Subsections

• Individual Sources
• General Properties

Discussion

Individual Sources

We restrict our comments here to the 18 sources found to be variable. Many of these sources have been observed in snapshot mode at 5 GHz with the Australia Telescope Compact Array (ATCA, Burgess 1998), and are also ATCA secondary phase calibrators.

MRC B0208-512: VLBI modelling shows a strong core (Preston et al 1989), and a jet-like feature (Tingay et al 1996). Detected as an X-ray source in the ROSAT All-Sky Survey (Brinkmann et al. 1994) and as a -ray source in the EGRET survey (Bertsch et al. 1993).

MRC B0537-441: See Paper I.

MRC B0943-761: Close

double at 5 GHz (Burgess 1998). Detected as an X-ray source in the ROSAT All-Sky Survey (Brinkmann et al 1994).

MRC B1151-348: Radio spectrum peaks at 200 MHz. A VLBI image shows a 90 mas double structure (King et al 1993).

MRC B1215-457: Compact steep-spectrum source with a strong, slightly resolved VLBI core (Preston et al 1989).

MRC B1234-504: Compact steep-spectrum source, with no optical counterpart on the UK Schmidt sky survey but possible stellar identification on a CCD image obtained at the Anglo-Australian Telescope (AAT) (Burgess 1998).

MRC B1424-418: Discordant flux densities measured at Parkes point to the source being variable at 5 GHz (Burgess, priv comm). VLBI modelling shows an unequal 23 mas double structure (Preston et al 1989).

MRC B1458-391: Compact steep-spectrum source in a crowded optical field; optical ID based on an AAT CCD image (Burgess 1998).

MRC B1549-790: VLBI image shows a curved structure, possibly a core plus jet (Murphy et al 1993).

MRC B1610-771: Quasar with a flat radio spectrum and very steep optical spectrum (Hunstead & Murdoch 1980). VLBI observations (Preston et al 1989) show a strong core surrounded by a 50 mas halo.

MRC B1718-649: The nearest GHz-peaked-spectrum source, with a radio spectrum peaking near 3 GHz. VLBI imaging shows two sub-parsec-scale components separated by 2 pc (Tingay et al. 1997).

MRC B1740-517: Crowded optical field; galaxy ID by di Serego Alighieri et al (1994) is confirmed by an AAT CCD image (Burgess 1998).

MRC B1827-360: Compact ultra-steep-spectrum source identified with a galaxy in a very crowded field.

MRC B1829-718: Candidate source for defining the VLBI astrometric reference frame (Ma et al 1998).

MRC B1854-663: Compact steep-spectrum source identified with a faint galaxy (Burgess 1998).

MRC B1921-293: See Paper I.

MRC B2052-474: Radio spectrum steep at low frequency, but flattens at high frequency; core dominated at 5 GHz, possibly triple (Burgess 1998). Detected as an X-ray source by the ROSAT All-Sky Survey (Brinkmann et al 1994).

MRC B2326-477: Detected as an X-ray source in the ROSAT All-Sky Survey (Brinkmann et al 1994). One of the set of defining sources for the VLBI astrometric reference frame (Ma et al 1997).

General Properties

If the observed variability is a result of refractive scintillation in the Galactic interstellar medium (ISM), then we expect some sort of dependence of one of modulation index

, characteristic timescale or their product, , on the Galactic latitude, b (e.g. Spangler et al. 1989; Ghosh & Rao 1992). However, apart from a weak tendency for larger to occur at lower |b|, there is no obvious correlation in our data. This is not surprising given the large uncertainties in arising from the irregular sampling of the light curves, and the fact that there are few variable sources at high latitudes (14 of the 18 variable sources have

).

An alternative indicator of the effects of the Galactic ISM is to test whether variable sources are more likely to be found at low latitudes. We consider this possibility in Figure 8, where we plot the ratio of variable sources (N_V) to variable plus steady sources (N_V + N_S) in different latitude bins. While the statistics are poor, there is a clear indication that sources are more likely to be variable at low latitudes, as found for northern hemisphere sources (Cawthorne & Rickett 1985; Gregorini et al. 1986). This is unlikely to be caused by selection effects associated with a dependence of spectral index on Galactic latitude (cf. Cawthorne & Rickett 1985), since the main criterion for source selection was angular size (). Thus the extensive monitoring data for the MOST calibrators provide good evidence that the variability observed at 843 MHz arises from scintillation in the local ISM.

While spectral index was not considered in selecting the majority of the sample, Table 1 shows that two-thirds of the variables have flat or inverted spectra (), consistent with source angular size being the main determinant of source variability. Surprisingly, the remaining third of the variables fall in the class of compact steep-spectrum (CSS) sources which are generally believed to be young sources still contained within their host galaxies, and not known to vary at high frequencies. The latter sources display a lower level of variability, as measured by the modulation index m, and in four of the six cases their V classification appears to be due to one-off events lasting 1 year.

To investigate the variability properties of the MOST calibrator sample as a whole, in Figure 9 we have plotted m versus for all 55 sources. This Figure shows a clear trend towards higher average modulation index as the radio spectrum flattens, with a suggestion of an upper envelope. Perhaps the simplest explanation for this behaviour in the unified model for powerful extragalactic radio sources is to link m and through the orientation of the radio axis to the line of sight (e.g. Orr & Browne 1982). We assume that the `core' of a classical triple source is the only part with components small enough in angular size to scintillate. If the core contribution dominates, as a consequence of Doppler boosting in the flat spectrum sources, even small fractional variations will be readily detected. However, the same fractional variations in the core of a steep-spectrum, lobe-dominated source will go undetected. We can therefore understand the trends in Figure 9 in a qualitative sense, and it is possible that a more detailed analysis may provide useful constraints on radio source models.

© Copyright Astronomical Society of Australia 1997