Lucyna Kedziora-Chudczer

PKS 0405–385: The Smallest Radio Quasar?

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 among extragalactic sources. If intrinsic to the source, these variations would imply a brightness temperature TB~1021 K, some 9 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 γ~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 that is less than 5 μas in size—a source size which implies a brightness temperature TB>5×1014 K, still far above the inverse Compton limit. Simply interpreted as a steady, relativistically beamed synchrotron source, this would imply a bulk Lorentz factor γ~103.

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.

Interstellar Scintillation of the Polarized Flux Density in Quasar PKS 0405–385

The remarkable rapid variations in radio flux density and polarization of the quasar PKS 0405-385 observed in 1996 are subject to a correlation analysis, from which characteristic timescales and amplitudes are derived. The variations are interpreted as interstellar scintillations (ISSs). The centimeter wavelength observations are in the weak scintillation regime for which models for the various auto- and cross-correlations of the Stokes parameters are derived and fitted to the observations. These are well modeled by ISS of a 30 × 22 ?as source, with about 180? rotation of the polarization angle along its long dimension. This success in explaining the remarkable intraday variability (IDVs) in polarization confirms that ISS gives rise to the IDV in this quasar. However, the fit requires the scintillations to be occurring much closer to the Earth than expected according to the standard model for the ionized interstellar medium (IISM). Scattering at distances in the range 3-30 pc is required to explain the observations. For our preferred distance of 25 pc the associated source model has a peak brightness temperature near 2 × 1013 K, which is about 25 times smaller than previously derived for this source. This reduces the implied Doppler factor in the relativistic jet, presumed responsible to ~75, which is still substantially higher than centimeter wavelength VLBI estimates for the Doppler factors in active galactic nuclei (AGNs).

First results from masiv: The Microarcsecond Scintillation-Induced Variability survey

We are undertaking a large-scale, microarcsecond scintillation-induced variability survey, MASIV, of the northern sky (decl. > 0°) at 4.9 GHz with the VLA. Our objective is to construct a sample of 100 to 150 scintillating extragalactic sources with which to examine both the microarcsecond structure and the parent populations of these sources, and to probe the turbulent interstellar medium responsible for the scintillation. We report on our first epoch of observations, which revealed variability on timescales ranging from hours to days in 85 of 710 compact flat-spectrum sources. The number of highly variable sources, those with rms flux density variations greater than 4% of the mean, increases with decreasing source flux density, but rapid, large-amplitude variables such as J1819+3845 are very rare. When compared with a model for the scintillation due to irregularities in an electron layer 500 pc thick, our preliminary results indicate maximum brightness temperatures ~1012 K, similar to those obtained from VLBI surveys even though interstellar scintillation is not subject to the same angular resolution limit.

Strong, Variable Circular Polarization in PKS 1519–273

We find strong (> 1%) circular polarization in the intraday-variable radio source PKS 1519–273. The source exhibits ∼ 12 hourly variability in all four Stokes parameters at 4.8 and 8.6 GHz, and longer timescale variability at 2.5 and 1.4 GHz. The characteristics and frequency dependence of the variability suggest that it is due to interstellar scintillation. VSOP limits on the distance to the scattering screen constrain the brightness temperature to TB > 5 × 1013 K. The fluctuations in total intensity are well-correlated with those in circular polarization, implying that the variable component of the source is -3.8 ± 0.4% circularly polarized at 4.8 GHz. The origin of the circular polarization is unclear.

Rapid interstellar scintillation of PKS 1257-326 : Two-station pattern time delays and constraints on scattering and microarcsecond source structure

We report measurements of time delays of up to 8 minutes in the centimeter-wavelength variability patterns of the intrahour scintillating quasar PKS 1257-326, as observed between the VLA and the ATCA on three separate epochs. These time delays confirm interstellar scintillation as the mechanism responsible for the rapid variability, at the same time effectively ruling out the coexistence of intrinsic intrahour variability in this source. The time delays are combined with measurements of the annual variation in variability timescale exhibited by this source to determine the characteristic length scale and anisotropy of the quasars intensity-scintillation pattern, as well as to attempt to fit for the bulk velocity of the scattering plasma responsible for the scintillation. We find evidence for anisotropic scattering and highly elongated scintillation patterns at both 4.9 and 8.5 GHz, with an axial ratio >10 : 1, extended in a northwest direction on the sky. The characteristic scale of the scintillation pattern along its minor axis is well determined, but the high anisotropy leads to degenerate solutions for the scintillation velocity. The decorrelation of the pattern over the baseline gives an estimate of the major-axis length scale of the scintillation pattern. We derive an upper limit on the distance to the scattering plasma of no more than 10 pc.

