Current Results from the RRFID Kinematic Survey: Apparent Speeds from the First Five Years of Data

We present current results from our ongoing project to study the parsec-scale relativistic jet kinematics of sources in the U.S. Naval Observatory's Radio Reference Frame Image Database (RRFID). The RRFID consists of snapshot observations using the VLBA plus up to 9 additional antennas at 8 and 2 GHz. The Image Database currently contains about 3000 images of 450 sources from 1994 to 2004, with some sources having images at 20 epochs or more. We have now completed analysis of the 8 GHz images for all sources observed at 3 or more epochs from 1994 to 1998. The completed analysis comprises 966 images of 87 sources, or an average of 11 epochs per source. Apparent jet speeds have been measured for these sources, and the resulting speed distribution has been compared with results obtained by other large VLBI surveys. The measured apparent speed distribution agrees with those found by the 2 cm survey and Caltech-Jodrell Bank (CJ) survey; however, when a source-by-source comparison is done with the 2 cm survey results, significant disagreement is found in the apparent speed measurements for a number of sources. This disagreement can be traced in most cases to either an insufficient time baseline for the current RRFID results, or to apparent component mis-identification in the 2 cm survey results caused by insufficient time sampling. These results emphasize the need for long time baselines and dense time sampling for multi-epoch monitoring of relativistic jets.Comment: 4 pages, To be published in the Proceedings of the 7th European VLBI Network Symposiu

The Celestial Reference Frame at 24 and 43 GHz. II. Imaging

We have measured the sub-milli-arcsecond structure of 274 extragalactic sources at 24 and 43 GHz in order to assess their astrometric suitability for use in a high frequency celestial reference frame (CRF). Ten sessions of observations with the Very Long Baseline Array have been conducted over the course of $\sim$5 years, with a total of 1339 images produced for the 274 sources. There are several quantities that can be used to characterize the impact of intrinsic source structure on astrometric observations including the source flux density, the flux density variability, the source structure index, the source compactness, and the compactness variability. A detailed analysis of these imaging quantities shows that (1) our selection of compact sources from 8.4 GHz catalogs yielded sources with flux densities, averaged over the sessions in which each source was observed, of about 1 Jy at both 24 and 43 GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25% relative to their mean values, with variations in the session-to-session flux density scale being less than 10%, (3) sources were found to be more compact with less intrinsic structure at higher frequencies, and (4) variations of the core radio emission relative to the total flux density of the source are less than 8% on average at 24 GHz. We conclude that the reduction in the effects due to source structure gained by observing at higher frequencies will result in an improved CRF and a pool of high-quality fiducial reference points for use in spacecraft navigation over the next decade.Comment: 63 pages, 18 figures, 6 tables, accepted by the Astronomical Journa

Growth Rates and Explosions in Sandpiles

We study the abelian sandpile growth model, where n particles are added at the origin on a stable background configuration in Z^d. Any site with at least 2d particles then topples by sending one particle to each neighbor. We find that with constant background height h <= 2d-2, the diameter of the set of sites that topple has order n^{1/d}. This was previously known only for h<d. Our proof uses a strong form of the least action principle for sandpiles, and a novel method of background modification. We can extend this diameter bound to certain backgrounds in which an arbitrarily high fraction of sites have height 2d-1. On the other hand, we show that if the background height 2d-2 is augmented by 1 at an arbitrarily small fraction of sites chosen independently at random, then adding finitely many particles creates an explosion (a sandpile that never stabilizes).Comment: 19 pages, 4 figures, to appear in Journal of Statistical Physics. v2 corrects the proof of the outer bound of Theorem 4.1 of arXiv:0704.068

