3 Results

3.1 Tabulated data

In Table 1 we have listed the sources monitored and the epoch of observations. Table 2 contains the peak flux densities and the calculated errors. Table 2 is available only in electronic form at the CDS, Strasbourg via anonymous ftp 130.79.128.5 or via http://cdsweb.u-strasbg.fr/Abstract.html.

In Table 2 we list the day of the observation, the frequency, peak flux density and error. The peak flux has been calculated from the Gaussian fit of the two orthogonal scans observed and is corrected for residual pointing errors of the telescope. The pointing corrected amplitudes are derived for each scanning direction independently and are averaged. The error includes uncertainties from the Gaussian fit parameters and the pointing results. The maximum error from both results is taken for the averaged amplitude. We convert the data into flux densities by using the mean of scaling factors found from all the observed calibrators during an observing session. We used the maximum and minimum of all these scaling factors and add the resulting flux density changes to the error. By this approach the resulting error includes a number of time dependent effects like changes in opacity, focal changes or gain variations of the antenna during an observing run. To take these effects into account is of particular importance when variability is under investigation. We note, however, that the errors resulting by our approach are systematically higher than those quoted from other flux density monitoring programs.

3.2 Comments on individual sources

Our sample includes two radio sources which have not been taken as identification for the respective $\gamma$-ray sources in the EGRET catalogues, but included in the list of Kanbach (1996). The association of 0738+549 with 2EG J0744+5438 has already been pointed out by Mukherjee et al. (1995). We also regard 0214+108 as a likely identification for 2EG J0216+1107. In Figs. 2 and 3 we show likelihood finemaps of both $\gamma$-ray sources (taken from Thompson et al. 1995) with the position of the radio source indicated. Both radio sources have also been listed as potential EGRET sources by Mattox et al. (1997) who used Bayesian statistics to evaluate the probability for a correct cross-identification of $\gamma$-ray and radio sources.

A few examples of radio and $\gamma$-ray light curves are shown for 0235+164 (Fig. 1), 0738+545 (Fig. 4), 3C 279 (Fig. 5) and 1611+343 (Fig. 6) in order to illustrate the rather different variability behaviour of sources in the sample. In general, the sampling of the $\gamma$-ray light curve is rather coarse if compared to the radio light curves and much short-term variability remains undetected. This needs to be taken into account when comparing the variability behaviour in both wavelength ranges.

  

Figure 1: Radio and $\gamma$-ray lightcurves for 0235+164

  

Figure 2: Proposed identification of 0214+108 (marked by a star) with the $\gamma$-ray source 2EG J0216+1107. The contours represent the statistical probability that a single source lies within the given contour

  

Figure 3: Proposed identification of 0738+545 (marked by a star) with the $\gamma$-ray source 2EG J0744+5438. The contours represent the statistical probability that a single source lies within the given contour

  

Figure 4: Radio and $\gamma$-ray lightcurves for 0738+545. The radio source shows remarkable spectral changes on short time scales

  

Figure 5: Radio and $\gamma$-ray lightcurves for 3C 279

  

Figure 6: Radio and $\gamma$-ray lightcurves for 1611+343

 

Table 3: Single measurements

We have added observations of MKN 501 in Table 1 which has not been identified as  an EGRET source  but has recently been seen at TeV $\gamma$-ray energies with the Whipple Cherenkov telescope (Quinn et al. 1996) or the HEGRA Cherenkov telescope (Bradbury et al. 1997). Also data for 3C 345 are included, since its strength in flux density and its high variability make it a potential $\gamma$-ray source candidate.

3.3 Single radio measurements

In Table 3 we list flux densities for six sources which have been observed once. Most of the data are from a program to observe quasi simultaneously the spectra of radio sources with soft X-rays counterparts as revealed by ROSAT (Reich et al., in preparation). We note that most of the sources listed in Table 1 are visible in the ROSAT all-sky survey.