Dramatic X-ray spectral variability of a Compton-thick type-1 QSO at \(z\sim 1\)

• Published in: arXiv.org
• Main Authors: Simm, Torben, Buchner, Johannes, Merloni, Andrea, Nandra, Kirpal, Shen, Yue, Erben, Thomas, Coil, Alison L, Willmer, Christopher N A, Schneider, Donald P
• Format: Paper
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 15.08.2018
• Subjects: Luminosity; Occultation; Reflection; X ray spectra; X-rays; XMM (spacecraft)
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1808.05207

We report on the discovery of a dramatic X-ray spectral variability event observed in a \(z\sim 1\) 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 \(\Gamma\sim 2\), a column density of \(N_{\mathrm{H}}\sim 5\times 10^{20}\,\mathrm{cm^{-2}}\), 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 \(N_{\mathrm{H}}\sim 2\times 10^{24}\,\mathrm{cm^{-2}}\) eclipsing the direct emission of the hot corona, implying an extreme \(N_{\mathrm{H}}\) 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 datasets 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.