We present measurements of the size of the Vela pulsar in 3 gates across the pulse, from observations of the distribution of intensity. We calculate the effects on this distribution of noise in the ...observing system, and measure and remove it using observations of a strong continuum source. We also calculate and remove the expected effects of averaging in time and frequency. We find that effects of variations in pulsar flux density and instrumental gain, self-noise, and one-bit digitization are undetectably small. Effects of normalization of the correlation are detectable, but do not affect the fitted size. The size of the pulsar declines from 440 +/- 90 km (FWHM of best-fitting Gaussian distribution) to less than 200 km across the pulse. We discuss implications of this size for theories of pulsar emission.
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.
The first VLBI images of water maser emission in the Circinus Galaxy AGN show both a warped, edge-on accretion disk and an outflow 0.1 to 1 pc from the central engine. The inferred central mass is ...1.3 million suns, while the disk mass may be on the order of 0.1 million suns, based on a nearly Keplerian rotation curve. The bipolar, wide-angle outflow appears to contain ``bullets'' ejected from within <0.1 pc of the central mass. The positions of filaments and bullets observed in the AGN ionization cone on kpc-scales suggest that the disk channels the flow to a radius of about 0.4 pc, at which the flow appears to disrupt the disk.
We have measured a time delay of 26 (+4/-5) days and a magnification ratio of 1.52 +/- 0.05 in the strong radio gravitational lens PKS 1830-211. The observations were made over the 18 month period ...from 1997 January to 1998 July with the Australia Telescope Compact Array at 8.6 GHz, and have shown that the source started a large flux density outburst around 1997 June.
We have observed the structure of the rapid variable radio source BL Lac (VRO 42.22.01) using long baseline interferometer systems with baselines up to 266 million wavelengths. Despite large ...variations in the total flux and in the overall size of this source, it has maintained an elongated brightness distribution, and the direction of elongation has not changed during the 1.3 years of observation. No simple model of stationary variable components or of separating, evolving components appears to fit all the data For this source. In particular, it apparently cannot be explained in terms of a stationary brightness distribution with a single variable component of very small angular size. The peak brightness temperature of VRO 42.22.01 is in excess of 5x 10/sup 12/. K at 11 cm wavelength, a value close to the limit set by inverse Compton scattering.
We present a new sample of Parkes half-Jansky flat-spectrum radio sources having made a particular effort to find any previously unidentified sources. The sample contains 323 sources selected ...according to a flux limit of 0.5 Jy at 2.7 GHz, a spectral index measured between 2.7 and 5.0 GHz of alpha > -0.5 (where S(v) is proportional to v to the power of alpha), Galactic latitude abs(b) > 20 deg and -45 deg < Declination (B1950) < +10 deg. The sample was selected from a region 3.90 steradians in area. We have obtained accurate radio positions for all the unresolved sources in this sample and combined these with accurate optical positions from digitised photographic sky survey data to check all the optical identifications. We report new identifications based on \R- and \Kn-band imaging and new spectroscopic measurements of many of the sources. We present a catalogue of the 323 sources of which 321 now have identified optical counterparts and 277 have measured spectral redshifts.
We describe measurements of the size of the Vela pulsar via scintillation, using both fits to the distribution of intensity and measurements of the modulation index. We briefly discuss systematic ...effects other than source size that can affect the distribution, including gain variations, self-noise, scintillation shot noise, and correlator saturation. Modulation index, a single number, can be biased by all of these, whereas the distribution of intensity is affected in different ways by different effects, providing means of distinguishing among them. Self-noise and gain variations are likely more important at long observing wavelengths, and correlator saturation and scintillation shot noise at short wavelengths. We find a size of about 500 km at decimeter wavelengths. Interestingly, this agrees with measurements of modulation index by Roberts & Ables at the same wavelength. Their results (and more recently that reported by Macquart et al.) suggest that size decreases with increasing wavelength. Although consistent with the observations, this conclusion is perhaps surprising from the standpoint of the traditional radius-to-frequency mapping. However, these measurements are of size rather than height; and of course systematic effects may play a role.
The quasar PKS 0637-753, the first celestial X-ray target of the Chandra X-ray Observatory, has revealed asymmetric X-ray structure extending from 3 to 12 arcsec west of the quasar, coincident with ...the inner portion of the jet previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a redshift of z=0.651, the jet is the largest (~100 kpc) and most luminous (~10^{44.6} ergs/s) of the few so far detected in X-rays. This letter presents a high resolution X-ray image of the jet, from 42 ks of data when PKS 0637-753 was on-axis and ACIS-S was near the optimum focus. For the inner portion of the radio jet, the X-ray morphology closely matches that of new ATCA radio images at 4.8 and 8.6 GHz. Observations of the parsec scale core using the VSOP space VLBI mission show structure aligned with the X-ray jet, placing important constraints on the X-ray source models. HST images show that there are three small knots coincident with the peak radio and X-ray emission. Two of these are resolved, which we use to estimate the sizes of the X-ray and radio knots. The outer portion of the radio jet, and a radio component to the east, show no X-ray emission to a limit of about 100 times lower flux. The X-ray emission is difficult to explain with models that successfully account for extra-nuclear X-ray/radio structures in other active galaxies. We think the most plausible is a synchrotron self-Compton (SSC) model, but this would imply extreme departures from the conventional minimum-energy and/or homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung models for the jet X-rays, unless multicomponent or ad hoc geometries are invoked.
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