Multiple components of the luminous compact X-ray source at the edge of Holmberg II observed by ASCA and ROSAT.
MIYAJI T., LEHMANN I. and HASINGER G.
Abstract (from CDS):
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☉) 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.