IUE observations of variability and differences in the UV spectra of double quasar 0957+561 a, b.

Abstract

The continuum flux from 2500-2800 A and the emission flux in the Lyman alpha line were observed to change substantially in component A of the double quasar but not in component B. The equivalent width of the line increased by 50 percent in A but not in B, which was probably not due to instrument effects. It was expected that these changes should recur in B after an appropriate time delay. The continuum flux ratio B/A started to increase, reached a maximum, and then recovered its intrinsic value. The observed changes were clearly seen to be confined to component A, probably the result of the lensed quasar being intrinsically variable, the change having been propagated along sight-line A but not along sightline B. This is supported by the fact that the UV radiation is more in step with visible radiation than with radio emission. Current theoretical estimates of the time delay are uncertain because of uncertainties in the mass distribution of the lensing galaxy.