1998AJ....115..451S

We present Hubble Space Telescope (HST) Faint Object Spectrograph spectra of the blazar 3C 279 obtained during two recent optical outbursts of this source, maximizing the sensitivity to weak foreground absorption systems at far-UV wavelengths for this sight line. Eighteen absorption systems were detected above an observed 3 σ limiting equivalent width of ∼0.3 Å. These systems include an unusual, partially thick Lyman limit + C IV + Mg II system at z_abs = 0.395. In retrospect, this Lyman limit system was detected in the cumulative IUE SWP scan of 3C 279 but was not noted previously. The column density (log N_H = 16.8 cm^–2), redshift (which places Lyα through the Lyman limit within the wavelength range observable with HST), location of the sight line ∼20 h^–1 kpc above the plane of an edge-on spiral galaxy, and apparent lack of velocity structure at 1 Å resolution make this system an ideal candidate for a low-z measurement of the cosmic abundance of deuterium relative to hydrogen. Amazingly, this absorption system in 3C 279 (1253-055) differs by less than 100 km.s^–1 from the velocity of a known metal line/21 cm absorber in another quasar, PKS 1229-021 (z_abs = 0.395) almost 7° (87.5 h^–1 Mpc) away on the sky. The 3C 279 sight line also contains a time-variable absorber intrinsic to 3C 279 (outflow velocity of ∼1200 km.s^–1) and a clump of four absorbers at z = 0.133-0.138. The 1996 epoch spectra, which immediately preceded a large flare at γ- and X-ray energies, showed the line flux of Lyα and N V emission in 3C 279 to be unchanged compared with an earlier epoch, despite a factor of 4 increase in the continuum emission level.