Steven J. Tingay

Rapid variability and annual cycles in the characteristic timescale of the scintillating source PKS 1257-326

Rapid radio intraday variability (IDV) has been discovered in the southern quasar PKS 1257-326. Flux density changes of up to 40% in as little as 45 minutes have been observed in this source, making it, along with PKS 0405 385 and J1819+3845, one of the three most rapid IDV sources known. We have monitored the IDV in this source with the Australia Telescope Compact Array (ATCA) at 4.8 and 8.6 GHz over the course of the last year and find a clear annual cycle in the characteristic timescale of variability. This annual cycle demonstrates unequivocally that interstellar scintillation is the cause of the rapid IDV at radio wavelengths observed in this source. We use the observed annual cycle to constrain the velocity of the scattering material and the angular size of the scintillating component of PKS 1257-326. We observe a time delay, which also shows an annual cycle, between the similar variability patterns at the two frequencies. We suggest that this is caused by a small (similar to10 muas) offset between the centroids of the 4.8 and 8.6 GHz components and may be due to opacity effects in the source. The statistical properties of the observed scintillation thus enable us to resolve source structure on a scale of similar to10 muas, resolution orders of magnitude higher than current VLBI techniques allow. General implications of IDV for the physical properties of sources and the turbulent interstellar medium are discussed.

Discovery of a Subparsec Radio Counterjet in the Nucleus of Centaurus A

A subparsec scale radio counterjet has been detected in the nucleus of the closest radio galaxy, Centaurus A (NGC 5128), with VLBI imaging at 2.3 and 8.4 GHz. This is one of the first detections of a VLBI counterjet and provides new constraints on the kinematics of the radio jets emerging from the nucleus of Cen A. A bright, compact core is seen at 8.4 GHz, along with a jet extending along P.A. 51°. The core is completely absorbed at 2.3 GHz. Our images show a much wider gap between the base of the main jet and the counterjet at 2.3 GHz than at 8.4 GHz and also that the core has an extraordinarily inverted spectrum. These observations provide evidence that the innermost 0.4-0.8 pc of the source is seen through a disk or torus of ionized gas which is opaque at low frequencies due to free-free absorption.

Estimates of the Free-Free Optical Depth toward the Subparsec-Scale Radio Source in Centaurus A

Multifrequency, near-simultaneous VLBA observations have been used to achieve matched resolution images of the inner 8 pc of Centaurus A, including the jet, the nucleus, and the counterjet, at a spatial resolution of approximately 0.1 pc. By comparing the three images, at 2.2, 5.0, and 8.4 GHz, and assuming a constant intrinsic spectral index between 2.2 and 8.4 GHz along each line of sight, it is possible, in principle, to separate the effects of synchrotron self-absorption intrinsic to the radio source from free-free absorption, which is extrinsic. Toward the unresolved Centaurus A nucleus, the observed spectral index between 2.2 and 5.0 GHz is inverted to an extent where free-free absorption is unavoidable, α = 3.8 ±, at its peak. Toward this region the optical depth to free-free absorption is estimated to be τff = 0.9 ± 0.4 at 2.2 GHz, giving an intrinsic spectral index of α = 2.0 ± 0.5, within the bounds explainable by synchrotron self-absorption. Away from the nucleus the remainder of the bright subparsec-scale radio jet has a spectral index close to -0.7 and is not affected by free-free absorption. We adopt a simple spherical geometry for the nuclear absorber with an upper limit on its radius of 0.016 pc, giving a constraint on the electron density and temperature, nT 9.3, where ne4 is the electron density in units of 104 cm-3 and T4 is the electron temperature in units of 104 K. Assuming T4 = 1 gives a lower limit for the total mass of the absorber of 0.01 M☉. Future observations of higher resolution and sensitivity will be required to more accurately constrain the properties of the nuclear absorber and detect any absorption toward the counterjet.

