies: General;
- Galaxies: Individual: Messier Number: M82;
- X-Rays: Binaries;
- Astrophysics
- E-Print:
- Accepted for publication in Astrophysical Journal Letters