1979A&AS...36..331Veron M.P. & Veron P.: Study of the 4C Catalogue of Radio Sources for +20{deg} 20{deg}, S>=2.5 Jy), are possible confusing sources, or are suspected of having a true flux density greater than 2.5 Jy at 178 MHz and to be confused or resolved. We have observed most of these sources at several frequencies, 318, 2700 and 5000 MHz (or at least those sources for which no flux densities at these frequencies were available in the literature), in order to get better radio position and to build accurate radio spectra to correct the original 4C flux densities for resolution or confusion effects. We present here 318 MHz flux densities for 415 of these sources measured in April 1978 with the 305 m Arecibo telescope of the National Astronomy and Ionospheric Center (operated by Cornell University under contract with the National Science Foundation). The observation and the reduction procedures were similar to those described by Condon and Jauncey (1974a). A six-minute-long sideral rate drift-scan was made at the declination of each source. A calibration source was observed about every three hours. The calibrators and their assumed flux densities are listed in table 1. A small bias čorrection of - 0.09 Jy was made to compensate for baseline underestimate (Condon and Jauncey 1974a). This correction may not be justified (see below). The flux density errors are dS_318 = [(0.15)^2 + (0.04 x S)^2]^{1/2}; for a discussion of these errors, see Condon and Jauncey (1974a). Table 1 Calibration sources Name S318 ---------------------------- 3C 28.0 9.0 3C 133.0 15.1 3C 166.0 8.7 3C 172.0 9.3 3C 234.0 19.4 3C 286.0 21.6 3C 287.0 12.9 3C 399.1 7.2 3C 400.1 7.3 3C 433.0 37.0 ---------------------------- The half-power beam width of the telescope is, at 318 MHz, 16 arcmin, allowing a total flux density to be measured since very few sources should be resolved with such a beam. Each time another Bologna source was within the beam, we have corrected for confusion by this source, extrapolating its 318 MHz flux density from the Bologna 408 MHz flux density, assuming a spectral index of {alpha}=0.75 ({alpha} being defined by S~{nu}^{-alpha}). However when the confusing source was less than 12 arcmin from the main source, we have rejected the measurement, the correction being too uncertain. To check if our flux measurements are on the Kellermann et al. (1969) scale, we have compared the 318 MHz flux densities with the Bologna 408 MHz flux densities (Colla et al. 1970, 1972, 1973; Fanti et al. 1974) and with the 4C 178 MHz flux densities (Pilkington and Scott 1965; Gower et al. 1967). Three hundred and sixty-eight sources appear in the 408 MHz Bologna survey (Colla et al. 1970, 1972 and 1973; Fanti et al. 1974) (excluding sources with a corrected 178 MHz flux density smaller than 2.5 Jy in order to avoid having in this comparison faint sources with large errors, but including 4C sources measured by Condon and Jauncey 1974b). The mean ratio of the flux densities is = 1.11+-0.01, which corresponds to a mean spectral index <{alpha}> = 0.42. Veron et al. (1974) have found =0.85 for the sources from the revised 3C catalogue (Bennett 1962). Assuming that this value is also correct for the 4C sources, we should have = 1.24. Four hundred and fourteen sources measured at 318 MHz have a 4C 178 MHz flux density not affected by confusion or resolution. For them we find =0.64+-0.01 ; however, according to Veron et al. (1974), the 4C flux densities should be multiplied by 1.100 +- 0.006, which changes this ratio to = 0.58+-0.01 instead of 0.61 if <{alpha}> =0.85. We conclude that to have consistent scales, the observed flux densities should be multiplied by the following factors: at 178 MHz 1.100+-0.006; at 318 MHz 1.05+-0.01 compared to 1.00+-0.10 found by Veron et al. (1974); at 408 MHz 0.94+-0.01 compared to 0.975+-0.030 found by Veron et al. (1974). This is in reasonable agreement with Condon and Jauncey (1974b) who have found that the 408 MHz Bologna flux densities have to be decreased by 13% (we find 11%) to put them on the same scale as the 318 MHz flux densities. However, the flux densities of the