Far-UV Spectral Variability in the Gravitational Lens Candidate UM 425

Abstract

A comparison of recently acquired far-UV spectra with data obtained nearly 10 months earlier indicates changes have occurred in both absorption and ionization levels associated with Broad Absorption Line (BAL) structure in the QSO UM 425, a candidate gravitationally lensed system. These observations are compared with the initial discovery of O VI and N V BAL structure previously found with the International Ultraviolet Explorer (IUE), that suggests outflow of highly ionized material with speeds up to ∼-12,000 km s^-1 in the QSO (z = 1.471) rest frame. Discrete absorption components in the O VI BAL trough, occurring at velocity intervals of ∼2100 km s^-1 up to the terminal speed, are also evident in the Lyman-α 1216 line profile, which is affected by similar absorption structures within the velocity-broadened N V λ1240 BAL trough. The greatest temporal change in relative optical depth occurs at velocity intervals which correspond to the BAL absorption components. Significantly, a decrease in O VI BAL absorption over the entire BAL velocity range was accompanied by both an increase in ionization level, indicated by enhanced 0 vi emission, and by a strengthening of other moderate-to high-excitation emission lines, that include S VI(1) λλ937,945, C III(1) λ977, N III(1) λ990, and S IV(1) λλ1063,1073. These observations are relevant to time-delay studies of UM 425 because the temporal changes observed in intrinsic QSO ionization are small compared with the estimated delay time between the images in the system.