Multiple Components of the Luminous Compact X-Ray Source at the Edge of Holmberg II Observed by ASCA and ROSAT

We report the results of the analysis of new ASCA observations and archival ROSAT data of the compact luminous X-ray source found at the edge of the nearby star-forming dwarf galaxy Holmberg II (UGC 4305) in the M81 group. We have found a number of new features in the X-ray properties of this source. Our new ASCA spectrum revealed that the X-ray emission extends to the hard band and can be best described by a power law with a photon spectral index Γ~1.9, while a kT~5 keV thermal plasma with a low abundance (~0.2 Z_solar) is also acceptable. The ASCA spectrum does not fit with a multicolor disk blackbody, unlike some off-nucleus X-ray sources with similar luminosities. The joint ASCA-ROSAT spectrum suggests two components to the spectrum: the hard power-law component and a warm thermal plasma (kT~0.3 keV). An additional absorption over that of our galaxy is required. The wobble correction of the ROSAT HRI image has clearly unveiled the existence of an extended component that amounts to 27%+/-5% of the total X-ray emission. These observations indicate that there are more than one component in the X-ray emission. The properties of the pointlike component indicate an accretion onto an intermediate-mass black hole unless a beaming is taking place. We argue that the extended component does not come from electron scattering and/or reflection by scattered optically thick clouds of the central radiation. Possible explanations of this X-ray source include multiple supernova remnants feeding an intermediate-mass black hole.