The variable X-ray spectrum of Markarian 766

I. Principal components analysis

¹ Dept. of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK e-mail: L.Miller1@physics.ox.ac.uk
² Dept. of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
³ X-Ray Astrophysics Laboratory, Code 662, Astrophysics Science Division, NASA/GSFC, Greenbelt, MD 20771, USA
⁴ Dept. of Physics and Astronomy, Johns Hopkins University, 3400 N Charles Street, Baltimore, MD 21218, USA
⁵ Catholic University of America, Washington DC 20064, USA

Received: 12 October 2006
Accepted: 14 November 2006

Abstract

Aims.We analyse a long XMM-Newton observation of the narrow-line Seyfert 1 galaxy Mrk 766, using the marked spectral variability on timescales >20 ks to separate components in the X-ray spectrum.

Methods.Principal components analysis is used to identify distinct emission components in the X-ray spectrum, possible alternative physical models for those components are then compared statistically.

Results. The source spectral variability is well-explained by additive variations, with smaller extra contributions most likely arising from variable absorption. The principal varying component, eigenvector one, is found to have a steep (photon index 2.4) power-law shape, affected by a low column of ionised absorption that leads to the appearance of a soft excess. Eigenvector one varies by a factor 10 in amplitude on time-scales of days and appears to have broad ionised Fe Kα emission associated with it: the width of the ionised line is consistent with an origin at ~100 gravitational radii. There is also a strong component of near-constant emission that dominates in the low state, whose spectrum is extremely hard above 1 keV, with a soft excess at lower energies, and with a strong edge at Fe K but remarkably little Fe Kα emission. Although this component may be explained as relativistically-blurred reflection from the inner accretion disc, we suggest that its spectrum and lack of variability may alternatively be explained as either (i) ionised reflection from an extended region, possibly a disc wind, or (ii) a signature of absorption by a disc wind with a variable covering fraction. Absorption features in the low state may indicate the presence of an outflow.