Recurring obscuration in NGC 3783
SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
2 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
3 Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel
4 Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
5 Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
6 Smithsonian Astrophysical Observatory, Cambridge, MA, USA
7 INAF-IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
8 Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
9 Department of Astronomy, University of Geneva, 16 Ch. d’Ecogia, 1290 Versoix, Switzerland
10 European Space Astronomy Centre, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
11 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
12 Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
13 Max Planck Institute für Extraterrestriche Physik, 85748 Garching, Germany
14 Institute of Astronomy, Madingley Road, CB3 0HA Cambridge, UK
Accepted: 25 August 2018
Context. Obscuration of the continuum emission from active galactic nuclei by streams of gas with relatively high velocity (>1000 km s−1) and column density (>3 × 1025 m−2) has been seen in a few Seyfert galaxies. This obscuration has a transient nature. In December 2016 we witnessed such an event in NGC 3783.
Aims. The frequency and duration of these obscuration events is poorly known. Here we study archival data of NGC 3783 in order to constrain this duty cycle.
Methods. We use archival Chandra/NuSTAR spectra taken in August 2016. We also study the hardness ratio of all Swift XRT spectra taken between 2008 and 2017.
Results. In August 2016, NGC 3783 also showed evidence of obscuration. While the column density of the obscuring material is ten times lower than in December 2016, the opacity is still sufficient to block a significant fraction of the ionising X-ray and extreme ultraviolet photons. From the Swift hardness ratio behaviour we find several other epochs with obscuration. Obscuration with columns >1026 m−2 may take place about half of the time. Also, in archival X-ray data taken by the Advanced Satellite for Cosmology and Astrophysics (ASCA) in 1993 and 1996 we find evidence of obscuration.
Conclusions. Obscuration of the ionising photons in NGC 3783 occurs more frequently than previously thought. This may not always have been recognised due to low-spectral-resolution observations, overly limited spectral bandwidth or confusion with underlying continuum variations.
Key words: X-rays: galaxies / galaxies: active / galaxies: Seyfert / galaxies: individual: NGC 3783 / techniques: spectroscopic
© ESO 2018