\documentstyle [adc,11pt]{article} % \newcommand{\dmv}{Documentation for the Computer-Readable Dataset} \newcommand{\cat}{Results of a Five--year Program of Multifrequency Monitoring\\ of Low--frequency Variable Radio Sources} % % INVESTIGATORS: % % K. J. Mitchell General Sciences Corporation % 6100 Chevy Chase Drive, Laurel MD 20707 % % Brian Dennison Department of Physics and Center for % Stochastic Processes in Science and Engineering % Virginia Tech, Blacksburg, Virginia 24061 % % J. J. Condon National Radio Astronomy Observatory % Edgemont Road, Charlottesville, Virginia 22903 % % Daniel R. Altschuler National Astronomy and Ionosphere Center % P. O. Box 995, Arecibo, Puerto Rico 00612 and % Faculty of Natural Sciences, % Department of Physics, University of Puerto Rico % Rio Piedras, Puerto Rico 00931 % % H. E. Payne Space Telescope Science Institute % 3700 San Martin Drive, Baltimore, MD 21218 % % S. L. O'Dell Space Science Lab, Marshall Space Flight Center % Huntsville, AL 35812 % % J. J. Broderick Department of Physics, Virginia Tech % Blacksburg, VA 24061 % % For further information please contact Brian Dennison at % dennison@astro.phys.vt.edu % \newcommand{\abbr}{LOWVAR} \newcommand{\catref}{Mitchell, et al. (1994)} \newcommand{\pcatref}{K. J. Mitchell, Brian Dennison, J. J. Condon, Daniel R. Altschuler,\\ H. E. Payne, S. L. O'Dell and J. J. Broderick} \newcommand{\docnum}{NSSDC/WDC-A-R\&S 93-**} % \begin{document} % \title{\cat} \author{(\pcatref)} \docauthor{Brian Dennison \linebreak {\it (Department of Physics and Center for Stochastic Processes in Science \\and Engineering, Virginia Tech, Blacksburg, Virginia 24061) }} \docno{\docnum} \date{} \date{October 1993} \maketitle \begin{abstract} The {\em Lowvar} dataset is the result of a detailed multifrequency monitoring program of 34 low--frequency variable radio sources. This consists of flux density measurements at 318, 430, 606, 880, and 1400 MHz over a five year period. The observations were carried out with the Arecibo 305--m radiotelescope and the Green Bank 91--m radiotelescope. \end{abstract} \section{Introduction and Source Reference} {\bf this documentation should be distributed with the data file.} \subsection{Introduction} These observations were motivated primarily by the need to understand the phenomenon of low frequency ($\nu < 1$ GHz) variability in extragalactic radio sources. The observed timescales typically imply brightness temperatures orders of magnitude in excess of the $10^{12}$ K Compton limit for stationary incoherent electron-synchrotron sources. Thus it appeared that our understanding of compact radio sources was seriously incomplete, or that some additional, poorly understood phenoma, such as extrinsic propagation effects, were occurring. In order to delineate the spectral properties of low-frequency variability, 34 low-frequency variable sources were monitored at five frequencies between 0.3 and 1.4 GHz between 1980.0 and 1985.0. These sources were previously found to be variable or probably variable in two- and three-epoch observations at 318 MHz of complete samples of extragalactic sources (Condon et al. 1979, Dennison et al. 1981). The flux density measurements were carried out at 318, 430 and 606 MHz using the NAIC\footnote{The National Astronomy and Ionosphere Center is operated by Cornell University under contract with the National Science Foundation.} Arecibo 305-m radio telescope, and at 880 and 1400 MHz using the NRAO\footnote{The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under contract with the National Science Foundation.