Follow-up observations at 16 and 33 GHz of extragalactic sources from WMAP 3-yr data: II – Flux density variability

Abstract

Using the Arcminute Microkelvin Imager (AMI) at 16 GHz and the Very Small Array (VSA)at 33 GHz to make follow-up observations of sources in the New Extragalactic WMAP PointSource catalogue, we have investigated the ?ux density variability in a complete sample of 97sources over time-scales of a few months to ˜1.5 yr.We ?nd that 53 per cent of the 93 sources, for which we have multiple observations, arevariable, at the 99 per cent con?dence level, above the ?ux density calibration uncertaintiesof ˜4 per cent at 16 GHz; the fraction of sources having varied by more than 20 per cent is15 per cent at 16 GHz and 20 per cent at 33 GHz. Not only is this common occurrence ofvariability at high frequency of interest for source physics, but also strategies for coping withsource contamination in cosmic microwave background work must take this variability intoaccount.There is no strong evidence of a correlation between variability and ?ux density for thesample as a whole. For those sources classi?ed as variable, the mean fractional rms variationin ?ux density increases signi?cantly with the length of time separating observation pairs.Using a maximum likelihood method, we calculate the correlation in the variability at the twofrequencies in a subset of sources classi?ed as variable from both the AMI and VSA dataand ?nd the Pearson product–moment correlation coef?cient to be very high (0.955±0.034).We also ?nd the degree of variability at 16 GHz (0.202 ± 0.028) to be very similar to that at33 GHz (0.224 ± 0.039).Finally, we have investigated the relationship between variability and spectral index, a33.75 13.9(whereS ??-a), and ?nd a signi?cant difference in the spectral indices of the variable sources(-0.06 ± 0.05) and non-variable sources (0.13 ± 0.04).