Intrinsic quasar variability and time delay determination in the lensed quasar UM673

We present the results of time delay measurements between the A and B components of the gravitationally lensed quasar UM673. The measurements are based on the brightness variations of the quasar components observed with the 1.5-m telescope of the Maidanak Observatory in the V, R and I bands during the 2003-2005 observational period. The amplitudes of variations in the R band are 0.046 and 0.122 mag for Component A and B, respectively. Due to the short visibility period of UM673, about 3 months in a year, the light curves of the quasar components have large annual gaps. It complicates the time delay estimation. From the cross-correlation analysis, we find that the brightness variations in component B follow the brightness variations in Component A by about 150 days. We also explore the possibility of measuring the wavelength-dependent time delay between the brightness variations in the V, R and I bands. We find that in agreement with reprocessing model of quasar variability, the brightness variation at longer wavelengths follows the brightness variations at shorter wavelengths. Finally, we show that the combination of the multiband data corrected for the wavelength-dependent time delay can help to improve the cross-correlation analysis of the light curves with large annual gaps.