X-ray spectral variability in PG 1535+547: the changing-look "soft X-ray weak" AGN

Context. PG 1535+547 is a bright Narrow Line Seyfert 1 galaxy, whose high-energy emission shows strong variability both in shape and flux. On the basis of ROSAT observations, it is classified as “soft X-ray weak quasi-stellar object (QSO)”, a class of objects whose X-ray-to-optical flux ratio is smaller than in typical QSOs. Their X-ray spectra are often characterized by highly-ionized, complex absorbers and/or reflection from the inner accretion disk, and the relative importance of the two is currently debated. Whatever the correct interpretation may be, the presence of such features implies that we are looking at matter located in the innermost regions of these AGN. Aims. We want to clarify the nature of the X-ray emission of PG 1535+547, and constrain the physical properties of its innermost regions, where this emission originates. Methods. We present new XMM-Newton observations of PG 1535+547 (90 ks exposure time, January 2006), from which we obtained two spectra European Photon Imaging Camera (EPIC) separated by about one week. We compare these spectra with those from a previous (November 2002) XMM-Newton observation. Results. These latest observations support the complex and variable nature of the X-ray emission of PG 1535+547. The broadband observed flux increases by a factor ∼2.3 in three years, and then decreases by a factor ∼1.3 in about one week. In the new spectra, strong absorption features at E < 3 keV and a complex spectral shape in the iron-line energy range are evident, coupled with a drop in the emission at higher energies. We describe all the different states in a consistent way, assuming either a warm absorber plus a relativistically-blurred ionized reflection, or a two-phase warm absorber partially covering the source with the addition of a scattered component. Conclusions. The observed variability can be ascribed mostly to warm absorbing gas in the innermost regions of PG 1535+547, which appears to vary in its physical properties on timescales of both years and days. In the blurred reflection scenario, all the analyzed states require a high fraction of reflection from the disk, calling for some mechanisms able to increase the reflection component with respect to the intrinsic continuum. Finally, the strong variability observed in the X-ray band opposed to a more constant emission at optical frequencies, changes the value of the X-ray-to-optical spectral index, implying that PG 1535+547 can not actually be classified as a soft X-ray weak active galactic nucleus (AGN).