Optical-to-Radio Time Delay in the Gravitationally Lensed QSO 0957+561

Abstract

Time delay determinations in astrophysics are used most often to find time shifts between variations of different spectral bands and or spectral lines in AGNs as well as time delays between different images of gravitationally lensed QSOs. Most often are used two different methods: CCF (Gaskell &Spark 1986) and DCF (Edelson & Krolik 1988), which are based respectively on line interpolation of data sets or binning of correlation coefficients. We have introduced several simple improvements to the CCF (Oknyanskij, 1994) and this modernized method MCCF combines best properties of these CCF and DCF methods. In addition we calculate in the MCCF line regression coefficients as functions of time shift. Here we use the same method, but generalized for the more complex case when the time delay is a linear function of time and a portion of response flux density is itself a power-low function of the delay. We apply this method to investigate optical-to-radio time delay in the double quasar 0957+561, which is a generally accepted case of gravitational lensing. Possibility for this correlation in Q0957+561 was first reported by Oknyanskij & Beskin (1993, here after 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). Here we use new data sets which were obtained during 1979–1994 to determine the gravitational lensing time delay τ_o (Haarsa et al. 1996). We will base our discussion below on the τ_o = 425 days which is preferred today, since we find that our results are virtually independent of which value in the interval of 410–550 days we take as. τ_o