Time delay between images of the lensed quasar UM673

Abstract

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.

Adaptive PSF fitting – a highly performing photometric method and light curves of the GLS H1413+117: time delays and micro-lensing effects

“…As we can see, there exists a brightness-colour dependence: a decrease in the brightness of the quasar leads to a reddening, and vice versa. This is an interesting observation and might possibly be explained as due to micro-lensing (as for the case of Q2237+0305, Wambsganss & Paczynski 1991) and/or as internal processes in the quasar itself (as in UM673, Koptelova et al 2012;Ricci et al 2013). To assert this statement, we must separately analyse the light curves and the colour curves of the lensed images.…”

“…We see two types of variations: short-term (with amplitude ∼0.1 mag) during separate seasons and long-term (with an amplitude larger than 0.15 mag) over the timescale of several seasons. The colour changes are also characteristic of quasars [as in UM673, Koptelova et al (2012)] -with an apparent flux decrease leading to some reddening, and vice versa -during an apparent flux increase, the system becomes bluer.…”

Flux and color variations of the doubly imaged quasar UM673

“…Nakos et al (2005) found that component "A" displayed possible evidence for microlensing. Koptelova et al (2008Koptelova et al ( , 2010a and Koptelova & Oknyanskij (2010) observed the object in the VRI bands and succeeded for the first time in determining a time delay: 150 +7 +42 −18 −36 days (at 68% and 95% confidence levels). Furthermore, Fadely & Keeton (2011) examined the wavelength dependence of the flux ratios for several gravitationally lensed quasars using K and L -band images obtained with the Gemini North 8m telescope, detecting no difference between the two flux ratios for the specific case of UM673 ("B"/"A" = 0.128 ± 0.002 in the K-band and 0.132 ± 0.006 in the L -band).…”

“…For a more accurate fitting, we used a bicubic interpolation of the images. Koptelova et al (2010bKoptelova et al ( , 2012 observed UM673 with the VRI filters, and derived the photometry without separately taking into account the magnitude of "B" and that of the lens galaxy "G". As the lens galaxy is located very close to the "B" component, and for comparison with other works, we also calculated the magnitudes of the "B"+"G" components as a simple superposition of their fluxes.…”

“…Finally, in a recent paper, Koptelova et al (2012) reestimated the determination of the time delay to a value of 89 ± 11 days using 2001-2011 VRI observations, and suggested the …”