The ATCA intraday variability survey of extragalactic radio sources

We present the results of an Australia Telescope Compact Array (ATCA) survey for intra-day variability (IDV) of the total and polarized flux densities of 118 compact, flat spectrum, extragalactic radio sources from the Parkes 2.7 GHz Survey. A total of 22 total flux density IDV sources were discovered and 15 sources were found to show IDV of their polarized flux density. We discuss the statistical properties of the IDV sources, including the distribution of source modulation indices, and the dependence of the variability amplitude on source spectral index and on Galactic position. We suggest interstellar scintillation (ISS) in the Galactic interstellar medium as the most likely mechanism for IDV. Even so, the inferred high brightness temperatures cannot be easily explained.

Modelling the spectra of planets, brown dwarfs and stars using vstar

We describe a new software package capable of predicting the spectra of Solar system planets, exoplanets, brown dwarfs and cool stars. The Versatile Software for Transfer of Atmospheric Radiation (vstar) code combines a line-by-line approach to molecular and atomic absorption with a full multiple scattering treatment of radiative transfer. vstar is a modular system incorporating an ionization and chemical equilibrium model, a comprehensive treatment of spectral line absorption using a data base of more than 2.9 billion spectral lines, a scattering package and a radiative transfer module. We test the methods by comparison with other models and benchmark calculations. We present examples of the use of vstar to model the spectra of terrestrial and giant planet in our own Solar system, brown dwarfs and cool stars.

A search for propylene oxide and glycine in Sagittarius B2 (LMH) and Orion

We have used the Mopra Telescope to search for glycine and the simple chiral molecule propylene oxide in the Sgr B2 (LMH) and Orion KL, in the 3-mm band. We have not detected either species, but have been able to put sensitive upper limits on the abundances of both molecules. The 3 sigma upper limits derived for glycine conformer I are 3.7 x 10(14) cm(-2) in both Orion-KL and Sgr B2 ( LMH), comparable to the reported detections of conformer I by Kuan et al. However, as our values are 3s upper limits rather than detections we conclude that this weighs against confirming the detection of Kuan et al. We find upper limits for the glycine II column density of 7.7 x 10(12) cm(-2) in both Orion-KL and Sgr B2 ( LMH), in agreement with the results of Combes et al. The results presented here show that glycine conformer II is not present in the extended gas at the levels detected by Kuan et al. for conformer I. Our ATCA results have ruled out the detection of glycine ( both conformers I and II) in the compact hot core of the LMH at the levels reported, so we conclude that it is unlikely that Kuan et al. have detected glycine in either Sgr B2 or Orion-KL. We find upper limits for propylene oxide abundance of 3.0 x 10(14) cm(-2) in Orion-KL and 6.7 x 10(14) cm(-2) in Sgr B2 (LMH). We have detected fourteen features in Sgr B2 and four features in Orion-KL which have not previously been reported in the interstellar medium, but have not been able to plausibly assign these transitions to any carrier.

Gravitational-Wave Detection Using Pulsars: Status of the Parkes Pulsar Timing Array Project

The first direct detection of gravitational waves may be made through observations of pulsars. The principal aim of pulsar timing-array projects being carried out worldwide is to detect ultra-low frequency gravitational waves (f ∼ 10 −9 -10 −8 Hz). Such waves are expected to be caused by coalescing supermassive binary black holes in the cores of merged galaxies. It is also possible that a detectable signal could have been produced in the inflationary era or by cosmic strings. In this paper, we review the current status of the Parkes Pulsar Timing Array project (the only such project in the Southern hemisphere) and compare the pulsar timing technique with other forms of gravitational-wave detection such as ground- and space-based interferometer systems.