X‐Ray and Ultraviolet Spectral Properties of the X‐Ray Transient Quasar PG 0844+349

Abstract

Despite the fact that quasars are generally strong X-ray emitters, ROSAT discovered several objects with only very weak X-ray emission. In this paper, the X-ray data from ASCA and ROSAT and the UV spectra from Hubble Space Telescope (HST) and IUE of one of these quasars, PG 0844+349, are analyzed. The ROSAT spectrum can be well fitted by a single power law with Galactic absorption. No spectral variations were observed during changes of the 0.1-2.4 keV X-ray flux by a factor of 10 between the ROSAT All-Sky Survey and pointed observations, separated by 6 months. The ASCA satellite found the object in a high state with a photon index of 1.98 and an Fe Kα line with EW ~ 300 eV. The X-ray flux in the 2-10 keV band is highly variable; the fastest variation detected is 60% in less than 2 × 104 s. The measured excess variance fits well the excess variance versus L2-10 keV relation for Seyfert 1 galaxies; the flux variability in the 0.5-2.0 keV band shows a slightly higher amplitude than in the 2-10 keV band. We show that the optical microvariability of this object can actually be driven by reprocessing of the variable X-ray flux if one-half of the absorbed X-rays are reradiated in the optical-to-UV band. A weak broad intrinsic absorption line (FWHM 800 km s-1), most likely Lyα absorption blueshifted by a velocity -6000 km s-1 relative to the quasar's rest frame, is found in the HST Faint Object Spectrograph spectrum. A similar C IV broad absorption line may also be present in the low-resolution IUE spectrum. Historic light curves in the X-ray, UV, and optical bands indicate that the variability amplitude in the UV and optical bands is much smaller than in the X-ray band. The QSO can be classified as X-ray weak only on one occasion out of five X-ray observations. An analysis of the long-term behavior of several other X-ray-weak objects shows no indications of a similar large X-ray variability.