We report on the variability of 443 flat-spectrum, compact radio sources monitored using the VLA for 3 days in four epochs at $4 month intervals at 5 GHz as part of the Micro-Arcsecond Scintillation-Induced Variability (MASIV) survey. Over half of these sources exhibited 2%Y10% rms variations on timescales over 2 days. We analyzed the variations by two independent methods and find that the rms variability amplitudes of the sources correlate with the emission measure in the ionized interstellar medium along their respective lines of sight. We thus link the variations with interstellar scintillation of components of these sources, with some (unknown) fraction of the total flux density contained within a compact region of angular diameter in the range 10Y50 as. We also find that the variations decrease for high mean flux density sources and, most importantly, for high-redshift sources. The decrease in variability is probably due either to an increase in the apparent diameter of the source or to a decrease in the flux density of the compact fraction beyond z $ 2. Here we present a statistical analysis of these results, and a future paper will discuss the cosmological implications in detail.

Rapid radio intra-day variability (IDV) has been discovered in the southern quasar PKS 1257−326. Flux density changes of up to 40% in as little as 45 minutes have been observed in this source, making it, along with PKS 0405−385 and J1819+3845, one of the three most rapid IDV sources known. We have monitored the IDV in this source with the Australia Telescope Compact Array (ATCA) at 4.8 and 8.6 GHz over the course of the last year, and find a clear annual cycle in the characteristic time-scale of variability. This annual cycle demonstrates unequivocally that interstellar scintillation is the cause of the rapid IDV at radio wavelengths observed in this source. We use the observed annual cycle to constrain the velocity of the scattering material, and the angular size of the scintillating component of PKS 1257−326. We observe a time delay, which also shows an annual cycle, between the similar variability patterns at the two frequencies.We suggest that this is caused by a small (∼ 10 µas) offset between the centroids of the 4.8 and 8.6 GHz components, and may be due to opacity effects in the source. The statistical properties of the observed scintillation thus enable us to resolve source structure on a scale of ∼ 10 microarcseconds, resolution orders of magnitude higher than current VLBI techniques allow. General implications of IDV for the physical properties of sources and the turbulent ISM are discussed.

We have observed profound variability in the radio flux density of the quasar PKS 0405-385 on timescales of less than an hour; this is unprecedented amongst extragalactic sources. If intrinsic to the source, these variations would imply a brightness temperature T B ∼ 10 21 K, some nine orders of magnitude larger than the inverse Compton limit for a static synchrotron source, and still a million times greater than can be accommodated with bulk relativistic motion at a Lorentz factor γ ∼ 10. The variability is intermittent with episodes lasting a few weeks to months.Our data can be explained most sensibly as interstellar scintillation of a source component which is < 5 µarcsec in size -a source size which implies a brightness temperature T B > 5 × 10 14 K, still far above the inverse Compton limit. Simply interpreted as a steady, relativistically beamed synchrotron source, this would imply a bulk Lorentz factor γ ∼ 10 3 .