Magnification Ratio of the Fluctuating Light in Gravitational Lens 0957+561

Radio observations establish the B/A magnification ratio of gravitationallens 0957+561 at about 0.75. Yet, for more than 15 years, the opticalmagnfication ratio has been between 0.9 and 1.12. The accepted explanation ismicrolensing of the optical source. However, this explanation is mildlydiscordant with (i) the relative constancy of the optical ratio, and (ii)recent data indicating possible non-achromaticity in the ratio. To study theseissues, we develop a statistical formalism for separately measuring, in aunified manner, the magnification ratio of the fluctuating and constant partsof the light curve. Applying the formalism to the published data of Kundi\'c etal. (1997), we find that the magnification ratios of fluctuating parts in boththe g and r colors agrees with the magnification ratio of the constant part ing-band, and tends to disagree with the r-band value. One explanation could beabout 0.1 mag of consistently unsubtracted r light from the lensing galaxy G1,which seems unlikely. Another could be that 0957+561 is approaching a causticin the microlensing pattern.Comment: 12 pages including 1 PostScript figur