1. The first gravitationally lensed (double) quasar Q0957+561A,B was discovered in 1979 by Walsh, Carswell and Weymann. However, the origin of the variability observed at optical wavelengths remained open to question over three decades. While some studies indicated that the variations may be due to intrinsic phenomena (e.g., accretion disc instabilities), other analyses suggested a relationship between optical variability and microlensing in the lensing galaxy (extrinsic scenario). Several exotic ingredients were also invoqued to explain the observations (e.g., a magnetic eternally collapsing object in the centre of the source quasar, a third component that is blended with the B component, and a cosmic string). During the third decade (1999-2009), we are conducting a multiband optical monitoring of Q0957+561A,B. The use of modern instrumentation and sophisticated photometric techniques allow us to obtain accurate and reliable brightness records of the quasar components, and thus, to significantly improve our knowledge of the nature of the observed optical fluctuations (see http://grupos.unican.es/glendama/index.htm). 1. INTRODUCTION “The First in the Third Decade: Jet-Accretion Disc Connection” A. Ullán (1), L.J. Goicoechea (2), V.N. Shalyapin (3), R. Gil-Merino (4), E. Koptelova (5), A. Oscoz (6), E. Mediavilla (6), M. Serra-Ricart (6) and D. Alcalde (6) (1) 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 (2) Departamento de Física Moderna, Universidad de Cantabria, Avda. de Los Castros s/n, 39005, Santander, Spain; e-mail: goicol@unican.es (3) Institute for Radiophysics and Electronics, National Academy of Sciences of Ukraine, 12 Proskura St., Kharkov, 61085, Ukraine; e-mail: vshal@ire.kharkov.ua (4) Instituto de Física de Cantabria (CSIC-UC), Avda. de Los Castros s/n, 39005, Santander, Spain; e-mail: gilmerino@ifca.unican.es (5) Sternberg Astronomical Institute, Universitetski pr. 13, 119992, Moscow, Russia: koptelova@xray.sai.msu.rukoptelova@xray.sai.msu.ru (6) Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38205, La Laguna, Tenerife, Spain: aoscoz@iac.es ; emg@iac.es ; mserra@ot.iac.es ; dalcalde@iac.esaoscoz@iac.esemg@iac.esmserra@ot.iac.es 2. ONGOING MONITORING WITH THE 2.0 m LIVERPOOL ROBOTIC TELESCOPE (LRT) 3. FIRST RESULTS In January 2005 we started a long-term programme with the LRT. The first monitoring period from January 2005 to July 2007 (that is called LQLM I) led to an important database in the g and r passbands. These LQLM I frames were analysed with two new photometric pipelines. The instrumental photometry pipeline is based on software to accurately model the lens system (quasar components and lensing galaxy). 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. Fig. 2. Normalized structure functions of the Q0957+561 luminosity in two different observer-frame periods: 1994-1996 (g band, APO) and 2005-2007 (g and r bands, LQLM I). The LQLM I observed brightness variations of Q0957+561A,B have an intrinsic origin (see Sect. 3), so they are due to the rest-frame UV variability of the distant quasar. We constructed normalized structure functions of the quasar luminosity at rest-frame wavelengths ~ 2100 Å and ~ 2600 Å. APO records also allow the structure function to be obtained at ~ 2100 Å, but 10 years ago in the observer's frame. From the LQLM I data, ~100-d time-symmetric and ~170-d time-asymmetric flares are produced at both rest-frame wavelengths. Taking into account measurements of time delays (see Sect. 3) and the existence of an EUV/radio jet, reverberation is probably the main mechanism of variability. Thus, two types of EUV/X-ray fluctuations would be generated within or close to the jet and later reprocessed by the disc gas in the two emission rings, at ~ 2100 Å and ~ 2600 Å. The ~100-d time-symmetric shots are also responsible for most of the ~ 2100 Å variability detected in the APO experiment (10 years ago). However, there is no evidence of asymmetric shots in the old UV variability. If reverberation is the involved mechanism of variability, this could mean an intermittent production of high-energy asymmetric fluctuations. The old records are also consistent with the presence of very short-lifetime (~ 10 d) symmetric flares, which may represent additional evidence of time evolution. In Fig. 2 we show the three normalized structure functions, as well as fits to Poissonian models for flare statistics. Fig. 1. Delay peaks of Q0957+561A,B obtained with the first LRT light curves of this double quasar. Left panel shows the delay between the A and B components. Right panel displays the delay between the g and r filters. The first LRT light curves of Q0957+561A,B show several prominent events and gradients, and some of them (in the g band) lead to a time delay between components  t BA = 417  2 d (1  ). We do not find evidence of extrinsic variability caused by microlensing or another physical mechanism. We also measure a time lag  t rg = 4.0  2.0 d (1  ) between a large event in the g band and the corresponding event in the r band (the g-band event is leading). Delay peaks (best delay values correspond to the minima) are shown in Fig. 1: delay between components (left panel) and delay between bands (right panel). Time delays from the new LRT records (LQLM I) and the Apache Point Observatory (APO) light curves in similar bands (1994-1996 period) indicate that most observed variations in Q0957+561A,B (amplitudes of ~ 100 mmag and timescales of ~ 100 d) are very probably due to reverberation within a gas disc around a supermassive black hole. 4. STRUCTURE FUNCTION ANALYSIS TO KNOW MORE ABOUT THIS PROJECT: ►Shalyapin, V.N., Goicoechea, L.J., Koptelova, E., Ullán, A. & Gil-Merino, R. 2008, A&A, 492, 401 ►Goicoechea, L.J., Shalyapin, V.N., Gil- Merino, R. & Ullán, A. 2008, A&A, 492, 411 5. NEW PRELIMINARY RESULTS We have used 515 R-band frames of Q0957+561A,B taken at IAC80 telescope between 1999 and 2005 and all available LRT images in the r-band (310 frames between 2005 and March 2009) in order to obtain a global perspective of the quasar variability at red wavelengths for the last 10 years. The R-band fluxes are transformed to the r-SDSS photometric system, and we then compare the time delay- and magnitude-shifted record of B (green circles in Fig. 3) and the fluxes of A (red circles in Fig. 3). The A variations are very similar to those of the B component 14 months later. This impressive (but preliminary!) agreement over the third decade confirms the intrinsic variability scenario, and rules out the presence of significant extrinsic fluctuations and the need for exotic ingredients. Fig. 3. Light curves of Q0957+561A,B from IAC80 and Liverpool telescopes in the 1999- 2009 period. The A component is unshifted (red circles), whereas the B component is shifted by the time delay and a magnitude offset.