Multiple delays in QSO 0957+561: observational evidence and interpretation

Abstract

Q0957+561A,B is a double-imaged quasar that has been intensively observed during the last 10 years in different optical bands and with several telescopes, and we concentrated on recent public data obtained at the Apache Point Observatory (APO) and the Teide Observatory (TO). When an intrinsic event appears in the light curve of Q0957+561A, its twin event (a similar feature) is seen in the brightness record of Q0957+561B, and thus, one can measure the corresponding time delay. The TO dataset includes two prominent twin events, which were detected with a time separation of 425 +/- 4 days. On the other hand, from the APO dataset, we found a clear evidence for two different time delays associated with two pairs of twin events: 417.0 +/- 0.6 (APO main twin events) and 432.0 +/- 1.9 days (APO secondary twin events), where the APO(main)-APO(secondary) difference delay is of - 15 +/- 2 days. In agreement with the Yonehara's idea, if the three pairs of twin events are originated inside a standard hybrid source (accretion disk and circumnuclear stellar region), the three measured time delays indicate that they do not come from a common zone in the source. Therefore, we can consider that the prominent features are caused by flares in a standard hybrid source and discuss on the size and nature of the region of flares. In this paper it is showed that the more plausible interpretation is that two of the three flares are generated at distances (from the central black hole) larger than 90 pc. Some stellar scenarios can explain the two flares far away from the black hole, while phenomena in the accretion disk cannot cause them.