1995A&A...299..365H

The large-redshift (z=3.8) quasar 1208+1011 has recently been discovered to be a gravitational lens candidate with a separation of 0.47" between the two imaged components (A and B). NOT (Nordic Optical Telescope) and HST (Hubble Space Telescope) studies from 1992 probing primarily the continuum light show that the amplification of A relative to B is almost achromatic with the canonical value A:B≃4±0.1. In this paper we present high-resolution optical images (FWHM ≃0.4-0".5)" from 1993 of the quasar. From our narrow-band CCD frames centred on the redshifted Lyα/Nv line at ∼5900A we find that A:B=3.85±0.2. Our broad-band I images uncontaminated by line-emission are found to be affected by PSF variation over a range of 20". However, by iteratively determining the PSF from the images themselves we show that the data can be accounted for by two point sources with A:B=3.35±0.2. These results imply that the continuum intensity ratio has decreased during 1992-1993 and that the amplification of the emission-line regions relative to the continuum emitting regions is different in the two components (under the gravitational lens hypothesis). The most conservative interpretation of these results is that 1208+1011 is a gravitationally lensed quasar in which component B is being microlensed, but the possibility that 1208+1011 is a binary quasar is not excluded by the present data.