New two-colour light curves of Q0957+561: time delays and the origin of intrinsic variations

¹ Institute for Radiophysics and Electronics, National Academy of Sciences of Ukraine, 12 Proskura St., Kharkov 61085, Ukraine e-mail: vshal@ire.kharkov.ua
² Departamento de Física Moderna, Universidad de Cantabria, Avda. de Los Castros s/n, 39005 Santander, Spain e-mail: goicol@unican.es
³ Sternberg Astronomical Institute, Universitetski pr. 13, 119992 Moscow, Russia e-mail: koptelova@xray.sai.msu.ru
⁴ Graduate Institute of Astronomy, Jhongli City, Taoyuan County 320, Taiwan e-mail: koptelova@astro.ncu.edu.tw
⁵ Robotic Telescopes Group, Centro de Astrobiología (CSIC-INTA), associated to the NASA Astrobiology Institute, Ctra de Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain e-mail: ullanna@inta.es
⁶ Instituto de Física de Cantabria (CSIC-UC), Avda. de Los Castros s/n, 39005 Santander, Spain e-mail: gilmerino@ifca.unican.es

Received: 23 June 2008
Accepted: 24 September 2008

Abstract

Aims. We extend the gr-band time coverage of the gravitationally lensed double quasar Q0957+561. New gr light curves permit us to detect significant intrinsic fluctuations, to determine new time delays, and thus to gain perspective on the mechanism of intrinsic variability in Q0957+561.

Methods. We use new optical frames of Q0957+561 in the g and r passbands from January 2005 to July 2007. These frames are part of an ongoing long-term monitoring with the Liverpool robotic telescope. We also introduce two photometric pipelines that are applied to the new gr frames of Q0957+561. The transformation pipeline incorporates zero-point, colour, and inhomogeneity corrections to the instrumental magnitudes, so final photometry to the 1–2% level is achieved for both quasar components. The two-colour final records are then used to measure time delays.

Results. The gr light curves of Q0957+561 show several prominent events and gradients, and some of them (in the g band) lead to a time delay between components = 417 ± 2 d (1σ). We do not find evidence of extrinsic variability in the light curves of Q0957+561. We also explore the possibility of a delay between a large event in the g band and the corresponding event in the r band. The gr cross-correlation reveals a time lag = 4.0 ± 2.0 d (1σ; the g-band event is leading) that confirms a previous claim of the existence of a delay between the g and r band in this lensed quasar.

Conclusions. The time delays (between quasar components and between optical bands) from the new records and previous ones in similar bands indicate that most observed variations in Q0957+561 (amplitudes of ~100 mmag and timescales of ~100 d) are very probably due to reverberation within the gas disc around the supermassive black hole.