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

Abstract

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. 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. 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 of 417 +/- 2 d (1 sigma). 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 of 4.0 +/- 2.0 d (1 sigma; 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. 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 about 100 mmag and timescales of about 100 d) are very probably due to reverberation within the gas disc around the supermassive black hole.

13 pages, 9 figures. Accepted for publication in A&A

References in corpus (5)

• The Sloan Digital Sky Survey Quasar Catalog IV. Fifth Data Release
• Testing thermal reprocessing in AGN accretion discs
• Microlensing variability in time-delay quasars
• Time delay of SBS 0909+532
• First robotic monitoring of a lensed quasar: intrinsic variability of SBS 0909+532

Cited by in corpus (12)

• Microlensing of the broad emission lines in 27 gravitationally lensed quasars. Broad line region structure and kinematics
• Doubly imaged quasar SDSS J1515+1511: time delay and lensing galaxies
• Revealing the structure of the lensed quasar Q 0957+561: I. Accretion disk size
• Near infrared and optical continuum emission region size measurements in the gravitationally lensed quasars Q0957+561 and SBS0909+532
• Revealing the structure of the lensed quasar Q 0957+561 III. Constraints on the size of the broad-line region
• Revealing the nonlinear behaviour of the lensed quasar Q0957+561
• Spectroscopy and polarimetry of the gravitationally lensed quasar Q0957+561
• Revealing the structure of the lensed quasar Q 0957+561: III. SMBH mass via gravitational redshift
• Structure function of the UV variability of Q0957+561
• Six Maxims of Statistical Acumen for Astronomical Data Analysis
• Gravitationally lensed QSOs in the ISSIS/WSO-UV era
• Spectra of faint sources in crowded fields with FRODOSpec on the Liverpool Robotic Telescope