Pawel Magdziarz

Simultaneous EUVE/ASCA/RXTE Observations of NGC 5548

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.

The average X-ray/gamma-ray spectrum of radio-quiet Seyfert 1s

We have obtained the average 1--500 keV spectrum of radio-quiet Seyfert 1s using data from EXOSAT, Ginga, HEAO, and GRO/OSSE. The spectral fit to the combined average EXOSAT and OSSE data is fully consistent with that for Ginga and OSSE, confirming results from an earlier Ginga/OSSE sample. The average spectrum is well-fitted by a power-law X-ray continuum with an energy spectral index of

Long-Term Monitoring of NGC 4151 by OSSE

\alpha \simeq 0.9

The Compton Mirror in NGC 4151

moderately absorbed by an ionized medium and with a Compton reflection component. A high-energy cutoff (or a break) in the the power-law component at a few hundred keV or more is required by the data. We also show that the corresponding average spectrum from HEAO A1 and A4 is fully compatible with that obtained from EXOSAT, Ginga and OSSE. These results confirm that the apparent discrepancy between the results of Ginga (with

Narrowband photometry of comet Okazaki-Levy-Rudenko 1989r

\alpha \simeq 0.9

The X-ray spectrum of the polar BY Cam

) and the previous results of EXOSAT and HEAO (with

Broad-band continuum and variability of NGC 5548

\alpha \simeq 0.7

Angle-dependent Compton reflection of X-rays and gamma-rays

) is indeed due to ionized absorption and Compton reflection first taken into account for Ginga but not for the previous missions. Also, our results confirm that the Seyfert-1 spectra are on average cut off in gamma-rays at energies of at least a few hundred keV, not at

Broad-band γ-ray and X-ray spectra of NGC 4151 and their implications for physical processes and geometry

\sim 40

A spectral decomposition of the variable optical, ultraviolet and X-ray continuum of NGC 5548

keV (as suggested earlier by OSSE data alone). The average spectrum is compatible with emission from either an optically-thin relativistic thermal plasma in a disk corona, or with a nonthermal plasma with a power-law injection of relativistic electrons.