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1999MNRAS.315..839Minns& Riley

MNRAS 315 (4), 839-855

Compact radio sources variable at 151 MHz - I. Catalogue of sources

A. R. Minns and J. M. Riley

The Cambridge Low-Frequency Synthesis Telescope has been used to produce a representative sample of low-frequency variable sources. 20 fields, each covering an area of approximately 9deg*9deg*cosec d, have been observed at 151 MHz at between 2 and 10 epochs over the period from 1984 to 1996. At each epoch, maps were made with rms noise levels of typically 10-15 mJy beam-1. From a total of ~6000 sources detected on these maps, 207 are found for which the flux density variations between at least two epochs appear significant at greater than the 3s level. A numerical model is used to assess the true significance of the variability, given the analysis method adopted. This shows that for about half of the sources which appear to vary by >3s the variability is genuine. For the other half it is caused by random statistical fluctuations; most of the spurious variables vary by [Image]3.5s between a single pair of epochs. A catalogue of the variable sources is presented, which includes an estimate of the probability that a given source is a genuine variable. Fractional flux density variations of between 5 and 100 per cent (typically 15-25 per cent) have been detected on a range of time-scales from 1 to 12 years.

2 Observations

The 20 7C fields used for variability studies are listed in Table 1, which gives field centres and observation epochs. Each field covers an area of approximately 9deg*9deg*cosec(d). Fields were observed when near to opposition to reduce the effects both of the ionosphere and of the Sun. The exact dates of observations differed from year to year, since usable data could be obtained only under stable ionospheric conditions. 7C surveys of some of these fields have already been published by McGilchrist et al. (1990), Lacy et al. (1995) and Pooley, Waldram & Riley (1998); surveys of the remainder of the fields will be given in a forthcoming paper.

5 The catalogue

The catalogue of 207 PLFV sources is presented in Table 7. For each source, the CLFST field, radio position, optical identification (where available), variability characteristics, 151-MHz flux density range, spectral indices and a spectral classification are given. References for positions and optical identifications are listed in Tables 8 and 9, respectively.

The 151-MHz flux densities have been indirectly scaled to the 6C flux scale. The required scalefactors were determined from the final 7C survey catalogue for each field using the method described by Visser et al. (1995).

Where there was no published optical identification, Digitized Sky Survey images were examined at the radio positions. An identification is reported if an optical object is seen with a separation from the radio position less than or equal to the error in the radio position or 5 arcsec, whichever is the larger. As discussed in Section 2, the sensitivity of the observations to flux density variations is a function of the brightness of the source; for the brightest sources (151-MHz flux density -1 Jy) variations at the 5 per cent level can be detected, whilst for the faintest sources the limit is much higher than this. Significant variations have been observed on a range of time-scales from 1 to 12 years. Amongst the 72 sources for which the chance of being a true variable is greater than 0.8, typical maximum flux density variations of 15-25 per cent are seen; eight objects show variations greater than 50 per cent, with one source varying by more than 100 per cent.

6 Conclusions

From a total of about 6000 sources mapped with the CLFST at 151 MHz, 207 have been found for which the flux density variations between at least two epochs appear significant at greater than the 3sigma level. Given the analysis method adopted, about half of these sources are likely to be genuine variables; a high proportion of these genuine variables have varied by 3.5sigma over more than one pair of epochs. Fractional flux density variations of between 5 and 100 per cent (typically 15-25 per cent) have been detected on a range of time-scales between 1 and 12 years. The results will be discussed in later papers.