We have presented data from three observing campaigns carried out at the VLA to investigate intraday variability of compact extragalactic radio sources. In total, we observed 14 flat-spectrum sources (some repeatedly); out of these only 3 (namely 0153+744, 0836+710, and 1928+738) did not show IDV in any of the observations. This confirms that this phenomenon occurs consistently in a significant number of compact radio sources (with a flat spectrum), although clear statistical conclusions are precluded by our target selection, which favored objects that had previously shown fast variability. Also, the detailed properties vary strongly between sources. In most cases, the variations in total flux density are accompanied by similar variability of the linear polarization. Very fast variations (i.e., on timescales 2days, type II) occur in about 40% of all cases. One should keep in mind, however, that this is a small, by no means complete sample of sources. More detailed statistics, including observations of a larger number of sources with the Effelsberg 100m telescope, will be presented separately (Kraus et al., in preparation).
The size of a variable source can be derived from typical timescales
assuming an intrinsic origin of the variability. In this case the linear size
cannot be much larger than
with the usual synchrotron
theory this leads to an expression for the brightness temperature of the source
(cf. Wagner & Witzel 1995)
In forthcoming papers (Kraus et al., in preparation) we will present the IDV observations carried out at the 100m telescope of the MPIfR in Effelsberg and a complete statistical analysis of all available observations.
We thank the NRAO staff for their expert assistance and advice, and A. Patnaik and K. Otterbein for help during the data analysis. The VLA is a facility of the NRAO, which is operated by Associated Universities Inc., under cooperative agreement with the NSF.
Copyright The European Southern Observatory (ESO)