X-ray observations of gravitationally lensed quasars: Evidence for a hidden quasar population

X-ray observations are presented of gravitationally lensed (GL) quasars with redshifts ranging between 1 and 4. The large magnification factors of GL systems allow us to investigate the properties of quasars with X-ray luminosities that are substantially lower than those of unlensed ones and also provide an independent means of estimating the contribution of faint quasars to the hard X-ray component of the cosmic X-ray background. Spectral indices have been estimated in the rest-frame energy bands 0.5-1 keV (soft), 1-4 keV (mid), and 4-20 keV (hard). Our spectral analysis indicates a flattening of the spectral index in the hard band for two radio-loud quasars in the GL quasar sample for which the data have moderate signal-to-noise ratio. These results are consistent with the reported spectral properties of non-lensed radio-loud quasars; however, there are no indications of spectral hardening toward fainter X-ray fluxes. We have identified a large fraction of broad absorption line (BAL) quasars among the GL quasar population. We find that approximately 35% of radio-quiet GL quasars contain BAL features, which is significantly larger than the 10% fraction of BAL quasars presently found in optically selected flux-limited quasar samples. We present a simple model that estimates the effects of attenuation and lens magnification on the luminosity function of quasars and that explains the observed fraction of GL BAL quasars. These observations suggest that a large fraction of BAL quasars are missed from flux-limited optical surveys. Modeling of several X-ray observations of the GL BAL quasar PG 1115 + 080 suggests that the observed large X-ray variability may be caused in part by a variable intrinsic absorber consistent with previously observed variability of the BAL troughs in the UV band. The observed large X-ray flux variations in PG 1115 + 080 offer the prospect of considerably reducing errors in determining the time delay with future X-ray monitoring of this system and hence constraining the Hubble constant H-0.