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

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 Δt BA = 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 Δt rg = 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.

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses

“…Lovell et al 1998), SDSS J1004+4112 (Fohlmeister et al 2007(Fohlmeister et al , 2008, QSO 0957+561 (e.g. Oscoz et al 2001;Shalyapin et al 2008;Fadely et al 2010). Two other systems are not presented here because we already published improved astrometry and time delays: WFI J2033-4723 (Vuissoz et al 2008) and HE 0435-1223 (Courbin et al 2011).…”

Spectra of faint sources in crowded fields with FRODOSpec on the Liverpool Robotic Telescope

“…12 with red (A image) and blue (B image) lines. This figure also includes independent energy fluxes from our photometric monitoring with RATCam in the Sloan g ′ and r ′ bands (Shalyapin et al 2008). The red (A image) and blue (B image) circles represent the g ′ r ′ -Sloan fluxes, which should be compared with spectral fluxes averaged over the g ′ and r ′ broad passbands around 4800 and 6225Å (see filter responses in Fig.…”

Multi-wave Monitoring of the Gravitational Lensed Quasar Q0957+561

“…The variability of the 'A' image can also be used to check our estimates from the 'B' image data. We use both the D 2 and χ 2 methods (Shalyapin et al 2008 and references therein) to estimate time delays with respect to the g-band record (1944Å in the source rest-frame). The inter-band delays from 'A' agree with the delays from 'B'.…”