We have observed profound variability in the radio flux density of the quasar PKS 0405-385 on timescales of less than an hour; this is unprecedented amongst extragalactic sources. If intrinsic to the ...source, these variations would imply a brightness temperature 10^21 K, some nine orders of magnitude larger than the inverse Compton limit for a static synchrotron source, and still a million times greater than can be accommodated with bulk relativistic motion at a Lorentz factor equal to 10. The variability is intermittent with episodes lasting a few weeks to months. Our data can be explained most sensibly as interstellar scintillation of a source component which is < 5 microarcsec in size - a source size which implies a brightness temperature > 5 times 10^14 K, still far above the inverse Compton limit. Simply interpreted as a steady, relativistically beamed synchrotron source, this would imply a bulk Lorentz factor 1000.
We show that in many methanol maser sources the masers are located in lines, with a velocity gradient along them which suggests that the masers are situated in edge-on circumstellar, or ...protoplanetary, disks. We present VLBI observations of the methanol maser source G309.92+0.48, in the 12.2 GHz transition, which confirm previous observations that the masers in this source lie along a line. We show that such sources are not only linear in space but, in many cases, also have a linear velocity gradient. We then model these and other data in both the 6.7 GHz and the 12.2 GHz transition from a number of star formation regions, and show that the observed spatial and velocity distribution of methanol masers, and the derived Keplerian masses, are consistent with a circumstellar disk rotating around an OB star. We consider this and other hypotheses, and conclude that about half of these methanol masers are probably located in edge-on circumstellar disks around young stars. This is of particular significance for studies of circumstellar disks because of the detailed velocity information available from the masers.
We report the results of a 5-GHz southern-hemisphere snapshot VLBI
observation of a sample of blazars. The observations were performed with the
Southern Hemisphere VLBI Network plus the Shanghai ...station in 1993 May.
Twenty-three flat-spectrum, radio-loud sources were imaged. These are the first
VLBI images for 15 of the sources. Eight of the sources are EGRET (> 100 MeV)
gamma-ray sources. The milliarcsecond morphology shows a core-jet structure for
12 sources, and a single compact core for the remaining 11. No compact doubles
were seen. Compared with other radio images at different epochs and/or
different frequencies, 3 core-jet blazars show evidence of bent jets, and there
is some evidence for superluminal motion in the cases of 2 blazars. The
detailed descriptions for individual blazars are given. This is the second part
of a survey: the first part was reported by Shen et al. (AJ 114(1997)1999).
Measurements of the properties of gravitational lenses have the power to tell
us what sort of universe we live in. The brightest known radio Einstein
ring/gravitational lens PKS 1830-211 (Jauncey et ...al., 1991), whilst obscured by
our Galaxy at optical wavelengths, has recently been shown to contain
absorption at the millimetre waveband at a redshift of 0.89 (Wiklind and
Combes, 1996a). We report the detection of a new absorption feature, most
likely due to neutral hydrogen in a second redshift system at z = 0.19.
Follow-up VLBI observations have spatially resolved the absorption and reveal
it to cover the NE compact component and part of the lower surface brightness
ring. This new information, together with existing evidence of the unusual VLBI
radio structure and difficulties in modeling the lensing system, points to the
existence of a second lensing galaxy along our line of sight and implies that
PKS 1830-211 may be a compound gravitational lens.
We report the results of a 5 GHz southern hemisphere VLBI survey of compact
extragalactic radio sources. These observations were undertaken with the SHEVE
array plus Shanghai station in November ...1992. A sample of 22 sources was
observed and images of 20 of them were obtained. Of the 20 sources imaged, 15
showed core-jet structure, one had a two-sided jet and 4 had only single
compact cores. Eleven of the 16 core-jet (including one two-sided jet) sources
show some evidence of bent jets. No compact doubles were found. A comparison
with previous images and the temporal variability of the radio flux density
showed evidence for superluminal motion in 4 of the sources. Five sources were
high energy (>100 MeV) gamma-ray sources. Statistical analysis showed the
dominance of highly polarized quasars among the detected gamma-ray sources,
which emphasizes the importance of beaming effect in the gamma-ray emission.
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