Issue A&A Volume 422, Number 1, July IV 2004 Page(s) 97 - 102 Section Extragalactic astronomy DOI http://dx.doi.org/10.1051/0004-6361:20040291

A&A 422, 97-102 (2004)
DOI: 10.1051/0004-6361:20040291

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

III. X-ray time variability

J. S. Kaastra¹, K. C. Steenbrugge¹, D. M. Crenshaw², S. B. Kraemer^{3, 4}, N. Arav⁵, I. M. George^{6, 7}, D. A. Liedahl⁸, R. L. J. van der Meer¹, F. B. S. Paerels⁹, T. J. Turner^{6, 7} and T. Yaqoob<sup>7, 10

1</sup>  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

(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.

Key words: galaxies: active -- galaxies: Seyfert -- galaxies: individual: NGC 5548 -- X-rays: individual: NGC 5548 -- X-rays: galaxies

Offprint request: J. S. Kaastra, J.S.Kaastra@sron.nl

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