Suzaku Observations of M 82 X-1 : Detection of a Curved Hard X-Ray Spectrum

Abstract

A report is presented on Suzaku observations of the ultra-luminous X-ray source X-1 in the starburst galaxy M 82, made three times in 2005 October for an exposure of ∼30ks each. The XIS signals from a region of radius 3' around the nucleus defined a 2--10keV flux of 2.1 × 10^-11erg s^-1cm^-2 attributable to point sources. The 3.2--10keV spectrum was slightly more convex than a power-law with a photon index of 1.7. In all observations, the HXD also detected signals from M 82 up to ∼20keV, at a 12--20keV flux of 4.4 × 10^-12erg s^-1cm^-2. The HXD spectrum was steeper than that of the XIS. The XIS and HXD spectra can be jointly reproduced by a cutoff power-law model, or similar curved models. Of the detected wide-band signals, 1/3 to 2/3 are attributable to X-1, while the remainder to other discrete sources in M82. Regardless of the modeling of these contaminants, the spectrum attributable to X-1 is more curved than a power-law, with a bolometric luminosity of (1.5--3) × 10⁴⁰erg s^-1. These results are interpreted as Comptonized emission from a black hole of 100--200 solar masses, radiating roughly at the Eddington luminosity.