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1992AZh....69..225Amirkhanyan+
1992SvA....36..115A

Complete sample of radio sources at 3.9, 4.8, 7.5 and 11.2 GHz from the Zelenchuk survey

(OCR+proof by H.Andernach 9+12/98)

V.R. Amirkhanyan, A.G. Gorshkov, and V.K. Konnikova

P.K. Shternberg State Astronomical Institute

Astron. Zh. 69, 225-237 (March-April 1992)

Fluxes at 3.9, 4.8, 7.5, and 11.2 GHz have been measured for a complete sample of radio sources from the Zelenchuk survey. The sample contains all sources with flux S_3.9 > 200 mJy in the declination range +4 to +6 degrees. The spectra of the sources in the 0.365-11.2 GHz range are analyzed.

1. INTRODUCTION

Observations in the RADOP program, the purpose of which is to construct a luminosity function for a sample of relatively strong radio sources at 3.9 GHz from the Zelenchuk survey, are being carried out at the P. K. Shternberg State Astronomical Institute (GAISh). The program includes a comprehensive investigation of objects in the sample in the radio and visible ranges. Optical identification of radio sources, UBVR photometry, and an investigation of spectra comprise the primary research in the visible. It has been suggested that about 85% of the objects will have magnitude mB < 23 [1].

The radio investigation of the sample has the aim of precise flux measurement, the construction of centimeter-wave spectra and, in the future, decimeter-wave spectra, the study of statistical properties, and the search for variable objects and an investigation of the nature of their variability. The radio investigation of sources in the sample was begun in 1984 at the RATAN-600 radio telescope at 3.9 and 7.5 GHz (Ref. 2) and continued in 1987-1988 [3]. In the present paper we give the results of observations of the sources in the sample at 3.9, 4.8, 7.5, and 11.2 GHz with the RATAN-600 in July-August and December 1990.

2. SAMPLE OF RADIO SOURCES

To obtain any statistical characteristics of objects that are free of selection effects, one must have samples of those objects. The samples can be generated on the basis of different parameters, depending on the purposes of the investigation. It is important that a generated sample contain all objects with the given parameters or a randomly selected fraction of them, which ensures the completeness of the sample, and that the accuracy in determining the parameters not vary over the population from which the sample is drawn; this determines the uniformity of the sample. Selection effects are minimized in such a sample.

The sample was originally generated from sources in the 3.9 GHz Zelenchuk survey [4]. The sample included survey sources with measured flux greater than 200 mJy at 3.9 GHz in the declination range +4 to +6 deg (epoch 1950.0) and at galactic latitudes |b| > 10 deg. After more refined processingS of survey data at 3.9 MHz, the sample was corrected, and that sample, containing 183 sources, is being used in the RADOP observations.

The sample is complete down to the 200 mJy flux level observed in the survey, but because of observational errors, the sample contains objects with lower true flux and is missing some objects with flux S >= 200 mJy. For example, half of the sources with a true flux of 200 mJy are not in the sample. Figure 1 shows the percentage content in the sample of sources with a true flux S. The sample's completeness must be taken into account in constructing any flux dependence - particularly the spectral index-flux dependence alpha(S).

3. OBSERVATIONS

Sources in the sample were observed- at 3.9 and 7.5 GHz (GAISh equipment) at the meridian using the south sector of the RATAN-600 with a plane reflector, and at 4.8 and 11.2 GHz (equipment from the Special Astronomical Observatory, Russian Academy of Sciences) at the meridian using the north sector. In Table 1 we give the parameters of the receivers and the antenna beam patterns: the receiver passband D_nu, the time constant r at the receiver output, the 3 dB beamwidth theta_RA and theta_DE in right ascension and declination, the distance phi between beam patterns, the receiver temperature sensitivity sigma_n for a 1 sec time constant, and the system flux sensitivity Delta_S per scan after processing. Because of the considerable offset of the feed horns relative to the optical axis at 4.8 GHz (the horns are centered at 11.2 GHz), the antenna beam pattern at 4.8 GHz is somewhat wider in right ascension than the calculated pattern.

Beam switching is used in all the receivers; those at 4.8 and 11.2 GHz, with transistor amplifiers at the input, are cooled to the temperature of liquid nitrogen, and those at 3.9 and 7.5 GHz, with parametric amplifiers at the input, are cooled to -40 K.

