Near-Field Radio Holography of Large Reflector Antennas

We summarise the mathematical foundation of the holographic method of measuring the reflector profile of an antenna or radio telescope. In particular, we treat the case, where the signal source is located at a finite distance from the antenna under test, necessitating the inclusion of the so-called Fresnel field terms in the radiation integrals. We assume a ``full phase'' system with reference receiver to provide the reference phase. We describe in some detail the hardware and software implementation of the system used for the holographic measurement of the 12m ALMA prototype submillimeter antennas. We include a description of the practicalities of a measurement and surface setting. The results for both the VertexRSI and AEC (Alcatel-EIE-Consortium) prototype ALMA antennas are presented.Comment: 14 pages, 14 figures, to appear in IEEE Antennas and Propagation Magazine, Vol. 49, No. 5, October 2007. Version 2 includes nice mug-shots of the author

Radio Emission Signatures in the Crab Pulsar

Our high time resolution observations of individual pulses from the Crab pulsar show that both the time and frequency signatures of the interpulse are distinctly different from those of the main pulse. Main pulses can occasionally be resolved into short-lived, relatively narrow-band nanoshots. We believe these nanoshots are produced by soliton collapse in strong plasma turbulence. Interpulses at centimeter wavelengths are very different. Their dynamic spectrum contains regular, microsecond-long emission bands. We have detected these bands, proportionately spaced in frequency, from 4.5 to 10.5 GHz. The bands cannot easily be explained by any current theory of pulsar radio emission; we speculate on possible new models.Comment: 26 pages, 10 figures, to appear in Ap

Photometric Variability and Astrometric Stability of the Radio Continuum Nucleus in the Seyfert Galaxy NGC 5548

The NRAO VLA and VLBA were used from 1988 November to 1998 June to monitor the radio continuum counterpart to the optical broad line region (BLR) in the Seyfert galaxy NGC 5548. Photometric and astrometric observations were obtained at 21 epochs. The radio nucleus appeared resolved, so comparisons were limited to observations spanning 10-60 days and 3-4 yr, and acquired at matched resolutions of 1210 mas = 640 pc (9 VLA observations), 500 mas = 260 pc (9 VLA observations), or 2.3 mas = 1.2 pc (3 VLBA observations). The nucleus is photometrically variable at 8.4 GHz by $33\pm5$% and $52\pm5$% between VLA observations separated by 41 days and 4.1 yr, respectively. The 41-day changes are milder ($19\pm5$%) at 4.9 GHz and exhibit an inverted spectrum ($\alpha \sim +0.3\pm0.1$, $S\propto \nu ^{+\alpha}$) from 4.9 to 8.4 GHz. The nucleus is astrometrically stable at 8.4 GHz, to an accuracy of 28 mas = 15 pc between VLA observations separated by 4.1 yr and to an accuracy of 1.8 mas = 0.95 pc between VLBA observations separated by 3.1 yr. Such photometric variability and astrometric stability is consistent with a black hole being the ultimate energy source for the BLR, but is problematic for star cluster models that treat the BLR as a compact supernova remnant and, for NGC 5548, require a new supernova event every 1.7 yr within an effective radius $r_e =$ 42 mas = 22 pc. A deep image at 8.4 GHz with resolution 660 mas = 350 pc was obtained by adding data from quiescent VLA observations. This image shows faint bipolar lobes straddling the radio nucleus and spanning 12 arcsec = 6.4 kpc. These synchrotron-emitting lobes could be driven by twin jets or a bipolar wind from the Seyfert 1 nucleus.Comment: with 9 figures, to appear in the Astrophysical Journal, 2000 March 10, volume 53

Constructing applicative functors

Applicative functors define an interface to computation that is more general, and correspondingly weaker, than that of monads. First used in parser libraries, they are now seeing a wide range of applications. This paper sets out to explore the space of non-monadic applicative functors useful in programming. We work with a generalization, lax monoidal functors, and consider several methods of constructing useful functors of this type, just as transformers are used to construct computational monads. For example, coends, familiar to functional programmers as existential types, yield a range of useful applicative functors, including left Kan extensions. Other constructions are final fixed points, a limited sum construction, and a generalization of the semi-direct product of monoids. Implementations in Haskell are included where possible

Evidence for Proportionate Partition Between the Magnetic Field and Hot Gas in Turbulent Cassiopeia A

