X-ray spectral variability in PG 1535+547: the changing look of a “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.