First Results from MASIV: The Microarcsecond Scintillation-induced Variability Survey

We are undertaking a large-scale, Micro-Arcsecond Scintillation-InducedVariability (MASIV) survey of the northern sky, Dec > 0 deg, at 4.9 GHz withthe VLA. Our objective is to construct a sample of 100 to 150 scintillatingextragalactic sources with which to examine both the microarcsecond structureand the parent populations of these sources, and to probe the turbulentinterstellar medium responsible for the scintillation. We report on our firstepoch of observations which revealed variability on timescales ranging fromhours to days in 85 of 710 compact flat-spectrum sources. The number of highlyvariable sources, those with RMS flux density variations greater than 4% of themean, increases with decreasing source flux density but rapid, large amplitudevariables such as J1819+3845 are very rare. When compared with a model for thescintillation due to irregularities in a 500 pc thick electron layer, ourpreliminary results indicate maximum brightness temperatures ~10E+12 K, similarto those obtained from VLBI surveys even though interstellar scintillation isnot subject to the same angular resolution limit.Comment: 18 pages, 5 figures. To appear in the Astronomical Journa