Survey of Data for Determining Scales of the Absolute Flux Densities in 10-180 MHz Range and Source Spectra in the Declination Strip 10° − 20°

Abstract

To determine the frequency spectra of radio sources the flux densities of these sources should be measured at the widest possible frequency range. The use of different telescope types, methods of measurements and calibrations led in a number of cases to the considerable difference in the data of various observatories. At the present time for frequencies above 400 MHz the scales of flux densities - S for - S ≥ 1 Jy = 10−26w m−2Hz−1 presented by different authors coincide accurate to 5%. Up to now for frequencies below 200 MHz a single scale of fluxes recognized by all radioastronomers is absent. We attempted to determine such a scale for frequencies 180-10 MHz. We used both analysis of the published data and the results of the new measurements obtained with the UTR-2 Radiotelescope in the declination strip 10° – 20° at frequencies 10.0; 12.6; 14.7; 16.7; 20, and 25 MHz. The method of these measurements and the obtained results are described in detail in [1]. The UTR-2 Radiotelescope has 5 beams in declination with half power beam width in a zenith direction 20′ × 20′ at 25 MHz [2]. The absolute values of the flux densities of all observed discrete radio sources are defined during the experiments. Minimum flux density measured at frequency 25 MHz – 15-20 Jy, with a signal-to-noise ratio equal to 3-4. During measurements about 300 radio sources were found. Comparing these data with our earlier measurements obtained with the UTR-1 Telescope [3] as well as with corrected data of the both Pentincton [4] at 10.02 and 22.25 MHz and the Clark-Lake Observatories at 26.3 MHz [5] and the results obtained in Cambridge at 38 MHz [6] and 178 MHz [7, 8] we have derived some correcting factors allowing to bring the data of different catalogues to a single scale. The method for determining such factors is described in [9]. It was shown that below 200 MHz where the influence of ionosphere is great, it is necessary to take into account two corrections : regular scale shift of the one catalogue to another z12 = 1/z21 (1/z21 is the average value of the relation between flux densities of the first catalogue and the second one) and a scale shift caused by the multiplicative scatter. A value is greater than unity and it can be determined comparing with each other three different catalogues. The comparison results of a number of catalogues with data obtained with UTR-2 are shown in table I.