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

T. Simm, J. Buchner, A. Merloni, K. Nandra, Y. Shen, T. Erben, A. L. Coil, C. N.A. Willmer, D. P. Schneider

Research output: Contribution to journalArticlepeer-review


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 $N-{mathrm{H}}sim 5times 10{20}, mathrm{cm{-2}}$, 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 $N-{mathrm{H}}sim 2times 10{24}, mathrm{cm{-2}}$ eclipsing the direct emission of the hot corona, implying an extreme NH 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.

Original languageEnglish (US)
Pages (from-to)4912-4917
Number of pages6
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Nov 1 2018


  • X-rays: galaxies
  • accretion, accretion discs
  • black hole physics
  • galaxies: active
  • methods: data analysis
  • quasars: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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