Observations of intrahour variable quasars: scattering in our Galactic neighbourhood

Abstract

Interstellar scintillation (ISS) has been established as the cause of the random variations seen at centimetre wavelengths in many compact radio sources on timescales of a day or less. Observations of ISS can be used to probe structure both in the ionized insterstellar medium of the Galaxy and in the extragalactic sources themselves down to μ as scales. A few quasars have been found to show large amplitude scintillations on unusually rapid, intrahour timescales. This has been shown to be due to weak scattering in very local Galactic ‘screens’, within a few tens of parsec of the Sun. The short variability timescales allow detailed study of the scintillation properties in relatively short observation periods with compact interferometric arrays. The three best-studied ‘intrahour variable’ quasars, PKS 0405−385, J1819+3845 and PKS 1257−326, have been instrumental in establishing ISS as the principal cause of intraday variability at centimetre wavelengths. Here we review the relevant results from observations of these three sources.

Keywords:

• IDV
• Interstellar scintillation
• Quasars

Acknowledgements

Bignall thanks the organizers of ‘Scattering and Scintillation in Radioastronomy’ for the invitation to attend and present a talk at the conference in Pushchino. We thank our colleagues, especially Barney Rickett, Ger de Bruyn, Jane Dennett-Thorpe and Mark Walker for sharing data and ideas presented in this paper. The ATCA is part of the Australia Telescope, which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. The Westerbork Synthesis Radio Telescope is operated by ASTRON (Netherlands Foundation for Research in Astronomy) with support from the Netherlands Foundation for Scientific Research (NWO). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.