The Subparsec-Scale Structure and Evolution of Centaurus A. II. Continued Very Long Baseline Array Monitoring

We present the results of continued 8.4 GHz Very Long Baseline Array monitoring of the subparsec-scale structure and evolution of Centaurus A, following on from the initial results presented in 1998 by Tingay et al. We include, for the first time, multiepoch VLBI images at 22.2 GHz that show that the jet is linear and well collimated on scales as small as 0.02 pc (~1000rs). Two components in the subparsec-scale jet continue to evolve slowly with a speed of 0.12c. We confirm that an additional component, close to the core, has no significant motion. Some evidence is seen for rapid variations within individual components, as noted dramatically in 1991–1992 by Tingay et al., albeit at a lower level of activity. Both the stationary behavior of the component close to the core and the internal variability of components in the subparsec-scale jet of Centaurus A may be explained as being due to the existence of shocks created in the wake of major component ejections from the nucleus, as simulated by Agudo et al. (published in 2001). Tentative evidence is found to suggest that two subparsec-scale counterjet components are in motion away from the nucleus. The estimated apparent speeds of the jet and counterjet components are consistent with the previously suggested likely jet viewing angle range, 50°–80°. We also compare our Centaurus A images with high-resolution VLBI images of M87 to show that the region of the Centaurus A jet in which collimation likely first occurs lies a factor of 10 below our current resolution limit. Future space VLBI missions at high frequency will be required to resolve this region.

The Parsec-Scale Structure and Evolution of PKS 0521−365

PKS 0521-365 is a relatively nearby active galactic nucleus possessing a bright compact radio source, strong extended radio and X-ray emission, broad and variable nuclear optical emission lines, and one of the best examples of an optical synchrotron jet. Modeling of the high-energy emission from this source, ratios of core to extended luminosity at radio and optical wavelengths, and the morphology of the extended radio and optical jet indicate that the emission from the nucleus of PKS 0521-365 is not likely to be strongly affected by relativistic boosting. We present multiepoch and multifrequency VLBI data for PKS 0521-365 that show that any apparent motion of components in the jet must be less than 1.2c, that the jet to counterjet surface brightness ratio on the parsec-scale is greater than 20, and that the peak brightness temperature of the source is greater than 1 × 1011 K at 5 GHz. While these results are consistent with the idea that PKS 0521-365 is not highly beamed, the combination of the apparent motion and jet-to-counterjet limits with Doppler-factor lower limits from synchrotron self-Compton and gamma-ray transparency models do not tightly constrain the jet speed or orientation to values which compel the conclusion that PKS 0521-365 is weakly beamed. The data are formally consistent with weak beaming as well as strong beaming. However, calculations by Pian et al. based on inhomogeneous jet models for the spectral energy distribution indicate that the Doppler factor is close to unity and place limits on the allowed ranges of jet speed and angle to the line of sight. These calculations are consistent with the VLBI data, although future higher sensitivity VLBI observations have the potential to test the results of these models more strictly. Although we find no significant support for superluminal motion in PKS 0521-365, we do find evidence for evolution internal to a component in the parsec-scale jet, similar to that seen on the subparsec-scale in Centaurus A and M87.

New Results from an ATCA Study of Intraday Variable Radio Sources

We are undertaking an observational program using the ATCA to monitor the intraday variability (IDV) of a sample of sources at 4.8 and 8.6 GHz. The sources were selected to include the known strong southern IDV sources plus a number of sources whose IDV was recently discovered. The present monitoring program will extend over a full year in order to search for any annual cycle that may be present in the long-term IDV characteristics of these sources. In this paper we discuss the observing strategy and data analysis, and present the first results from our observations.

LBADR: The LBA Data Recorder

The LBA (Long-Baseline Array) is an ad-hoc network of radio telescopes within Australia and is the only VLBI array in the Southern Hemisphere. Since 2004 all experiments have been recorded using standard computer hard-disks, replacing the aging tape based S2 system. The recorder developed for this, the LBADR, comprises largely of commercial off-the-shelf components and is closeley related to the PC-EVN system

The VSOP Survey: Final individual results

In February 1997 the Japanese radio astronomy satellite HALCA was launched to provide the space-borne element for the VSOP mission. HALCA provided linear baselines three-times greater than that of ground arrays, thus providing higher resolution and higher AGN brightness temperature measurements and limits. Twenty-five percent of the scientific time of the mission was devoted to the “VSOP survey” of bright, compact, extra-galactic radio sources at 5 GHz. A complete list of 294 survey targets were selected from pre-launch surveys, 91% of which were observed during the satellite’s lifetime. The major goals of the VSOP Survey are statistical in nature: to determine the brightness temperature and approximate structure, to provide a source list for use with future space VLBI missions, and to compare radio properties with other data throughout the electro-magnetic spectrum. All the data collected have now been analysed and is being prepared for the final image Survey paper. In this paper we present details of the mission, and some statistics of the images and brightness temperatures.