ROSAR observations of the Sombrero galaxy: Discovery of an X-ray active nucleus

The analysis of a high-resolution X-ray image of NGC 4594, obtained with the ROSAT HRI, shows at least three components of the X-ray emission. The most striking one is a pointlike source associated with the LINER nucleus, with (0.1-2.4) keV L(X) similar to 3.5 x 10(40) ergs s(-1). Comparison of the X-ray, H alpha, and radio emission of this source with those of active galactic nuclei suggests that it may be a low-luminosity version of the latter. If the X-ray emission is due to accretion onto the 5 x 10(8) M. black hole that may be present at the nucleus of NGC 4594, the accretion rates must be very low, since the luminosity is extremely sub-Eddington. Alternatively, the source may be heavily obscured and only reprocessed photons are detected in the ROSAT range. We also detect clumpy emission associated with the disk of NGC 4594, and diffuse (or not resolved) emission from the bulge. With the present data, including a reanalysis of the archival ROSAT PSPC observation of this galaxy, we cannot determine how much of this diffuse emission is due to a hot interstellar medium, and how much is instead due to the integrated emission of a population of evolved stellar sources.