Ultraviolet spectral variability and the Ly alpha forest in the lensed quasar Q0957+561

Far-ultraviolet spectra of the gravitational lens components Q0957+561A, B were obtained with the Hubble Space Telescope Faint Object Spectrograph (FO S) at five equally spaced epochs, one every 2 weeks. We confirm the flux va riability of the quasar's Ly alpha and O VI lambda 1037 emission lines in I UE spectra reported in earlier work of Dolan et al. The fluxes in these lin es vary on a timescale of weeks in the observer's rest frame, independently of each other and of the surrounding continuum. The individual spectra of each image were co-added to investigate the prope rties of the Ly alpha forest along the two lines of sight to the quasar. Ab sorption lines having equivalent width W-lambda greater than or equal to 0. 3 Angstrom in the observer's frame not previously identified by Michalitsia nos et al. as interstellar lines, metal lines, or higher order Lyman lines were taken to be Ly alpha forest lines. The existence of each line in this consistently selected set was then verified by its presence in two archival FOS spectra with similar to 1.5 times higher signal to noise than our co-a dded spectra. Ly alpha forest lines with W-lambda greater than or equal to 0.3 A appear at 41 distinct wavelengths in the spectra of the two images. O ne absorption line in the spectrum of image A has no counterpart in the spe ctrum of image B, and one line in image B has no counterpart in image A. Ba sed on the separation of the lines of sight over the redshift range searche d for Ly alpha forest lines, the density of the absorbing clouds in the dir ection of Q0957 + 561 must change significantly over a distance R = 160(-70 )(+120) h(50)(-1) kpc in the simplified model where the absorbers are treat ed as spherical clouds and the characteristic dimension, R, is the radius. (We adopt H-0 = 50 h(50) km s(-1) Mpc(-1), q(0) = 1/2, and Lambda = 0 throu ghout this paper.) The 95% confidence interval on R extends from 50 to 950 h(50)(-1) kpc. We show in the Appendix that the fraction of Lya forest lines that appear i n only one spectrum can be expressed as a rapidly converging power series i n 1/r, where r the ratio of the radius of the cloud to the separation of th e two lines of sight at the redshift of the cloud. This power series can be rewritten to give r in terms of the fraction of Ly alpha forest wavelength s that appear in the spectrum of only one image. A simple linear approximat ion to the solution that everywhere agrees with the power series solution t o better than 0.8% for r greater than or equal to 2 is derived in the Appen dix.