Time delay between images of the lensed quasar UM673^⋆,⋆⋆

¹ Department of Physics, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., 106 Taipei, Taiwan
e-mail: ekaterina@phys.ntu.edu.tw; chiuehth@phys.ntu.edu.tw
² Graduate Institute of Astronomy, National Central University, Jhongli City, Taoyuan County 320, Taiwan
e-mail: wchen@astro.ncu.edu.tw
³ Sternberg Astronomical Institute (SAI), Moscow M.V. Lomonosov State University, Universitetskii pr. 13, 119992 Moscow, Russia
e-mail: artamon@sai.msu.ru; oknyan@sai.msu.ru
⁴ Ulugh Beg Astronomical Institute of the Uzbek Academy of Sciences, Astronomicheskaya 33, 100052 Tashkent, Uzbekistan
e-mail: nurit2006@yahoo.com
⁵ Institute of Radio Astronomy, Krasnoznamennaya 4, 61002 Kharkov, Ukraine
⁶ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Sart Tilman, B5c, 4000 Liège, Belgium

Received: 4 February 2011
Accepted: 11 June 2012

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_-16⁺⁵_-29⁺¹⁴ 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.