Warm absorber and truncated accretion disc in IRAS 05078+1626

¹ Astronomical Institute, Academy of Sciences, Boční II 1401, 14131 Prague, Czech Republic e-mail: svoboda@astro.cas.cz
² Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12161 Prague, Czech Republic
³ European Space Astronomy Centre of ESA, PO Box 78, Villanueva de la Cañada, 28691 Madrid, Spain

Received: 12 November 2009
Accepted: 24 December 2009

Abstract

Context. X-ray observations of unabsorbed active galactic nuclei provide an opportunity to explore the innermost regions of supermassive black hole accretion discs.

Aims. Our goal in this paper is to investigate the central environment of a Seyfert 1.5 galaxy IRAS 05078+1626.

Methods. We studied the time-averaged spectrum obtained with the EPIC and RGS instruments onboard XMM-Newton.

Results. A power law continuum (photon index Γ 1.75) dominates the 2–10 keV energy range. A narrow iron K α spectral line is clearly seen, presumably originating in a distant torus, but no broad relativistic component was detected. However, the power law and the iron K α line alone do not provide a satisfactory fit in the soft X-ray band whose spectrum can be explained by the combination of three components: a) a cold photoelectric absorber with column density N_H ≈ 10²¹ cm^-2. This gas could be located either in outer parts of the accretion disc, at the rim of the torus or farther out in the host galaxy; b) a warm absorber with high ionization parameter (log ξ ≈ 2.2) and column density (N_H ≈ 10²⁴ cm^-2); c) an ionised reflection where the reflecting gas could be either in the inner wall of a warm absorber cone or in an ionised accretion disc.

Conclusions. The first X-ray spectroscopic measurement of IRAS05078+1626 unveils some of the standard ingredients in Seyfert galaxies, such as a power law primary continuum, modified by reflection from the accretion disc and by the effect of complex, multi-phase obscuration. However, data constrains the accretion disc, if present, not to extend closer than to 60 gravitational radii from the black hole.