2005ApJ...619...19G

We present VR magnification ratios of QSO 0957+561 that are inferred from the GLITP light curves of Q0957+561A and new frames taken with the 2.56 m Nordic Optical Telescope about 14 months after the GLITP monitoring. To extract the fluxes of the two close components, two different photometric techniques are used, pho2comC and psfphot. From the two photometric approaches and a reasonable range for the time delay in the system (415-430 days), we do not obtain achromatic optical continuum ratios, but ratios depending on the wavelength. Our final global measurements Δm_AB(λ_V)=0.077±0.023 mag and Δm_AB(λ_R)=0.022±0.013 mag (1 σ intervals) are in agreement with the Oslo group results (using the same telescope in the same seasons but another photometric task and only one time delay of about 416 days). These new measurements are consistent with differential extinction in the lens galaxy, the Lyman limit system, the damped Lyα system, or the host galaxy of the QSO. The possible values for the differential extinction and the ratio of total to selective extinction in the V band are reasonable. Moreover, crude probability arguments suggest that the ray paths of the two components cross a similar dusty environment, including a network of compact dust clouds and compact dust voids. As an alternative (in fact, the usual interpretation of the old ratios), we also try to explain the new ratios as being caused by gravitational microlensing in the deflector. From magnification maps for each of the gravitationally lensed images, using different fractions of the surface mass density represented by the microlenses as well as different sizes and profiles of the V-band and R-band sources, several synthetic distributions of [Δm_AB(λ_V), Δm_AB(λ_R)] pairs are derived. In some gravitational scenarios, there is an apparent disagreement between the observed pair of ratios and the simulated distributions. However, several microlensing pictures work well. To decide between either extinction, microlensing, or a mixed scenario (extinction + microlensing), new observational and interpretation efforts are required.