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.

arXiv comment13 pages, 9 figures. Accepted for publication in A&A

References in corpus (28)

• The u'g'r'i'z' Standard Star Network
• The Fifth Data Release of the Sloan Digital Sky Survey
• The Ensemble Photometric Variability of ~25000 Quasars in the Sloan Digital Sky Survey
• The Sloan Digital Sky Survey Quasar Catalog IV. Fifth Data Release
• Testing thermal reprocessing in AGN accretion discs
• A Robust Determination of the Time Delay in 0957+561A,B and a Measurement of the Global Value of Hubble's Constant
• Quantitative Interpretation of Quasar Microlensing Light Curves
• The Optical Properties of Gravitational Lens Galaxies as a Probe of Galaxy Structure and Evolution
• A new direct method for measuring the Hubble constant from reverberating accretion discs in active galaxies
• The gravitational lens MG0414+0534: a link between red galaxies and dust
• Improved Parameters and New Lensed Features for Q0957+561 from WFPC2 Imaging
• Microlensing variability in time-delay quasars
• BVRI photometry of QSO 0957+561A, B: Observations, new reduction method and time delay
• The Angstrom Project: a microlensing survey of the structure and composition of the bulge of the Andromeda galaxy
• Time delay in QSO 0957+561 from 1984-99 optical data
• Time delay of SBS 0909+532
• New aperture photometry of QSO 0957+561; application to time delay and microlensing
• First robotic monitoring of a lensed quasar: intrinsic variability of SBS 0909+532
• Image reconstruction technique and optical monitoring of the QSO2237+0305 from Maidanak Observatory in 2002 -- 2003
• Multiple delays in QSO 0957+561: observational evidence and interpretation
• Source Size Limitation from Variabilities of a Lensed Quasar
• Short-timescale Fluctuations in the Difference Light Curves of QSO 0957+561A,B: Microlensing or Noise?
• Accretion Disc Structure and Orientation in the Lensed and Microlensed Q0957+561 Quasar
• Ultraviolet structure in the lensed QSOs 0957+561
• Five nights of intensive R- and V-band photometry of QSO 0957+561A,B
• GLITP Optical Monitoring of QSO 0957+561: VR Light Curves and Variability
• A New Method to Map Flares in Quasars
• Estimation of time delays from unresolved photometry

Cited by in corpus (21)

• A New Microlensing Event in the Doubly-Imaged Quasar Q0957+561
• Microlensing of the broad emission lines in 27 gravitationally lensed quasars. Broad line region structure and kinematics
• New database for a sample of optically bright lensed quasars in the northern hemisphere
• Improved Constraints on the Gravitational Lens Q0957+561. I. Weak Lensing
• Accretion Onto the Supermassive Black Hole in the High-redshift Radio-loud AGN 0957+561
• Time delay between images of the lensed quasar UM673
• 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
• Microlensing variability in FBQ 0951+2635: short-timescale events or a long-timescale fluctuation?
• Discovery of the optically bright, wide separation double quasar SDSS J1442+4055
• A 5.5-year robotic optical monitoring of Q0957+561: substructure in a non-local cD galaxy
• 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
• Ultraviolet Fe II Emission in Fainter Quasars: Luminosity Dependences, and the Influence of Environments
• Six Maxims of Statistical Acumen for Astronomical Data Analysis
• Spectra of faint sources in crowded fields with FRODOSpec on the Liverpool Robotic Telescope
• Gravitationally lensed QSOs in the ISSIS/WSO-UV era