DISCOVERY OF STRONG X-RAY VARIABILITY IN AN H-II-TYPE GALAXY

We present the first detection of X-ray variability in an H II type ga laxy. The standard line ratio diagnostics of the galaxy, IRAS 15564+63 59, all point to the H II nature of the source. On the other hand, in terms of X-ray flux and spectral variability, the source is very simil ar to very active AGN. The shortest doubling time scale found in the R OSAT (0.1-2.4keV) energy band is only about 1500 seconds. Amplitude va riations of a factor of 10 are detected on a time scale of about half a day. The small size of the emitting region, less than 2.10(-5)pc, de duced from the rapid variations can be taken as the strongest proof fo r the presence of an active nucleus in the H II type galaxy IRAS 15564 +6359. Our analysis of the X-ray spectra indicate that the flux variat ions are connected with strong spectral changes. We detected a signifi cant hardening of the ROSAT spectrum with increasing source intensity. Spectral simulations show that the ROSAT spectrum can be represented by a combination of a steep thermal starburst component and a flatter power law AGN contribution. On the basis of this model we can disentan gle the (0.1-2.4keV) luminosity emitted in the starburst from the lumi nosity emerging from the AGN component as a function of varying source intensity.