The Gaussian Plasma Lens in Astrophysics. Refraction

We consider the geometrical optics for refraction of a distant radio source by an interstellar plasma lens, with application to a lens with a Gaussian electron column density profile. The refractive properties of the lens are specified completely by a dimensionless parameter, alpha, which is a function of the wavelength of observation, the lens' electron column density, the lens-observer distance, and the transverse diameter of the lens. Relative motion of the observer and lens produces modulations in the source's light curve. Plasma lenses are diverging so the light curve displays a minimum, when the lens is on-axis, surrounded by enhancements above the unlensed flux density. Lensing can also produce caustics, multiple imaging, and angular position wander of the background source. If caustics are formed, the separation of the outer caustics can constrain alpha, while the separation of the inner caustics can constrain the size of the lens. We apply our analysis to 0954+654, a source for which we can identify caustics in its light curve, and 1741-038, for which polarization observations were obtained during and after the scattering event. We find general agreement between modelled and observed light curves at 2.25 GHz, but poor agreement at 8.1 GHz. The discrepancies may result from a combination of lens substructure or anisotropic shape, a lens that only grazes the source, or unresolved source substructure. Our analysis places the following constraints on the lenses: Toward 0954+654 (1741-038) the lens was 0.38 AU (0.065 AU) in diameter, with a peak column density of 0.24 pc cm^{-3} (1E-4 pc cm^{-3}) and an electron density of 1E5 cm^{-3} (300 cm^{-3}). The angular wander caused by the lens was 250 mas (0.4 mas) at 2.25 GHz. For 1741-038, we place an upper limit of 100 mG on the lens' magnetic field.Comment: 26 pages, LaTeX2e using AASTeX macro aaspp4, 11 PostScript figures; to be published in Ap

USNO Analysis Center for Source Structure Report

This report summarizes the activities of the United States Naval Observatory Analysis Center for Source Structure for the 2012 calendar year and the activities planned for the year 2013

Characterisation of Long Baseline Calibrators at 2.3 GHz

We present a detailed multi-epoch analysis of 31 potential southern hemisphere radio calibrators that were originally observed as part of a program to maintain the International Celestial Reference Frame (ICRF). At radio wavelengths, the primary calibrators are Active Galactic Nuclei (AGN), powerful radio emitters which exist at the centre of most galaxies. These are known to vary at all wavelengths at which they have been observed. By determining the amount of radio source structure and variability of these AGN, we determine their suitability as phase calibrators for long baseline radio interferometry at 2.3 GHz. For this purpose, we have used a set of complementary metrics to classify these 31 southern sources into five categories pertaining to their suitability as VLBI calibrators. We find that all of the sources in our sample would be good interferometric calibrators and almost ninety per cent would be very good calibrators.Comment: 9 pages, 7 Figures, accepted MNRA

Relativistic Jets in the Radio Reference Frame Image Database. I. Apparent Speeds from the First 5 Years of Data

We present the results of an analysis of relativistic jet apparent speeds from VLBI images in the Radio Reference Frame Image Database (RRFID). The images are snapshot VLBI images at 8 and 2 GHz using the VLBA, plus up to 10 additional antennas that provide global VLBI coverage. We have analyzed the 8 GHz images from the first 5 years of the database (1994-1998), for all sources observed at three or more epochs during this time range. This subset comprises 966 images of 87 sources. The sources in this subset have an average of 11 epochs of observation over the years 1994-1998, with the best-observed sources having 19 epochs. About half of the sources in this RRFID kinematic survey have not been previously studied with multiepoch VLBI observations. We have measured apparent speeds for a total of 184 jet components in 77 sources, of which the best-measured 94 component speeds in 54 sources are used in the final analysis. The apparent speed distribution shows a peak at low apparent speeds (consistent with stationary components), a tail extending out to apparent speeds of about 30c, and a mean apparent speed of 3.6c. A total of 36 of the sources in this paper are also included in the 2 cm VLBA survey by Kellermann et al., with similar angular resolution, sensitivity, and time range. For those sources, we present a detailed component-by-component comparison of the apparent speeds measured by the 2 cm survey and those measured in this paper. Many of the independent apparent speed measurements agree very well, but for approximately 25% of the components we find significant differences in the apparent speeds measured by the two surveys. The leading cause of these discrepancies is differences in how the two surveys have identified jet components from epoch to epoch