HST/COS observations of the newly discovered obscuring outflow in NGC 3783

¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA
e-mail: gak@stsci.edu
² SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁴ Max-Planck-Institut für Extraterrestriche Physik, Gießenbachstraße, 85748 Garching, Germany
⁵ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D bus 2401, 3001 Leuven, Belgium
⁶ Department of Physics, Virginia Tech, Blacksburg, VA, 24061, USA
⁷ Department of Physics, Technion-Israel Institute of Technology, 32000 Haifa, Israel
⁸ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
¹⁰ INAF-IASF Bologna, Via Gobetti 101, 40129 Bologna, Italy
¹¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
¹² Department of Astronomy, University of Geneva, 16 Ch. d’Ecogia, 1290 Versoix, Switzerland
¹³ European Space Astronomy Centre, PO Box 78 28691 Villanueva de la Cañada, Madrid, Spain
¹⁴ Department of Astronomy, The Ohio State University, 140 West 18th Ave., Columbus, OH, 43210, USA
¹⁵ Center for Cosmology & AstroParticle Physics, The Ohio State University, 191 West Woodruff Ave., Columbus, OH, 43210, USA
¹⁶ Univ. Grenoble Alpes, CNRES, IPAG, 38000 Grenoble, France
¹⁷ INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
¹⁸ Institute of Astronomy, Madingley Road, CB3 0HA Cambridge, UK

Received: 26 September 2018
Accepted: 29 October 2018

Abstract

Aims. To understand the nature of transient obscuring outflows in active galactic nuclei, we use simultaneous multiwavelength observations with XMM-Newton, NuSTAR, the Hubble Space Telescope (HST), and the Max Planck Gesellschaft/European Southern Observatory (ESO) 2.2 m telescope triggered by soft X-ray absorption detected by Swift.

Methods. We obtained ultraviolet spectra on 2016-12-12 and 2016-12-21 using the Cosmic Origins Spectrograph (COS) on HST simultaneously with X-ray spectra obtained with XMM-Newton and NuSTAR. We modeled the ultraviolet spectra to measure the strength and variability of the absorption, and used photoionization models to obtain its physical characteristics.

Results. We find new components of broad, blue-shifted absorption associated with Lyα, N V, Si IV, and C IV in our COS spectra. The absorption extends from near-zero velocities in the rest-frame of the host galaxy to −6200 km s⁻¹. These features appear for the first time in NGC 3783 at the same time as heavy soft X-ray absorption seen in the XMM-Newton X-ray spectra. The X-ray absorption has a column density of ∼10²³ cm⁻², and it partially covers the X-ray continuum source. Combining the X-ray column densities with the UV spectral observations yields an ionization parameter for the obscuring gas of log ξ = 1.84_−0.2^+0.4 erg cm s⁻¹. Despite the high intensity of the UV continuum in NGC 3783, F(1470 Å) = 8 × 10⁻¹⁴ erg cm⁻² s⁻¹ Å⁻¹>, the well known narrow UV absorption lines are deeper than in earlier observations in unobscured states, and low ionization states such as C III appear, indicating that the narrow-line gas is more distant from the nucleus and is being shadowed by the gas producing the obscuration. Despite the high continuum flux levels in our observations of NGC 3783, moderate velocities in the UV broad line profiles have substantially diminished.

Conclusions.We suggest that a collapse of the broad line region has led to the outburst and triggered the obscuring event.