RADIO VARIABILITY IN A COMPLETE SAMPLE OF EXTRAGALACTIC SOURCES AT 151-MHZ

Variability in the 15 1 -MHz flux density of radio sources, on time-scales of 1 and 5 yr, has been investigated using the Cambridge Low Frequency Synthesis Telescope for a sample of approximately 1050 sources with S151 > 0.3 Jy. Over a time-scale of 1 yr, between 0.4 and 1.0 per cent of the sources show variability at a level > 10 per cent; the five sources which definitely vary have steep radio spectra which turn over at approximately 400 MHz. Over a time-scale of 5 yr, between 1 and 2 per cent of the sources show variability at the same level; of the 16 sources which definitely vary, six have flat spectra and two have steep spectra which turn over at approximately 400 MHz. This strong correlation between variability and radio spectral properties is reflected in estimates of the proportion of variable sources in each spectral class at 151 MHz. Amongst steep-spectrum sources in general, variability is rare. occurring in 0.5-1 per cent of the population over time-scales up to 5 yr, but in one subset of this population, namely those sources whose spectra turn over at approximately 400 MHz, variability is much more common, with 25 per cent being variable on a time-scale of 1 yr. An even higher proportion of the flat-spectrum population is variable, with up to 40 per cent of the sources varying on time-scales greater-than-or-equal-to 5 yr. The variability in the steep spectrum population is probably extrinsic in origin, being consistent with the predictions of the models of refractive interstellar scintillation.