We develop an absolute flux density scale for cm-wavelength astronomy by combining accurate flux density ratios determined by the VLA between the planet Mars and a set of potential calibrators with the Rudy thermophysical emission model of Mars, adjusted to the absolute scale established by WMAP. The radio sources 3C123, 3C196, 3C286 and 3C295 are found to be varying at a level of less than ∼5% per century at all frequencies between 1 and 50 GHz, and hence are suitable as flux density standards. We present polynomial expressions for their spectral flux densities, valid from 1 to 50 GHz, with absolute accuracy estimated at 1 -3 % depending on frequency. Of the four sources, 3C286 is the most compact and has the flattest spectral index, making it the most suitable object on which to establish the spectral flux density scale. The sources 3C48, 3C138, 3C147, NGC7027, NGC6542, and MWC349 show significant variability on various timescales. Polynomial coefficients for the spectral flux density are developed for 3C48, 3C138, and 3C147 for each of the seventeen observation dates, spanning 1983 through 2012. The planets Venus, Uranus, and Neptune are included in our observations, and we derive their brightness temperatures over the same frequency range.
The flux-density scale of Perley & Butler is extended downward to ∼50 MHz by utilizing recent observations with the Karl G. Jansky Very Large Array (VLA) of 20 sources between 220 MHz and 48.1 GHz, and legacy VLA observations at 73.8 MHz. The derived spectral flux densities are placed on an absolute scale by utilizing the Baars et al. values for Cygnus A (3C405) for frequencies below 2 GHz, and the Mars-based polynomials for 3C286, 3C295, and 3C196 from Perley & Butler above 2 GHz. Polynomial expressions are presented for all 20 sources, with accuracy limited by the primary standards to 3%–5% over the entire frequency range. Corrections to the scales proposed by Perley & Butler, and by Scaife & Heald are given.
Measurement of the time delay between multiple images of a gravitational lens system is potentially an accurate method of determining the Hubble constant over cosmological distances. One of the most promising candidates for an application of this technique is the system B0218+357 which was found in the Jodrell Bank/VLA Astrometric Survey (JVAS). This system consists of two images of a compact radio source, separated by 335 milliarcsec, and an Einstein ring which can provide a strong constraint on the mass distribution in the lens. We present here the results of a three-month VLA monitoring campaign at two frequencies. The data are of high quality and both images show clear variations in total flux density, percentage polarization and polarization position angle at both frequencies. The time delay between the variations in the two images has been calculated using a chi-squared minimization to be 10.5 ± 0.4 days at 95 per cent confidence, with the error being derived from Monte-Carlo simulations of the light curves. Although mass modelling of the system is at a preliminary stage, taking the lensing galaxy to be a singular isothermal ellipsoid and using the new value for the time delay gives a value for the Hubble constant of 69 +13 −19 km s −1 Mpc −1 , again at 95 per cent confidence.
We discuss 6 GHz JVLA observations covering a volume-limited sample of 178 low redshift (0.2 < z < 0.3) optically selected QSOs. Our 176 radio detections fall into two clear categories: (1) About 20% are radio-loud QSOs (RLQs) having spectral luminosities L 6 10 23.2 W Hz −1 primarily generated in the active galactic nucleus (AGN) responsible for the excess optical luminosity that defines a bona fide QSO. (2) and radio sizes 10 kpc, and we suggest that the bulk of their radio emission is powered by star formation in their host galaxies. "Radio silent" QSOs (L 6 10 21 W Hz −1 ) are rare, so most RQQ host galaxies form stars faster than the Milky Way; they are not "red and dead" ellipticals. Earlier radio observations did not have the luminosity sensitivity L 6 10 21 W Hz −1 needed to distinguish between such RLQs and RQQs. Strong, generally double-sided, radio emission spanning ≫ 10 kpc was found associated with 13 of the 18 RLQ cores having peak flux densities S p > 5 mJy beam −1 (log(L) 24). The radio luminosity function of optically selected QSOs and the extended radio emission associated 1 The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.-2 -with RLQs are both inconsistent with simple "unified" models that invoke relativistic beaming from randomly oriented QSOs to explain the difference between RLQs and RQQs. Some intrinsic property of the AGNs or their host galaxies must also determine whether or not a QSO appears radio loud.
We present the integrated polarization properties of the four compact radio sources 3C48, 3C138, 3C147 and 3C286, from 1 to 50 GHz, over a 30-year time frame spanning 1982 to 2012. These four sources are commonly used as flux density and polarization calibrators for cm-wave interferometers. Using the polarized emission of Mars, we have determined that the true position angle of the linearly polarized emission of 3C286 rises from 33 degrees at 8 GHz to 36 degrees at 45 GHz. There is no evidence for a change in the position angle over time. Using these values, the position angles of the intergrated polarized emission from the other sources are determined as a function of frequency and time. The fractional polarization of 3C286 is found to be slowly rising, at all frequencies, at a rate of ∼ 0.015%/year. The fractional polarizations of 3C48, 3C138, and 3C147 are all slowly variable, with the variations clearly correlated with changes in the total flux densities of these sources.
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