calibration sources used are on the Kellermann et al. (1969) scale (Condon and Jauncey, 1974a), which at this frequency is close to the scale used by Veron et al. (1974); if we do not apply the correction of -0.09 Jy for baseline underestimate, the ratios and are increased by 4.0% ; in that case, assuming that the calibrators used are really on the scale of Kellermann et al. (1969), we still get a good agreement for the faint sources. So we suspect that the correction for baseline underestimate is not necessary, in which case the correction factor to bring the measured 318 MHz flux densities into the scale of Veron et al. (1974) is only 1.01 +- 0.01. However to make our measurements directly comparable with those measured by Condon and Jauncey (1974a and b), we have applied this -0.09 Jy correction to the flux densities in table 2. This has to be kept in mind when using this table. The ratio of the flux densities S_318/S)408 has a dispersion {sigma}=14%. A dispersion of the spectral index {sigma}_{alpha}=0.10 corresponds to a dispersion of 2.5% of the flux density ratio. For a flux density of 2 Jy the uncertainty of the flux densities is 8.5% at both 318 and 408 MHz, which correspond to a dispersion of 12% for the ratio; this is in agreement with the observed value {sigma}= 14%. Fourteen sources already measured by Condon and Jauncey (1974b) have been remeasured. Our values are in general 4.6% higher, which is not statistically different. The dispersion is 12%. For one source (4C22.21 = PKS 0820+22), the difference is rather large (we found S_318 = 3.34 instead of 3.98); but this source has a flat spectrum (Veron et al. 1974) and could be variable, although at 18 cm Webber et al. (1978) did not find it to be so. The results are listed in table 2. Columns 1 and 2 give the 4C and another name for each source; columns 3 and 4, the assumed 1950.0 right ascension and declination; the positions given here are not 4C positions ; some of them have been remeasured by us (unpublished) at 2.7 or 5.0 GHz with the Bonn 100 meter telescope, and are accurate to about 5 arcsec; the others are the best available in the literature. In no case the error in declination is larger than 35 arcsec, which produce a negligible error on the flux density measurement. Columns 5 and 6 give the 318 MHz flux densities and the errors computed with the formula given above. Column 7 refers to notes given at the end of the table. A few sources observed near the sun show interplanetary scintillation ; this is indicated in the notes in a very qualitative way, as it gives some information on the compactness of the sources. Two sources (B2 0157+21 and 4C 32.13 = B2 0239+32) were scintillating so strongly (~100%) that no useful flux densities could be measured for them; they do not appear in the table. The authors thank Dr. H. Craft, director. of the Arecibo Observatory, for his hospitality, and Drs. M.M. Davis and R.D. Sramek for their help at the telescope. REFERENCES Bennett, A.S.: 1962, MmRAS 68, 163. Colla, G., Fanti, C., Fanti, R., Ficarra, A., Formiggini, L., Gandolfi, E., Grueff, G., Lan, C., Padrielli, L., Roffi, G., Tomasi, P. and Vigotti, M. : 1970, A&AS 1, 281. Colla, G., Fanti, C., Fanti, R., Ficarra, A., Formiggini, L., Gandolfi, E., Lan, C., Marano, B., Padrielli, L. and Tomasi, P. : 1972, A&AS 7, 1. Colla, G., Fanti, C., Fanti, R., Ficarra, A., Formiggini, L., Gandoffi, E., Gioia, I., Lan, C., Marano, B., Padrielli, L. and Tomasi, P.: 1973, A&AS 11, 291. Condon, J.J. and Jauncey, D.L. : 1974a, AJ 79, 437. Condon, J.J. and Jauncey, D.L. : 1974b, AJ 79, 1220. Fanti, C., Fanti, R., Ficarra, A. and Padrielli, L. : 1974, A&AS 18, 147. Gower, J.F.R., Scott, P.F. and Wills, D. : 1967, MmRAS 71, 49. Kellermann, K.I., Pauliny-Toth, I.I.K. and Williams, P.J.S. : 1969, ApJ 157, 1. Pilkington, J.D.H. and Scott, P.F. : 1965, MmRAS 69, 183. Veron, M.P., Veron, P. and Witzel, A. : 1974, A&AS 13, 1. Webber, J.C., DeNoyer, L.K., Yang, KS. and Swenson, Jr, G.W. : 1978, AJ 83, 900. M.P. Veron & P. Veron Observatoire de Paris, F-92190 Meudon (France) European Southern Observatory, c/o CERN CH-1211 Geneva 23 (Switzerland)