In each scan of a source, the receiver gains are monitored against the signals from semiconductor noise generators. Most of the sources were observed nine times: three times at the declination of the source and three times each at declinations +/-6' from the central declination at 3.9 and 7.5 GHz and +1' (or 30") at 4.8 and 11.2 GHz. The declinations of sources in the Zelenchuk survey have been obtained with low precision, so declinations obtained for the given sources: in the Texas survey (J. N. Douglas, private communication), with precision no worse than several arcseconds, were used for the observations. Over 80% of the sources in our sample are present in the Texas survey; other sources were observed at declinations obtained in the MIT-Green Bank survey [6] at 4775 GHz or in the Zelenchuk survey. Observing at three declinations enables us to eliminate the sizable declination errors that are found for some sources in the Texas survey due to skips to adjacent lobes of the interferometer pattern, and to reduce the dependence of the measured flux on possible antenna pointing errors. All observations were processed by two-dimensional optimal filtering. [3] The calibration source P 2127+04 was observed and processed by the same procedure as the other sources; its flux at 3.9, 4.8, 7.5, and 11.2 GHz were taken to be 2.4, 2.15, 1.6, and 1.3 Jy, respectively.

4. RESULTS

The flux from 179 radio sources in the sample is given in Table 2. Twenty-five of the sources were not observed at 4.8 and 11.2 GHz for lack of observing time. The double sources 1648+050 and 2310+050 were omitted from the table because they are not resolved by the adopted processing method.

In columns 1-3 we give the names and coordinates of the sources at epoch 1950.0. The coordinates from the Texas survey are given for sources present in that survey. The peak fluxes from the sources at 3.9, 4.8, 7.5, and 11.2 GHz are given in millijanskys in columns 4, 6, 8, and 10, and the flux measurement errors in millijanskys in columns 5, 7, 9, and 11. The flux errors are dominated mainly by receiver noise, instability of the calibration signal, and by the number of observations. In columns 12-14 we give the spectral indices between frequencies 0.365 - 3.9, 3.9 - 7.5, and 3.9-11.2 GHz. Spectral indices are not given for sources that are known to be extended, the fluxes from which are underestimated.

Fluxes. In Fig. 2 we compare the fluxes at 4.8 GHz in the present paper and fluxes from the same sources obtained in the MIT-Green Bank survey at the same frequency. Flat-spectrum sources, alpha < 0.5 (S ~ nu^{-alpha}), many of which display variability, are shown as circles. Most points lie within the 95% confidence interval (+/-2 sigma); the points for flat-spectrum sources for which the flux has changed over the decade between the MIT-Green Bank and the present observations lie outside that interval.

A comparison of the fluxes at 3.9 and 7.5 GHz with those measured [2] at the same frequencies in 1985, plus an analysis of the data, have shown that the flux scales at 7.5 GHz almost coincide, while the scales at 3.9 GHz differ by a factor of 1.1, the fluxes in the 1985 measurements being higher. The spread of the fluxes from nonvariable sources does not exceed the calculated range of random errors for the most part.

Spectra. It is well known that the spectral-index distribution P(alpha) hardly depends on flux for radio sources detected in meter-wave surveys [7], but the alpha(S) dependence is significant for samples of sources from both decameter-wave and centimeter-wave [9] surveys. In the decameter-wave range the flattening of spectra in the high-flux range S > 80 Jy at 25 MHz is due to sources with self-absorption, which have a normal spectrum at higher frequencies. A similar relationship in the centimeter-wave range is due to flat-spectrum sources. In the low-flux range (S < 10 mJy), the P(alpha) distribution for samples of sources from.any surveys have similar parameters.

Our measurements make it possible to obtain the distribution of two-frequency spectral indices in the centimeter-wave range, and the inclusion of data from the Texas survey at 365 MHz expands the range of two-frequency indices to the meter-wave range.

In Table 3 we give the mean two-frequency spectral indices of sources from the sample between frequencies 0.365--3.9, 3.9--7.5, and 3.9--11.2 GHz, for a number of 3.9 GHz flux ranges. The indices between 3.9 and 4.8 GHz were not considered because of the closeness of the frequencies. In determining alpha we considered only sources with S>=200 mJy in the given series of measurements, thereby cutting out some sources with a true flux S < 200 mJy, but also some variable objects, unfortunately. Nor did we consider manifestly extended sources.

The first noteworthy fact is that for sources with S>=200 mJy remains constant to within the measurement errors for all of the intervals of two-frequency indices. This confirms the conclusion of [2] that the spectra of a considerable fraction of radio sources can be approximated by a simple power law from the meter-wave to the short centimeter-wave range. That conclusion is also valid for sources with power-law spectra and alpha > 0.5 (last row in Table 3), and for some flat-spectrum sources.

