THE GRAVITATIONAL LENS SYSTEM Q0957+561 IN THE ULTRAVIOLET

Photometric and polarimetric observations of both images of the gravit ationally lensed quasar Q0957 + 561 (z(em) = 1.41) were obtained in th e UV in 1993 with the High Speed Photometer on board the Hubble Space Telescope. The images exhibited no significant polarization in a bandp ass centered on 2770 Angstrom (observer's frame); p less than or equal to 3.2% (2 sigma upper limit) in each image. The ratio of the flux de nsity in image A to that in image B in late 1993 had a constant value, 1.021 +/- 0.008, in four different UV bandpasses between 1400 Angstro m and 3040 Angstrom (observer's frame). These results are consistent w ith the prediction of the gravitational lens interpretation that the p hotometric ratio of the images measured simultaneously should be indep endent of frequency. Reprocessed archival spectra of the two images ob tained between 1981 and 1983 by the International Ultraviolet Explorer show that the photometric ratio of A to B varies between 0.96 and 2.0 in the Ly alpha emission line, and between 0.77 and 1.8 in the O VI 3 ,1037 emission line (quasar rest frame). The photometric ratio of A to B at any single epoch is often significantly different in the two emi ssion lines. Accepting the system as a gravitational lens implies that in the quasar the flux in the Ly alpha emission line can vary indepen dently of the flux in the O VI emission line.