We present a deep X-ray observation of the young Galactic supernova remnant Cas A, acquired with the ROSAT High Resolution Imager. This high dynamic range (232 ks) image reveals low-surface-brightness X-ray structure, which appears qualitatively similar to corresponding radio features. We consider the correlation between the X-ray and radio morphologies and its physical implications. After correcting for the inhomogeneous absorption across the remnant, we performed a point by point (4" resolution) surface brightness comparison between the X-ray and radio images. We find a strong (r = 0.75) log-log correlation, implying an overall relationship of $\log(\Sigma_{_{\rm X-ray}}) \propto (2.21\pm0.05) \times \log(\Sigma_{_{\rm radio}})$. This is consistent with proportionate partition (and possibly equipartition) between the local magnetic field and the hot gas --- implying that Cas A's plasma is fully turbulent and continuously amplifying the magnetic field.Comment: 8 pages with embedded bitmapped figures, Accepted by ApJ Letters 5/1/9

Cicrumnuclear Supernova Remnants and HII Regions in NGC 253

Archival VLA data has been used to produce arcsecond-resolution 6- and 20-cm images of the region surrounding the nuclear 200-pc (~15") starburst in NGC 253. Twenty-two discrete sources stronger than 0.4 mJy have been detected within ~2 kpc (~3') of the galaxy nucleus; almost all these sources must be associated with the galaxy. None of the radio sources coincides with a detected X-ray binary, so they appear to be due to supernova remnants and H II regions. The region outside the central starburst has a derived radio supernova rate of <~0.1/yr, and may account for at least 20% of the recent star formation in NGC 253. Most of the newly identified sources have steep, nonthermal radio spectra, but several relatively strong thermal sources also exist, containing the equivalent of tens of O5 stars. These stars are spread over tens of parsecs, and are embedded in regions having average ionized gas densities of 20-200/cm^3, much lower than in the most active nuclear star-forming regions in NGC 253 or in the super star clusters seen in other galaxies. The strongest region of thermal emission coincides with a highly reddened area seen at near-infrared wavelengths, possibly containing optically obscured H II regions.Comment: 17 pages, 3 postscript figures, AASTeX format, in press for Astronomical Journal, July 200

A Wide Field, Low Frequency Radio Survey of the Field of M31: I. Construction and Statistical Analysis of the Source Catalog

We present here the results of a 325 MHz radio survey of M31, conducted with the A-configuration of the Very Large Array. The survey covered an area of 7.6 deg$^2$, and a total of 405 radio sources between \la6\arcsec and 170\arcsec in extent were mapped with a resolution of 6\arcsec and a 1$\sigma$ sensitivity of $\sim$0.6 \mjyb. For each source, its morphological class, major axis $\theta_M$, minor axis $\theta_m$, position angle $\theta_{PA}$, peak flux $I$, integrated flux density $S$, spectral index $\alpha$ and spectral curvature parameter $\phi$ were calculated. A comparison of the flux and radial distribution -- both in the plane of the sky and in the plane of M31 -- of these sources with those of the XMM--LSS and WENSS radio surveys revealed that a vast majority of sources detected are background radio galaxies. As a result of this analysis, we expect that only a few sources are intrinsic to M31. These sources are identified and discussed in an accompanying paper.Comment: 29 pages with 4 tables and 10 figures (JPEGs), accepted for publication in ApJS. Full-resolution images available on reques

A Compact Extreme Scattering Event Cloud Towards AO 0235+164

We present observations of a rare, rapid, high amplitude Extreme Scattering Event toward the compact BL-Lac AO 0235+164 at 6.65 GHz. The ESE cloud is compact; we estimate its diameter between 0.09 and 0.9 AU, and is at a distance of less than 3.6 kpc. Limits on the angular extent of the ESE cloud imply a minimum cloud electron density of ~ 4 x 10^3 cm^-3. Based on the amplitude and timescale of the ESE observed here, we suggest that at least one of the transients reported by Bower et al. (2007) may be attributed to ESEs.Comment: 11 pages, 2 figure

Mapping the CMB II: the second flight of the QMAP experiment

We report the results from the second flight of QMAP, an experiment to map the cosmic microwave background near the North Celestial Pole. We present maps of the sky at 31 and 42 GHz as well as a measurement of the angular power spectrum covering the l-range 40-200. Anisotropy is detected at about 20 sigma and is in agreement with previous results at these angular scales. We also report details of the data reduction and analysis techniques which were used for both flights of QMAP.Comment: 4 pages, with 5 figures included. Submitted to ApJL. Window functions and color figures are available at http://pupgg.princeton.edu/~cmb/welcome.htm