MULTIWAVELENGTH SPECTROSCOPY OF ACTIVE GALACTIC NUCLEI

Abstract

ABSTRACT This thesis comprises extensive observational studies of two active galactic nuclei: the bright QSO H1821+643 and the Seyfert 1 galaxy NGC 3516. We report on the first simultaneous spectroscopy of the nearby (z = 0.297) QSO H1821+643 in teh optical, UV, and X-ray bands, in addition to optical, X-ray, and radio mapping. The QSO, which had not been detected at radio frequencies before, exhibits a detailed radio structure in which two separate components are distinguished. In the soft X-ray observations we separated (both spectroscopically and spatially) the emission originating in H1821+643 from that from the nearby hot planetary nucleus K1-16, thus isolating the intrinsic soft X-ray excess in the QSO. In the coadded UV spectrum we detected a Lyman alpha absorption line at z=0.225, which we identify with a foreground galaxy. The combined spectroscopy shows a strong optical/UV bump with a flat slope in the UV (alpha -0.96, fv approaches Valpha) and a steep excess (alpha -4) in the soft X-ray band in addition to a typical hard slope of alpha -0.8 between 0.5 and 10 keV. Standard accretion-disk models can account for the observed optical/UV emission, but fail to fit both the soft X-ray excess and the optica/UV bump. We also report on the first simultaneous UV and hard X-ray observations of the Seyfert 1 galaxy NGC 3516, which were carried out to investigate the connection between the exceptionally strong and variable absorption in the X-ray and ultraviolet bands. The X-ray spectrum is complex, and can be fitted satisfactorily with a warm-absorber model, a partial-covering model, or a cold-reflector model. Lower limits on the column densities of the X-ray and UV absorbers are established at 4 X 1022 and 2 X 1019 cm-2, respectively. Assuming that the variability in the C IV lambda-1550 absorption is due to changes in ionization level, we find that the UV absorbers should be located closer than 9 pc from the ionizing source for log NH 20, and at smaller distances for higher column densities, thus excluding the narrow-line region. Although the absence of short-term variability in absorption during the simultaneous X-ray and UV observations preempted a direct test of the common origin of the UV and X-ray absorption, indirect arguments lead to the conclusion that these absorptions probably do not arise in the same location.