David B. Shaffer

Publications (2)31.2 Total impact

• Source

  Available from: adsabs.harvard.edu

  Article: High dynamic range radio observations of PKS 1413+135: A BL Lacertae object with a parsec-scale counterjet

  Eric S. Perlman, John T. Stocke, David B. Shaffer, Chris L. Carilli, Chopo Ma
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  ABSTRACT: We report the results of three high dynamic range, high-resolution radio observations with the Very Large Array (VLA), US Very Long Base Interferometry (VLBI) Network, and partially completed Very Long Baseline Array (VLBA) on the peculiar BL Lacertae objects PKS 1413+135. The VLA observations (resolution approximately 1.5 sec) reveal that PKS 1413+135 has no kiloparsec-scale extended structure to a dynamic range limit of 10,000:1. However, its milliarcsecond-scale structure appears to be a triple (i.e., a core, jet plus 'counterjet') at both 3.6 and 18 cm, a unique structure among BL Lac objects but similar to a recently discovered class of VLBI 'mini-triples.' The presence of a counterjet at fluxes comparable to the main jet is incompatible with the standard relativistically beamed jet model for BL Lac objects and with the high value of core dominance exhibited by this source at arcsec resolution. This suggests a nonstandard interpretation in which the radio source lies far in the background of the spiral galaxy it is projected upon so that its VLBI structure may be affected by gravitational lensing. At 18 cm, the core is much weaker than at 3.6 cm suggesting free-free absorption by the high-column-density gas found along this sightline by previous infrared(IR)/optical, X-ray and redshifted H I 21 cm observations. We discuss the roles that free-free absorption and gravitational lensing may have for PKS 1413+135 and other GHz-peaked spectrum radio sources.
  05/1994;
• Article: Very-Long-Baseline Radio Interferometry: The Mark III System for Geodesy, Astrometry, and Aperture Synthesis

  ALAN E. E. ROGERS, ROGER J. CAPPALLO, HANS F. HINTEREGGER, JAMES I. LEVINE, EDWIN F. NESMAN, JOHN C. WEBBER, ALAN R. WHITNEY, THOMAS A. CLARK, CHOPO MA, JAMES RYAN, [......], TOMAS A. HERRING, IRWIN I. SHAPIRO, CURTIS A. KNIGHT, DAVID B. SHAFFER, NANCY R. VANDENBERG, RICHARD LACASSE, ROBERT MAUZY, BENNO RAYHRER, BRUCE R. SCHUPLER, J. C. PIGG
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  ABSTRACT: The Mark III very-long-baseline interferometry (VLBI) system allows recording and later processing of up to 112 megabits per second from each radio telescope of an interferometer array. For astrometric and geodetic measurements, signals from two radio-frequency bands (2.2 to 2.3 and 8.2 to 8.6 gigahertz) are sampled and recorded simultaneously at all antenna sites. From these dual-band recordings the relative group delays of signals arriving at each pair of sites can be corrected for the contributions due to the ionosphere. For many radio sources for which the signals are sufficiently intense, these group delays can be determined with uncertainties under 50 picoseconds. Relative positions of widely separated antennas and celestial coordinates of radio sources have been determined from such measurements with 1 standard deviation uncertainties of about 5 centimeters and 3 milliseconds of arc, respectively. Sample results are given for the lengths of baselines between three antennas in the United States and three in Europe as well as for the arc lengths between the positions of six extragalactic radio sources. There is no significant evidence of change in any of these quantities. For mapping the brightness distribution of such compact radio sources, signals of a given polarization, or of pairs of orthogonal polarizations, can be recorded in up to 28 contiguous bands each nearly 2 megahertz wide. The ability to record large bandwidths and to link together many large radio telescopes allows detection and study of compact sources with flux densities under 1 millijansky.
  Science 01/1983; 219(4580):51-54. · 31.20 Impact Factor