X-ray and ultraviolet spectral properties of the X-ray transient quasar PG0844+349

Despite the fact that quasars are generally strong X-ray emitters, ROSAT di scovered 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 Te lescope (HST) and IUE of one of these quasars, PG 0844+349, are analyzed. T he ROSAT spectrum can be well fitted by a single power law with Galactic ab sorption. 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 poi nted observations, separated by 6 months. The ASCA satellite found the obje ct in a high state with a photon index of 1.98 and an Fe K alpha line with EW similar to 300 eV. The X-ray flux in the 2-10 keV band is highly variabl e; the fastest variation detected is 60% in less than 2 x 10(4) s. The meas ured excess variance fits well the excess variance versus L2-10kev 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 op tical microvariability of this object can actually be driven by reprocessin g of the variable X-ray flux if one-half of the absorbed X-rays are reradia ted in the optical-to-UV band. A weak broad intrinsic absorption line (FWHM similar or equal to 800 km s(-1)), most likely Ly alpha absorption blueshi fted by a velocity similar or equal to -6000 km s(-1) relative to the quasa r's rest frame, is found in the HST Faint Object Spectrograph spectrum. A s imilar 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 in dicate that the variability amplitude in the UV and optical bands is much s maller 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-te rm behavior of several other X-ray-weak objects shows no indications of a s imilar large X-ray variability.