Discovery of Associated Absorption Lines in an X-Ray Warm Absorber: HST-FOS Observations of MR2251-178

The presence of a ``warm absorber'' was first suggested to explain spectral variability in an X-ray spectrum of the radio-quiet QSO MR2251-178. A unified picture, in which X-ray warm absorbers and ``intrinsic'' UV absorbers are the same, offers the opportunity to probe the nuclear environment of active galactic nuclei. To test this scenario and understand the physical properties of the absorber, we obtained a UV spectrum of MR2251-178 with the Faint Object Spectrograph (FOS) onboard the Hubble Space Telescope (HST). The HST spectrum clearly shows absorption due to Ly-alpha, NV and CIV, blueshifted by 300 km/sec from the emission redshift of the QSO. The rarity of both X-ray and UV absorbers in radio-quiet QSOs suggests these absorbers are physically related, if not identical. Assuming the unified scenario, we place constraints on the physical parameters of the absorber and conclude the mass outflow rate is essentially the same as the accretion rate in MR2251-178.