Discovery of associated absorption lines in an X-ray warm absorber: HubbleSpace Telescope faint object spectrograph observations of MR 2251-178

The presence of a "warm absorber" was first suggested to explain spectral v ariability in an X-ray spectrum of the radio-quiet quasi-stellar object (QS O) MR 2251-178. A unified picture, in which X-ray warm absorbers and "intri nsic" UV absorbers are the same, offers the opportunity to probe the nuclea r environment of active galactic nuclei. To test this scenario and understa nd the physical properties of the absorber, we obtained a UV spectrum of MR 2251-178 with the Faint Object Spectrograph on board the Hubble Space Tele scope (HST). The HST spectrum clearly shows absorption due to Ly alpha, N V , and C IV, blueshifted by 300 km s(-1) from the emission redshift of the Q SO. The rarity of both X-ray and UV absorbers in radio-quiet QSOs suggests these absorbers are physically related, if not identical. Assuming the unif ied scenario, we place constraints on the physical parameters of the absorb er and conclude the mass outflow rate is essentially the same as the accret ion rate in MR 2251-178.