1994A&A...291..403B

We present the first detection of X-ray variability in an HII type galaxy. The standard line ratio diagnostics of the galaxy, IRAS 15564+6359, all point to the HII nature of the source. On the other hand, in terms of X-ray flux and spectral variability, the source is very similar to very active AGN. The shortest doubling time scale found in the ROSAT (0.1-2.4keV) energy band is only about 1500 seconds. Amplitude variations 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 2x10^–5pc, deduced from the rapid variations can be taken as the strongest proof for the presence of an active nucleus in the HII type galaxy IRAS 15564+6359. Our analysis of the X-ray spectra indicate that the flux variations are connected with strong spectral changes. We detected a significant 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 disentangle the (0.1-2.4keV) luminosity emitted in the starburst from the luminosity emerging from the AGN component as a function of varying source intensity.