Dramatic X-ray spectral variability of a Compton-thick type-1 QSO at z ∼ 1. Issue 4 (17th August 2018)

• Record Type: Journal Article
• Authors: Simm, T; Buchner, J; Merloni, A; Nandra, K; Shen, Y; Erben, T; Coil, A L; Willmer, C N A; Schneider, D P
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 480:Issue 4(2018)
• Page Start: 4912
• Page End: 4917
• Publication Date: 2018-08-17
• Subjects: accretion, accretion discs -- black hole physics -- methods: data analysis -- galaxies: active -- quasars: general -- X-rays: galaxies; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/sty2208
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We report on the discovery of a dramatic X-ray spectral variability event observed in a $z$ ∼ 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 Γ ∼ 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 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.