The XMM -Newton view of AGN with intermediate-mass black holes

Abstract

We have observed with XMM-Newton four radiatively efficient active type 1 galaxies with black hole masses <106 M⊙, selected optically from the Sloan Digital Sky Survey and previously detected in the ROSAT All Sky Survey. Their X-ray spectra closely resemble those of more luminous Seyferts and quasars, powered by accretion on to much more massive black holes and none of the objects is intrinsically absorbed by cold matter totally covering the source. We show here that their soft X-ray spectrum exhibits a soft excess with the same characteristics as that observed ubiquitously in radio-quiet Seyfert 1 galaxies and type 1 quasars, both in terms of temperatures and strength. This is highly surprising because the small black hole mass of these objects should lead to a thermal disc contribution in the soft X-rays and not in the ultraviolet (as for more massive objects) with thus a much more prominent soft X-ray excess. Moreover, even when the soft X-ray excess is modelled with a pure thermal disc, its luminosity turns out to be much lower than that expected from accretion theory for the given temperature. While alternative scenarios for the nature of the soft excess (namely smeared ionized absorption and disc reflection) cannot be distinguished on a pure statistical basis, we point out that the absorption model produces a strong correlation between absorbing column density and ionization state, which may be difficult to interpret and is most likely spurious. Moreover, as pointed out before by others, absorption must occur in a fairly relativistic wind which is problematic, especially because of the enormous implied mass outflow rate. As for reflection, it does only invoke standard ingredients of any accretion model for radiatively efficient sources such as a hard X-rays source and a relatively cold (though partially ionized) accretion disc, and therefore seems the natural choice to explain the soft excess in most (if not all) cases. The reflection model is also consistent with the additional presence of a thermal disc component with the theoretically expected temperature (although, again, with smaller-than-expected total luminosity). We also studied in some detail the X-ray variability properties of the four objects. The observed active galaxies are among the most variable in X-rays and their excess variance is among the largest. This is in line with their relatively small black hole mass and with expectations from simple power spectra models. © 2009 RAS.