Abstract

A set of 11 contemporaneous IUE and GINGA observations of NGC 5548 reveal the existence of correlated variations of its hard X-ray (2–10 keV) and Ultraviolet (1200−3300 Å) flux over time scales of 2 days to one year. This is best explained in the framework of a model where the X-rays irradiate a cold (T ∼ 105K) accretion disk. Only a tiny fraction of the irradiating flux is compton reflected back to the observer in the form of a hard X-ray tail while the bulk of the X-rays are absorbed in the disk and eventually re-emitted as thermal radiation in the Ultraviolet. The absence of a detectable phase delay between the two bands together with the absence of rapid (∼ hours) fluctuations of the UV flux further constrain the X-ray source to lie between 200 and 1400 Schwarzschild radii above the disk. The thermal reprocessing model provides a natural explanation for the simultaneity of the optical and UV variations in NGC 5548 and may solve most of the problems facing the accretion disk model.

Introduction

The presence of a strong Iron Kα line near 6.4 keV and a “hard tail” above 10 keV is a common property of the X-ray spectrum of Seyfert 1 galaxies [1]. This has been interpreted as evidence for reprocessing of the X-rays by a “cold” (T ∼ 105 K) accretion disk [2]. In this model, less than 10% of the irradiating photons are compton scattered back to the observer forming a spectral hump centred near 15keV while the bulk of the X-rays are absorbed in the disk.