XMM-Newton observation of the NLS1 galaxy Ark 564

I. Spectral analysis of the time-average spectrum

¹ Physics Department, University of Crete, PO Box 2208, 710 03 Heraklion, Crete, Greece e-mail: jhep@physics.uoc.gr
² Max–Planck–Institut für extraterrestrische Physik, Giessenbachstrasse, 85740 Garching, Germany
³ MSSL, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁴ School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
⁵ Sterrenkundig Instituut, Universiteit van Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands

Received: 1 May 2006
Accepted: 29 September 2006

Abstract

Context.We present the results from the spectral analysis of the time-average spectrum of the Narrow Line Seyfert 1 (NLS1) galaxy Ark 564 from a ~100 ks XMM-Newton observation.

Aims.Our aim is to characterize accurately the shape of the time-average, X-ray continuum spectrum of the source and search for any emission and/or absorption features in it.

Methods.We use the XMM-Newton data to obtain the X-ray spectrum of the source and we fit various spectral models to it.

Results.The time-average,  keV spectrum is well fitted by a power-law of slope 2.43. We detect a weak (equivalent width ~80 eV) emission line at ~6.7 keV, which implies emission from ionized iron. There is no compelling evidence for significant broadening of the line.We also detect a possible Doppler shifted absorption line at 8.1 keV. At energies 2 keV, the spectrum is dominated by a smooth soft excess component which can be fitted well either by a model consisting of two black body components (kT ~ 0.15 and 0.07 keV) or by a model consisting of a black body plus reflection from a relativistically-blurred, photoionized disc. We detect a broad, shallow flux deficit in the  keV band, reminiscent of the iron unresolved transition array (UTA) features. We detect neither a strong absorption edge around 0.7 keV nor an emission line around 1 keV.

Conclusions.The soft excess emission is consistent with being reflected emission from a geometrically flat disc, with solar abundances, illuminated by an isotropic source. The weakness of the iron line emission can be explained by relativistic blurring. The UTA feature implies the presence of warm material with a column density of   10²⁰ cm^-2. If the absorption line at 8.1 keV corresponds to FeXXVI Kα, it suggests the presence of highly ionized absorbing material with  cm^-2, outflowing at a high velocity of ~0.17c.