Incoherent fast variability of X-ray obscurers

The case of NGC 3783

¹ N. Copernicus Astronomical Center of the Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
e-mail: bdemarco@camk.edu.pl
² Inter-University Centre for Astronomy and Astrophysics (IUCAA), Ganeshkhind, Pune 411007 Maharashtra, India
³ INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
⁴ Dipartimento di Matematica e Fisica, Università Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA
⁷ Department of Physics, Technion-Israel Institute of Technology, 32000 Haifa, Israel
⁸ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁹ INAF-IASF Bologna, Via Gobetti 101, 40129 Bologna, Italy
¹⁰ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 Utrecht, The Netherlands
¹¹ Italian Space Agency (ASI), Via del Politecnico snc, 00133 Roma, Italy
¹² European Space Astronomy Centre, PO Box 78, 28691 Villanueva de la Caada, Madrid, Spain
¹³ Leiden Observatory, Leiden University, PO Box 9513, 2300 Leiden, The Netherlands
¹⁴ School of Physics and Astronomy and Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel
¹⁵ Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK
¹⁶ University of California, San Diego, Center for Astrophysics and Space Sciences, 9500 Gilman Dr, La Jolla, CA 92093-0424, USA
¹⁷ Department of Astronomy, University of Geneva, 16 Ch. d′Ecogia, 1290 Versoix, Switzerland
¹⁸ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁹ European Space Agency/ESTEC, Keplerlaan 1, 2201 Noordwijk, The Netherlands
²⁰ Institute of Astronomy, Madingley Road, CB3 0HA Cambridge, UK

Received: 6 August 2019
Accepted: 28 November 2019

Abstract

Context. Obscuration events caused by outflowing clumps or streams of high column density and low ionised gas, shown to absorb the X-ray continuum heavily, have been witnessed across a number of Seyfert galaxies.

Aims. We report on the X-ray spectral-timing analysis of the December 2016 obscuration event in NGC 3783, which was aimed at probing variability of the X-ray obscurer on the shortest possible timescales. The main goals of this study are to obtain independent constraints on the density and, ultimately on the distance of the obscuring gas, as well as to characterise the impact of variable obscuration on the observed X-ray spectral-timing characteristics of Seyfert galaxies.

Methods. We carried out a comparative analysis of NGC 3783 during unobscured (using archival 2000–2001 XMM-Newton data) and obscured states (using XMM-Newton and NuSTAR data from the 2016 observational campaign). The analysed timescales range between ten hours and about one hour. This study was then generalised to discuss the signatures of variable obscuration in the X-ray spectral-timing characteristics of Seyfert galaxies as a function of the physical properties of the obscuring gas.

Results. The X-ray obscurer in NGC 3783 is found to vary on timescales between about one hour to ten hours. This variability is incoherent with respect to the variations of the X-ray continuum. A fast response (on timescales shorter than about 1.5 ks) of the ionisation state of the obscuring gas to the short timescale variability of the primary X-ray continuum provides a satisfactory interpretation of all the observed X-ray spectral-timing properties. This study enabled us to put independent constraints on the density and location of the obscuring gas. We found the gas to have a density of n_e >  7.1 × 10⁷ cm⁻³, which is consistent with a location in the broad line region.