Aims. We study brightness variations in the double lensed quasar UM673 (Q0142-100) with the aim of measuring the time delay between its two images. Methods. We combined our previously published observational data of UM673 obtained during the 2003-2005 seasons at the Maidanak Observatory with archival and recently observed Maidanak and CTIO UM673 data. We analyzed the V, R and I-band light curves of the A and B images of UM673, which cover ten observational seasons from August 2001 to November 2010. We also analyzed the time evolution of the difference in magnitudes (flux ratio) between images A and B of UM673 over more than ten years. Results. We find that the quasar exhibits both short-term (with an amplitude of ∼0.1 mag in the R band) and long-term (with an amplitude of ∼0.3 mag) variability on timescales of about several months and several years, respectively. These brightness variations are used to constrain the time delay between the images of UM673. From a cross-correlation analysis of the A and B quasar light curves and an error analysis we measure a mean time delay of 89 days with an rms error of 11 days. Given the input time delay of 88 days, the most probable value of the delay that can be recovered from light curves with the same statistical properties as the observed R-band light curves of UM673, is 95 +5 −16 +14−29 days (68% and 95% confidence intervals). Analysis of the V − I color variations and the V, R and I-band magnitude differences of the quasar images does not show clear evidence for microlensing variations between 1998 and 2010.

We present the results of an intensive ultraviolet monitoring campaign on the Seyfert 1 galaxy NGC 4151, as part of an effort to study its short time-scale variability over a broad range in wavelength. The nucleus of NGC 4151 was observed continuously with the International Ultraviolet Explorer (IUE) for 9.3 days, yielding a pair of LWP and SWP spectra every ∼70 minutes, and during four-hour periods for 4 days prior to and 5 days after the continuous monitoring period.The sampling frequency of the observations is an order of magnitude higher than that of any previous UV monitoring campaign on a Seyfert galaxy.The continuum fluxes in bands from 1275 Å to 2688 Å went through four significant and welldefined "events" of duration 2 -3 days during the continuous monitoring period. We find that the amplitudes of the continuum variations decrease with increasing wavelength, which extends a general trend for this and other Seyfert galaxies to smaller time scales (i.e., a few days). The continuum variations in all of the UV bands are simultaneous to within an accuracy of about 0.15 days, providing a strict constraint on continuum models. The emission-line light curves show only one major event during the continuous monitoring (a slow rise followed by a shallow dip), and do not correlate well with continuum light curves over the (short) duration of the campaign, because the time scale for continuum variations is apparently smaller than the response times of the emission lines.

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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.

Aims. We considered the possibility of measuring time delays between components of the multiplied quasar QSO2237+0305 and between V and R band variations. Methods. The analysis is based on the flux variations of four components observed by the OGLE collaboration and Maidanak group during the last quarter of 2003. The observed gradients of the brightness variations in this period for OGLE data in V band are 3.4 mmag/day, 2.2 mmag/day, 2.4 mmag/day, and 1.1 mmag/day for the components A, B, C, and D, respectively. The variations are probably intrinsic source variations. The basic method used for time-delay measurements is the cross-correlation technique. To estimate the uncertainty of the time-delay measurements, Monte Carlo simulations were carried out. Results. The calculations showed the impossibility of unambiguously measuring the differential time delays between the components. The observations at shorter wavelengths seem to be the only way to achieve robust time delay measurements in the system. The wavelength-dependent time delays can be used to provide the evidence of an accretion-disk structure of the central optical variable source. The observed V and R band variations of component C show good correlation with the correlation coefficient of 0.83. However, the obtained time delay, about 16.2 days, and its accuracy are far from reliable.