Dramatic X-ray spectral variability of a Compton-thick type-1 QSO at z ∼ 1

We report on the discovery of a dramatic X-ray spectral variability event observed in a z similar to 1 broad line type-1 QSO. The XMM Newton spectrum from the year 2000 is characterized by an unobscured power-law spectrum with photon index of Gamma similar to 2, a column density of N-H similar to 5 x 10(20) cm(-2), and no prominent reflection component. Five years later, Chandra captured the source in a heavily-obscured, reflection-dominated state. The observed X-ray spectral variability could be caused by a Compton-thick cloud with N-H similar to 2 x 10(24) cm(-2) eclipsing the direct emission of the hot corona, implying an extreme N-H variation never before observed in a type-1 QSO. An alternative scenario is a corona that switched off in between the observations. In addition, both explanations require a significant change of the X-ray luminosity prior to the obscuration or fading of the corona and/or a change of the relative geometry of the source/reflector system. Dramatic X-ray spectral variability of this kind could be quite common in type-1 QSOs, considering the relatively few data sets in which such an event could have been identified. Our analysis implies that there may be a population of type-1 QSOs which are Compton-thick in the X-rays when observed at any given time.