A variable-density absorption event in NGC 3227 mapped with Suzaku and Swift

¹ Dr. Remeis-Sternwarte & Erlangen Centre for Astroparticle Physics, Universität Erlangen-Nürnberg, Sternwartstrasse 7, 96049 Bamberg, Germany
e-mail: tobias.beuchert@sternwarte.uni-erlangen.de
² Lehrstuhl für Astronomie, Universität Würzburg, Emil-Fischer-Straße 31, 97074 Würzburg, Germany
³ Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0424, USA
⁴ Centro de Astrobiología (CSIC–INTA), Dep. de Astrofísica, European Space Astronomy Centre, PO Box 78, Villanueva de la Cañada, 28691 Madrid, Spain
⁵ Instituto de Astronomía, Universidad Nacional Autónoma de México (UNAM), 04510 Ciudad de México, D.F. México
⁶ European Space Agency, European Space Astronomy Centre, PO Box 78, Villanueva de la Cañada, 28691 Madrid, Spain
⁷ Physics and Astronomy Department, UCLA, Los Angeles, CA 90095-1562, USA
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
⁹ Instituto de Astronomía, Universidad Nacional Autónoma de México, Km 103, Carret, Tijuana-Ensenada, 22860 Ensenada, BC, Mexico ( PO Box 439027, San Ysidro, CA, 92143, USA )
¹⁰ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain

Received: 19 June 2015
Accepted: 10 August 2015

Abstract

Context. The morphology of the circumnuclear gas accreting onto supermassive black holes in Seyfert galaxies remains a topic of much debate. As the innermost regions of active galactic nuclei (AGN) are spatially unresolved, X-ray spectroscopy, and in particular line-of-sight absorption variability, is a key diagnostic to map out the distribution of gas.

Aims. Observations of variable X-ray absorption in multiple Seyferts and over a wide range of timescales indicate the presence of clumps/clouds of gas within the circumnuclear material. Eclipse events by clumps transiting the line of sight allow us to explore the properties of the clumps over a wide range of radial distances from the optical/UV broad line region (BLR) to beyond the dust sublimation radius. Time-resolved absorption events have been extremely rare so far, but suggest a range of density profiles across Seyferts. We resolve a weeks-long absorption event in the Seyfert NGC 3227.

Methods. We examine six Suzaku and 12 Swift observations from a 2008 campaign spanning five weeks. We use a model accounting for the complex spectral interplay of three absorbers with different levels of ionization. We perform time-resolved spectroscopy to discern the absorption variability behavior. We also examine the IR to X-ray spectral energy distribution (SED) to test for reddening by dust.

Results. The 2008 absorption event is due to moderately-ionized (log ξ ~ 1.2–1.4) gas covering 90% of the line of sight. We resolve the density profile to be highly irregular, in contrast to a previous symmetric and centrally-peaked event mapped with RXTE in the same object. The UV data do not show significant reddening, suggesting that the cloud is dust-free.

Conclusions. The 2008 campaign has revealed a transit by a filamentary, moderately-ionized cloud of variable density that is likely located in the BLR, and possibly part of a disk wind.