High-Fidelity VLA Imaging of the Radio Structure of 3C273

3C273, the nearest bright quasar, comprises a strong nuclear core and a bright, one-sided jet extending ~ 23 arcseconds to the SW. The source has been the subject of imaging campaigns in all wavebands. Extensive observations of this source have been made with the Very Large Array and other telescopes as part of a campaign to understand the jet emission mechanisms. Partial results from the VLA radio campaign have been published, but to date, the complete set of VLA imaging results has not been made available. We have utilized the VLA to determine the radio structure of 3C273 in Stokes I, Q, and U, over the widest possible frequency and resolution range. The VLA observed the source in all four of its configurations, and with all eight of its frequency bands, spanning 73.8 MHz to 43 GHz. The data were taken in a pseudo-spectral line mode to minimize the VLA's correlator errors, and were fully calibrated with subsequent self-calibration techniques to maximise image fidelity. Images in Stokes parameters I, Q, and U, spanning a resolution range from 6 arcseconds to 88 milliarcseconds are presented. Spectral index images, showing the evolution of the jet component are shown. Polarimetry demonstrates the direction of the magnetic fields responsible for the emission, and rotation measure maps show the RM to be very small with no discernible trend along or across the jet. This paper presents a small subset of these images to demonstrate the major characteristics of the source emission. A library of all ~500 images has been made available for open, free access by interested parties.Comment: 9 pages, 17 figure

A new technique to improve RFI suppression in radio interferometers

Radio interferometric observations are less susceptible to radio frequency interference (RFI) than single dish observations. This is primarily due to : (1)fringe-frequency averaging at the correlator output and (2) bandwidth decorrelation of broadband RFI. Here, we propose a new technique to improve RFI suppression of interferometers by replacing the fringe-frequency averaging process with a different filtering process. In the digital implementation of the correlator, such a filter should have cutoff frequencies $< 10^{-6}$ times the frequency at which the baseband signals are sampled. We show that filters with such cutoff frequencies and attenuation $>$ 40 dB at frequencies above the cutoff frequency can be realized using multirate filtering techniques. Simulation of a two element interferometer system with correlator using multirate filters shows that the RFI suppression at the output of the correlator can be improved by 40 dB or more compared to correlators using a simple averaging process.Comment: 12 pages, 7 figures; Invited talk given at IVS Symposium in Korea -- New Technologies in VLBI, Korea, Nov 2002; to appear in the conference proceedings (Added answers to the questions during the discussion session

VLA Observations of the Gravitational Lens System Q2237+0305

We report observations of the four-image gravitational lens system Q2237+0305 with the VLA at 20 cm and 3.6 cm. The quasar was detected at both frequencies (\approx 0.7 mJy) with a flat spectrum. All four lensed images are clearly resolved at 3.6 cm, and the agreement of the radio and optical image positions is excellent. No radio emission is detected from the lensing galaxy, and any fifth lensed quasar image must be fainter than \sim 20% of the A image flux density. Since the optical quasar images are variable and susceptible to extinction, radio flux ratios provide the best measurement of the macrolensing magnification ratios. The radio B/A and C/A image flux ratios are consistent with the observed range of optical variations, but the D/A ratio is consistently higher in the radio than in the optical. The radio ratios are consistent with magnification ratios predicted by lens models, and weaken alternative interpretations for Q2237+0305. More accurate radio ratios can distinguish between the models, as well as improve our understanding of both microlensing and extinction in this system.Comment: 1 postscript file, 13 pages. To appear in AJ (1996.09), Submitted 1996.03.13, Accepted 1996.05.2

Relativistic and slowing down: the flow in the hotspots of powerful radio galaxies and quasars

Pairs of radio emitting jets with lengths up to several hundred kiloparsecs emanate from the central region (the `core') of radio loud active galaxies. In the most powerful of them, these jets terminate in the `hotspots', compact high brightness regions, where the jet flow collides with the intergalactic medium (IGM). Although it has long been established that in their inner ($\sim$parsec) regions these jet flows are relativistic, it is still not clear if they remain so at their largest (hundreds of kiloparsec) scales. We argue that the X-ray, optical and radio data of the hotspots, despite their at-first-sight disparate properties, can be unified in a scheme involving a relativistic flow upstream of the hotspot that decelerates to the sub-relativistic speed of its inferred advance through the IGM and viewed at different angles to its direction of motion. This scheme, besides providing an account of the hotspot spectral properties with jet orientation, it also suggests that the large-scale jets remain relativistic all the way to the hotspots.Comment: to appear in ApJ

Gamma-ray Bursts, Classified Physically

From Galactic binary sources, to extragalactic magnetized neutron stars, to long-duration GRBs without associated supernovae, the types of sources we now believe capable of producing bursts of gamma-rays continues to grow apace. With this emergent diversity comes the recognition that the traditional (and newly formulated) high-energy observables used for identifying sub-classes does not provide an adequate one-to-one mapping to progenitors. The popular classification of some > 100 sec duration GRBs as ``short bursts'' is not only an unpalatable retronym and syntactically oxymoronic but highlights the difficultly of using what was once a purely phenomenological classification to encode our understanding of the physics that gives rise to the events. Here we propose a physically based classification scheme designed to coexist with the phenomenological system already in place and argue for its utility and necessity.Comment: 6 pages, 3 figures. Slightly expanded version of solicited paper to be published in the Proceedings of ''Gamma Ray Bursts 2007,'' Santa Fe, New Mexico, November 5-9. Edited by E. E. Fenimore, M. Galassi, D. Palme