Highly ionized disc and transient outflows in the Seyfert galaxy IRAS 18325–5926

¹ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain
e-mail: kazushi.iwasawa@icc.ub.edu
² ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
³ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁴ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁵ Centro de Astrobiologia (CSIC-INTA), Dep. de Astrofísica, ESAC, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁶ X-ray Astrophysics Laboratory and CRESST, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 22 December 2015
Accepted: 10 June 2016

Abstract

We report on strong X-ray variability and the Fe K-band spectrum of the Seyfert galaxy IRAS 18325–5926 obtained from the 2001 XMM-Newton EPIC pn observation with a duration of ~120 ks. While the X-ray source is highly variable, the 8–10 keV band shows larger variability than that of the lower energies. Amplified 8–10 keV flux variations are associated with two prominent flares of the X-ray source during the observation. The Fe K emission is peaked at 6.6 keV with moderate broadening. It is likely to originate from a highly ionized disc with an ionization parameter of log ξ ≃ 3. The Fe K line flux responds to the main flare, which supports its disc origin. A short burst of the Fe line flux has no relation to the continuum brightness, for which we have no clear explanation. We also find transient, blueshifted Fe K absorption features that can be identified with high-velocity (~0.2c) outflows of highly ionized gas, as found in other active galaxies. The deepest absorption feature appears only briefly (~1 h) at the onset of the main flare and disappears when the flare declines. The rapid evolution of the absorption spectrum makes this source peculiar among the active galaxies with high-velocity outflows. Another detection of the absorption feature also precedes the other flare. The variability of the absorption feature partly accounts for the excess variability in the 8–10 keV band where the absorption feature appears. Although no reverberation measurement is available, the black hole mass of ~2 × 10⁶M_⊙ is inferred from the X-ray variability. When this mass is assumed, the black hole is accreting at around the Eddington limit, which may fit the highly ionized disc and strong outflows observed in this galaxy.