The situation for flat-spectrum sources is illustrated by the alpha(3.9-7.5)-alpha(7.5-11.2) two-color diagram in Fig. 3. The two diagonal parallel lines show the limits within which 95% of the sources with power-law spectra should lie, given the mean error in spectral index. For sources lying above this range, the spectra steepen with increasing frequency, and for sources below it they flatten. The numbers of sources in these ranges are about equal, i.e., about a third of the sources obey a power law. The curvature of the spectra of the remaining two-thirds of the sources is most likely due to their variability: the fluxes were measured at different stages of development of a flare. These sources may also have power-law spectra in the steady state, since the mean spectral indices of all flat-spectrum objects are equal, at least for two frequency ranges: = -0.05+/-0.05 and = -0.08+/-0.06.

The data in Table 3 confirm the existence of the alpha(S) dependence for the sample of sources from the high-frequency survey (nu = 3.9 GHz). The mean spectral index of the objects decreases with increasing flux, i.e., the fraction of flat-spectrum sources increases. The somewhat larger in the flux ranges 0.2-0.3 Jy and 0.3-0.5 Jy in comparison with the mean spectral indices in the centimeter-wave range is explained by the high limiting flux S_lim = 200 mJy in the Texas survey, so that some of the flat-spectrum objects are missing from that survey.

The alpha(S) dependence obtained, including the increase in , is in good agreement with the predictions of the model considered in [10]. In the model it was shown that the alpha(S) dependence appears because of the non-powerlaw nature of the radio source counts and the considerable spread of P(alpha), and is not a consequence of cosmological evolution of radio sources.

The mean spectral indices calculated in the model are also given in Table 3. The limiting flux of 200 mJy in the Texas survey was taken into account in calculating . The value of is the same for all of the two-frequency indices in the centimeter-wave range and is given in the last column. That value allows for the incompleteness of the sample in the 0.2-0.3 Jy range.

The agreement between the observed and model mean spectral indices is good in all but the 0.5-5.0 Jy flux range, in which the model value = 0.43 is considerably higher than the observed value = 0.26. This disagreement may be explained by the influence of variable sources, which predominate in this flux range. It is well known that the amplitude of variability decreases as one moves from the short centimeter-wave range to longer wavelengths, so the flattening is greater for the spectra of sources in which activity appears at the shortest wavelengths at the time of observation than the steepening of the spectra of sources for which the activity occurs at longer wavelengths at this time. Moreover, flares may cover the entire observing range, which also leads to flattening of the spectra. Such asymmetry in the variation of the spectral indices of variable sources, in our opinion, also leads to a decrease in the observed for strong radio sources relative to the model value.

Our observations of the sample since 1980 lead us to believe that the sources marked by asterisks Table 2 are variable. The investigation of the variability of those sources is continuing, and the results will be published in a future paper.

1) V.K. Konnikova, Gos. Astron. Inst. Im. Shternberga, Preprint No. 8, Moscow (1989).

2) V.K. Konnikova and V. N. Sidorenkov, Astron. Zh. 65, 263 (1988) [Sov. Astron. 32, 134 (1988)].

3) V.R. Amirkhanyan, A.G. Gorshkov, and V.K. Konnikova, Pis'ma Astron. Zh. 15, 876 (1989).

4) V.R. Amirkhanyan, A. G.. Gorshkov, A. A. Kapustkin, et al., Soobsch. Spets. Astrofiz. Obs. 47, 5 (1985)

5) V.R. Amirkhanyan, A. G. Gorshkov, A. A. Kapustkin, et al., Catalog of Radio Sources in the Zelenchak Sky Survey in the 0 to +14 deg Declination Range [in Russian], Moskow Gos. Univ., Moscow (1989).

6) C.L. Bennett, C.R. Lawrence, B.F. Burke, et al., ApJS 61, 1 (1986).

7) I.I.K. Pauliny-Toth and K.I. Kellerrnann, AJ 77, 560 (1977)

8) K.P. Sokolov, Astron. Zh. 66, 1121 (1989) [Sov. Astron. 33, 579 (1989)].

9) J. J. Condon, ApJ 287, 461 (1984).

10) A. G. Gorshkov, Astron. Zh. 68, 1121 (1991) [Sov. Astron. 35, 562 (1991)].

Translated by Edward U. Oldham