Variable radio echoes from a jet in QSO 0957+561

Time delay determinations in astrophysics are used most often to find time delays between flux density variations of different spectral bands and/or spectral lines in AGNs and different images of gravitationally lensed QSOs. Here we consider a new algorithm for a complex case, when the time delay is itself a linear function of time and the intensity of echo response is power function of the delay. We apply this method to investigate optical-to-radio delay in the double quasar 0957+561, which is a generally accepted case of gravitational lensing.Radio-optical correlation in QSO 0957+561 was first reported by Oknyanskij and Beskin (1993, hereafter OB) on the basis of radio observations made in the years 1979 to 1990. OB used an idea to take into account the known gravitational lensing time delay to get combined radio and optical light curves and then to use them for determination of the possible radio-from-optical time delay. It was found this way that radio variations (5 MHz) followed optical ones by about 6.4 years with high level of correlation (0.87). Using new radio data (Haarsmaet al., 1996), for the interval 1979–1994 we find nearly the same value for the optical-to-radio delay as has been found before. Additionally we suspect that the time delay value is linearly increasing at about 110 days per year while the portion of reradiated flux in the radioresponse is decreasing.Obtained results indicate that the optical and radio emitting regions are physically related, but have distinct size scales, locations and possibly radiation mechanisms. We conclude that the results can be explained by simple model were the variable radio source is ejected from the central part of the QSO compact component and that the changing time delay between the optical and radio light curves is consequence of light travel effect.