The micro-arcsecond scintillation-induced variability (MASIV) survey. II. The first four epochs

Creators: Lovell, J. E. J.; Rickett, B. J.; Macquart, J.-P.; Jauncey, D. L.; Bignall, H. E.; Kedziora-Chudczer, L.; Ojha, R.; Pursimo, T.; Dutka, M.; Senkbeil, C.; Shabala, S.

Abstract

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%-10% 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 10-50 μas. 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.

Additional Information

© 2008 The American Astronomical Society. Received 2008 April 18; accepted 2008 August 7. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. We are extremely grateful for the technical support provided by NRAO staff at Socorro; in particular we would like to thank Ken Sowinski, Miller Goss, Mark Claussen, and Jim Ulvestad for helping to implement the five subarrays at the VLA. This research has made use of the NASA/ IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We also made use of data from the Wisconsin Hα Mapper, which is funded by the National Science Foundation. B. J. R. thanks the US NSF for funding under grant AST 05-07713. He also thanks both the Cavendish Astrophysics group at Cambridge and the ATNF, Epping, for hospitality.

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