Simultaneous X-ray and ultraviolet spectroscopy of the Seyfert galaxy NGC 5548

III. X-ray time variability

¹ SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands, USA
² Department of Physics and Astronomy, Georgia State University, Astronomy Offices, One Park Place South SE, Suite 700, Atlanta, GA 30303, USA
³ Catholic University of America, USA
⁴ Laboratory for Astronomy and Solar Physics, NASA's Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA
⁵ CASA, University of Colorado, 389 UCB, Boulder, CO 80309-0389, USA
⁶ Joint Center for Astrophysics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁷ Laboratory for High Energy Astrophysics, Code 660, NASA's Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ Physics Department, Lawrence Livermore National Laboratory, PO Box 808, 41, Livermore, CA 94550, USA
⁹ Columbia Astrophysics Laboratory, Columbia University, 538W. 120th Street, New York, NY 10027, USA
¹⁰ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA

Corresponding author: J. S. Kaastra, J.S.Kaastra@sron.nl

Received: 18 February 2004
Accepted: 12 April 2004

Abstract

The Seyfert 1 galaxy NGC 5548 was observed for a week by Chandra using both the HETGS and LETGS spectrometers. In this paper we study the time variability of the continuum radiation. During our observation, the source showed a gradual increase in flux over four days, followed by a rapid decrease and flattening of the light curve afterwards. Superimposed upon these relatively slow variations several short duration bursts or quasi-periodic oscillations occured with a typical duration of several hours and separation between 0.6–0.9 days. The bursts show a delay of the hard X-rays with respect to the soft X-rays of a few hours. We interpret these bursts as occurring due to a rotating, fluctuating hot spot at approximately 10 gravitational radii; the time delay of the hard X-rays from the bursts agrees with the canonical picture of Inverse Compton scattering of the soft accretion disk photons on a hot medium that is relatively close to the central black hole.