Simultaneous ROSAT, GINGA, VLA, IUE, and Optical Observations of the Bright Quasar H1821+643

We report on the first simultaneous optical, UV, and X-ray spectroscopy of the low-redshift (z = 0.297), high-luminosity QSO H 1821 + 643. This multiwavelength campaign was complemented by optical imaging and radio mapping with the VLA. Although there was no significant variability during the 40 day monitoring period in either the UV or X-ray bands, a number of new results were obtained. There is significant variability in the optical, UV, and X-ray bands on time scales of years, both in flux level and spectral shape. In the co-added ultraviolet spectrum we detected a Lyman-α absorption line at z = 0.225. In the radio map the QSO is extended in the north-south direction at 20 cm, and can be resolved into two separate components at 6 cm. In the ROSAT data we separated (both spatially and spectroscopically) the emission originating in H1821 + 643 from the nearby hot white dwarf K1 - 16, and thus we isolated the intrinsic soft X-ray excess in the QSO. The IUE and ROSAT data were combined with new optical spectroscopy and hard X-ray data from Ginga, to reveal a strong optical/UV bump with a flat slope (α~ -0.96, f_v_ is proportional to V^alpha^) in the UV, a steep soft X-ray excess (α~ -4) between ~0.15 keV and ~0.5 keV, and a typical hard X-ray slope of α~ -0.8 extending up to 10 keV. While the optical/UV bump and the soft X-ray excess can each be fit satisfactorily with standard accretion disk models, a joint fit to the optical/UV/X-ray bump cannot be achieved. This suggests that either the optical/UV or the soft X-ray emission do not arise in an accretion disk, or that the standard bare disk model should be modified (possibly by inclusion of electron scattering) to account for both the strong, flat UV emission and the steep, soft X-ray excess.