SimultaneousEUVE/ASCA/RXTEObservations of NGC 5548

Abstract

We present simultaneous observations by EUVE, ASCA, and RXTE of the type?1 Seyfert galaxy NGC?5548. These data indicate that variations in the EUV emission (at ~0.2 keV) appear to lead similar modulations in higher energy (1 keV) X-rays by ~10-30 ks. This is contrary to popular models which attribute the correlated variability of the EUV, UV and optical emission in type?1 Seyferts to reprocessing of higher energy radiation. This behavior instead suggests that the variability of the optical through EUV emission is an important driver for the variability of the harder X-rays which are likely produced by thermal Comptonization. We also compute the power density spectra at the various energy bands probed by these observations. Over 10-300 ks timescales, the emission in EUV shows about a factor of two greater rms variability than that of the 2-20 keV RXTE-PCA band?18. ? 1.4% versus 7.4 ? 0.6%. On longer timescales, we construct a PDS from 1-12 keV RXTE-ASM data which shows evidence for a break at about 6 ? 10-8 Hz. Furthermore, we find that the combined RXTE-ASM/PCA power spectrum is remarkably similar in shape to PDSs found for the low/hard states of Galactic black hole candidates such as Cygnus X-1. The implied scaling factor of ~106 is comparable to the expected mass ratio for these two objects. In addition, we investigate the spectral characteristics of the fluorescent iron K? line and Compton reflection emission. In contrast to prior measurements of these spectral features, we find that the iron K? line has a relatively small equivalent width (WK? ~ 100 eV) and that the reflection component is consistent with a covering factor which is significantly less than unity (?/2? ~ 0.4-0.5). Notably, although the 2-10?keV X-ray flux varies by ~?25% and the derived reflection fraction appears to be constant throughout our observations, the flux in the iron K? line is also constant. This behavior is difficult to reconcile in the context of standard Compton reflection models.