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1986A&AS...65..485Roger+

Spectral Flux Densities of Radio Sources at 22 MHz

OCR+proof by H.Andernach 9/98

Astron. Astrophys. Suppl. Ser. 65, 485-496 (1986)

R. S. Roger, C. H. Costain and D. I. Stewart

Dominion Radio Astrophysical Observatory, Herzberg Institute of Astrophysics, Box 248, Penticton, B.C. Canada V2A

Received November 29, accepted December 24, 1985

Summary.

We present a compilation of the flux densities at 22.25 MHz for 395 radio sources. The flux densities are derived from three or more observations of each source. The sources were measured over a period of several years with the T-shaped radiotelescope at the Dominion Radio Astrophysical Observatory.

Key words: radio sources: general-radio continuum-galaxies: radio.

1. Introduction. The 22 MHz T-shaped radiotelescope at the Dominion Radio Astrophysical Observatory (Costain et al., 1969) was used for several years both to survey galactic background emission and for observations of discrete (mostly unresolved) radio sources stronger than the confusion limit of ~ 30 Jy. The flux densities for 220 sources measured with the telescope have been reported (Roger et al., 1969 (Paper I; Roger et al., 1973). In this paper we present a final list of the flux densities scaled both from the specific observations of individual sources and from scans of the survey of galactic emission. The list comprises revised flux densities of sources listed in paper I and new flux densities for sources measured subsequently.

2. The 22 MHz telescope.

The telescope consists of an array of 624 dipoles arranged in the form of a "T" above a reflecting screen of area 65000 m^2. An instantaneous beam of size 66' (EW) by 102' (NS) at the zenith was produced by phase-switching the east-west arm with the north arm and detecting the correlated component. The signal from the dipoles in the overlap area common to both arms was split and fed separately to both parts, thus ensuring that the correlated output contained all spatial components down to the resolution of the telescope. The array could be phased to move the beam off the zenith in the meridian plane with consequent north-south broadening of the beam proportional to the secant of the zenith angle. In the "survey" mode-of-operation the telescope beam was cycled through phasings for five adjacent declinations in a time short compared to the sample time in the hour-angle dimension. Since there was no provision for phasing the array in the east-west direction, all observations were made at meridian transit. Further details of the telescope are described by Costain et al. (1969).

3. The observing conditions.

Observations of point sources at 22 MHz can be severely affected by conditions in the Earth's ionosphere. Small-scale irregularities in electron density cause the signal to scintillate on a time scale of 10 to 100 s. Large-scale gradients in density, with both regular and irregular components, produce refraction which at times can amount to 20'. Since ionospheric conditions are not readily predictable, it is often necessary to observe each source on ten or more occasions in order to obtain sufficient measurements free of propagation defects to provide a reliable estimate of flux density.

Owing to ionospherically propagated man-made interference, regular observations during daytime hours were possible only during the years near sunspot minimum, when critical frequencies in the F-region are too low to sustain such propagation. These daytime observations required a correction for D-region absorption, typically amounting to 15 % near midday. This correction was derived from 22 MHz riometer measurements made on site (see Paper I). Nighttime absorption was generally less than 4 % and was not corrected for.

4. The flux densities.

Because of the "total-power" nature of the detected output, most radio sources (S <=200 Jy) appear as small deflections on top of the non-thermal galactic background emission. Relatively isolated sources were scaled "by computer" with a cubic fit to the background emission before and after transit (Paper I). However, because of the proximity of nearby sources and structure in the background emission, the majority of sources were "hand-scaled" from the right-ascension scans.

All flux densities are measured with respect to a reference flux density for Cygnus A (3C405) of 29100 Jy at 22 MHz. A complete description of this scale and comparisons with other low-frequency scales are given in Roger et al. (1973). That paper reported substantial agreement between this (Penticton) scale and the Clark Lake 26.3 MHz scale (Viner and Erickson, 1975). A discrepancy between these two scales and the 20-25 MHz Grakovo UTR-1 scale (Braude et al., 1970) has been largely resolved with the newly calibrated Grakovo UTR-2 scale (Braude, 1978). The continuity of the calibration was ensured by the injection of a controlled level of correlated noise in alternate integration samples. Table 1 contains the compilation of the flux densities. The sources are ordered in right ascension with a designation (Column 1) representing the 1950 coordinates. Column 2 contains other catalogue designations. The flux densities and their errors are listed in columns 3 and 4. Column 5 indicates the number of separate observations which contribute to the final flux density. A nearby source listed in column 6 may affect the accuracy of the measurement. Doubtful identification of a confusing source is indicated by parentheses. A nearby source which is not listed in higher frequency catalogues is shown as a "not previously catalogued source (NPCS)" together with its 1950 right ascension. A cross (X) in column 7 indicates that the complexity of the galactic background emission in the neighbourhood of the source may have affected the measurement. Specific sources or their sidelobes (sl) which may contribute to the background complexity are listed in column 7. Notes on individual sources are indicated in column 8 and follow the table. The notes include cross references to other low-frequency measurements made with the Clark Lake 26 MHz radiotelescope (Viner and Enckson, 1975) and the Grakovo UTR-2 telescope (Braude et al., 1978,1979, 1980, 1985).

The Dominion Radio Astrophysical Observatory is operated as a national facility by the National Research Council of Canada.

References

Braude, S. Ya.: 1977, IAU Symp. 74, eds. D. L. Jauncey (Reidel, Dordrecht), p. 9.

Braude, S. Ya., Megn, A. V., Rashkovski, S. L., Ryabov, B. P., Sharykin, N. K., Sokolov, K. P., Tkatchenko, A. P., Zhouck, I. N.: 1978, Ap&SS 54, 37.

Braude, S. Ya., Megn, A. V., Ryabov, B. P., Zhouck, I. N.: 1970, Ap&SS 8, 275.

Braude, S. Ya., Megn, A. V., Sokolov, K. P., Tkachenko, A. P., Sharykin, N. K.: 1979, Ap&SS 64, 73.

Braude, S. Ya., Miroshnitchenko, A. P., Sokolov, K. P., Sharykin, N. K.: 1981, Ap&SS 74, 409.

Braude, S. Ya., Sharykin, N. K., Sokolov, K. P., Zakcharenko, S. M.: 1985, Ap&SS 111, 237.

Costain, C. H., Lacey, J. D., Roger, R. S.: 1969, Ieee Trans. Antennas Propag. Ap-17, 162.

Roger, R. S., Bridle, A. H., Costain, C. H.: 1973, Astron. J. 78, 1030.

Roger, R. S., Costain, C. H., Lacey, J. D.: 1969, Astron. J. 74, 366.

Viner, M. R., Erickson, W. C.: 1975, Astron. J. 80, 931.