Introduction

Many compact extragalactic radio sources show variations in their radio flux density as a function of time. At high frequencies ( GHz) this variability is usually interpreted as being intrinsic to the source (e.g. Qian et al 1995, although see Kedziora-Chudczer et al 1997). Variability at lower frequencies (e.g. Hunstead 1972; Ghosh & Rao 1992) is normally attributed to refractive interstellar scintillation, in which the intensity variations are caused by distortions of the wavefront by electron density gradients in an intervening screen of material (Shapirovskaya 1978; Rickett et al 1984). There is evidence that the parameters of such variability depend on the Galactic latitude of the source (Spangler et al. 1989; Ghosh & Rao 1992), suggesting that the material causing the scintillation is in our own Galaxy.

In some sources, both intrinsic variability and scintillation may be occurring at the same time (e.g. Mitchell et al 1994). Such sources show large but uncorrelated variations at high and low frequencies. At frequencies  GHz, one might expect both effects to occur; however, variability in this region of the spectrum is largely unexplored. The Molonglo Observatory Synthesis Telescope (MOST; Mills 1981; Robertson 1991) operates at a frequency of 843 MHz, and is thus well placed to study this regime. For calibration purposes, the MOST monitors the flux density of 10 compact extragalactic sources every day. Thus the full record of MOST calibrations, running from 1984 until the commencement of the Wide Field Project in 1996 (Large et al 1994), forms an ideal database with which to study variability in this intermediate frequency range.

A preliminary analysis of three MOST calibrators was made by Campbell-Wilson & Hunstead (1994), hereafter Paper I. It was shown that flux density measurements with a relative accuracy of 2% could be extracted from the database. Over the period from 1990.1 to 1993.7, the source MRC B0409-752 was shown to be stable, while MRC B0537-441 and MRC B1921-293 were found to be highly variable. In this paper we now report on all 55 calibrators used by the MOST, over a thirteen year period. In Section 2 we explain how we process the calibrator measurements in order to produce light curves for each source, and then determine whether a source is variable or not. In Section 3 we present light curves for all 55 sources, plus structure functions for those sources found to be variable. In Section 4 we discuss some individual sources in our sample, and consider whether any of the observed properties correlate with Galactic latitude.

© Copyright Astronomical Society of Australia 1997