ROSAT observations of the dwarf star-forming galaxy Holmberg II (UGC 4305)

We present ROSAT PSPC and HRI observations of the dwarf irregular galaxy Holmberg II (UGC 4305). This is one of the most luminous dwarf galaxies (L-X similar to 10(40) erg s(-1)) detected in the ROSAT All-Sky Survey. The X-ray emission comes from a single unresolved point source, coincident with a large HII region that emits intense radio emission. The source is variable on both year and day time-scales, clearly favouring accretion into a compact object rather than a supernova remnant or a superbubble interpretation for the origin of the X-ray emission. However, its X-ray spectrum is well-fitted by a steep power law or alternatively by a Raymond-Smith spectrum with kT similar to 0.8 keV, lower than the temperature of X-ray binaries in nearby spiral galaxies.