2 The radio observations

Since autumn 1991 we have systematically monitored strong flat spectrum variable radio sources as potential counterparts of extragalactic CGRO sources at several frequencies with the Effelsberg 100-m telescope. The method of observations, the applied data reduction and the calibration have been already described in Paper I. In brief, the observations are made via orthogonal cross-scans and the peak flux densities have been obtained by fitting a Gaussian to the observed data.

The calibration of the data is based on 3C 286 with assumed flux densities of 14.4 Jy, 10.4 Jy, 7.5 Jy, 4.5 Jy and 2.15 Jy at 1.4 GHz, 2.695 GHz, 4.75 GHz, 10.55 GHz and 32 GHz, respectively. 3C 138 served as a secondary calibrator and its flux density was measured every few months in respect to 3C 286 because of its weak variability. Occasionally also NGC 7027, 3C 48 and 3C 295 have been used as secondary calibrators in respect to 3C 286. The data obtained at other frequencies than those listed have been calibrated by interpolation of the flux densities of 3C 286. The Effelsberg 100-m telescope has an angular resolution (HPBW) of 9$.\mkern-4mu^\prime$4 at 1.4 GHz, 4$.\mkern-4mu^\prime$3 at 2.695 GHz, 2$.\mkern-4mu^\prime$4 at 4.75 GHz, 1$.\mkern-4mu^\prime$15 at 10.55 GHz and 0$.\mkern-4mu^\prime$45 at 32 GHz. The 1.4 GHz receiver (tunable up to 1.7 GHz) is installed in the primary focus of the telescope, while the 2.695 GHz, 4.75 GHz, 10.55 GHz and 32 GHz receivers are located in the secondary focus.

The occasionally used receivers at 2.3 GHz and 8.6 GHz are also installed in the secondary focus, while the maser receiver operating in the 22/23 GHz band, the 3.3 GHz and 30 GHz receivers are mounted in the prime focus. More details about the different receiving systems have been compiled by Schmidt & Zinz (1994).

The 4.75 GHz receiver has been replaced by a new 4.85 GHz receiving system in August 1995. A shift of the centre frequency from 10.55 GHz to 10.45 GHz was made in early 1995 due to severe interference caused by the TV-satellite ASTRA  1D.