} 91-m radio telescope. Partial results of these observations on selected sources were reported by Payne et al. (1982), Altschuler et al. (1984), Dennison et al. (1984a), and Dennison et al. (1984b). The complete results (tabulated in the accompanying dataset) and the observations are described in detail by Mitchell et al. (1993). The {\em Lowvar} dataset is expected to be useful for studies of radio source variability caused by extrinsic and intrinsic mechanisms, as both appear to play a role in the observed variations. These data may also be of use in comparing variations at different wavebands. \subsection {Source Reference} \begin{references} \item ``Results of a Five--year Program of Multifrequency Monitoring of Low--frequency Variable Radio Sources'' 1993, Mitchell, K. J., Dennison, B., Condon, J. J., Altschuler, D. R., Payne, H. E., O'Dell, S. L., and Broderick, J. J., ApJS (submitted). \end {references} \section {Structure and Content of the {\em Lowvar} Dataset} The {\em Lowvar} dataset is distributed with this document as a single file, containing 3067 records, of 25 bytes each. Each record consists of source name (IAU), a frequency code, date of observation, measured flux density and error. The records are ordered by source right ascension, frequency, and date of observation. The user of these data is cautioned that the quoted errors do not reflect uncertainties in absolute scale, which vary with source and frequency, but are constant in time. Hence small temporal variations can be identified, although in some cases the absolute scale may be uncertain by $\approx 10\%$. The source reference (Mitchell et al. 1993) should be consulted before attempting to use these data. The file format of a record is given Table 1. To assure correct transcription of the data \begin{table} [htbp] \begin{center} \begin{tabular}{ c l l l l } \hline & & & & \\ Bytes & Format & Units & Label & Description \\ \hline 1--7 & A7 & --- & Name & Source Name \\ 8 & I1 & --- & c\_Freq & [1-5]! Frequency Code \\ & & & & \phantom{xxxxx}1 for 318 MHz \\ & & & & \phantom{xxxxx}2 for 430 MHz \\ & & & & \phantom{xxxxx}3 for 606 MHz \\ & & & & \phantom{xxxxx}4 for 880 MHz \\ & & & & \phantom{xxxxx}5 for 1400 MHz \\ 9--14 & & & & Date of Observation \\ 9--10 & I2 & yr & Year & \phantom{xxxxx}Year $-$ 1900 \\ 11--12 & I2 & --- & Month & \phantom{xxxxx}Month \\ 13--14 & I2 & d & Day & \phantom{xxxxx}Day \\ 16--20 & F5.2 & Jy & S & Flux Density \\ 21--25 & F5.2 & Jy & e\_S & Flux Density Error \\ \hline \end{tabular} \caption{Data Format for {\em Lowvar} dataset} \end{center} \label{T:form} \end{table} \section{Notes to the Catalog} The source 0723--00 is suspected of having a large scale error at 430 MHz, owing to the large zenith angle ($19.5^\circ$) used at that frequency, which is near the Arecibo telescope limit. This source is included, however, as real and significant variability is clearly present at all frequencies, including 430 MHz. \section{Acknowledgments} We thank the technical staffs of the NAIC at the Arecibo Observatory and the NRAO in Green Bank. This research was supported by NSF grants AST 81--17864 to Virginia Polytechnic Institute and State University and AST 87--15532 to the University of Puerto Rico. \section{References} \begin{references} \item Altschuler, D. R., Broderick, J. J., Condon, J. J., Dennison, B. K., Mitchell, K. J., O'Dell, S. L. and Payne, H. E. 1984, AJ, 89, 1784 \item Condon, J. J., Ledden, J. E., O'Dell, S. L., \& Dennison, B. 1979, AJ, 84, 1 \item Dennison, B., Broderick, J. J., Ledden, J. E., O'Dell, S. L., \& Condon, J. J. 1981, AJ, 86, 1604 \item Dennison, B., Broderick, J. J., O'Dell, S. L., Mitchell, K. J., Altschuler, D. R., Payne, H. E., \& Condon, J. J. 1984a, ApJL, 281, L55 \item Dennison, B., Broderick, J. J., O'Dell, S. L., Mitchell, K. J., Altschuler, D. R., Payne, H. E., \& Condon, J. J. 1984b, in VLBI and Compact Radio Sources --- IAU Symposium No. 110, ed. R. Fanti, K. I. Kellermann, and G. Setti (Dordrecht:Reidel), 309 \item Mitchell, K. J., Dennison, B., Condon, J. J., Altschuler, D. R., Payne, H. E., O'Dell, S. L., and Broderick, J. J. 1993, ApJS (submitted) \item Payne, H. E., Altschuler, D. R., Broderick, J. J., Condon, J. J., Dennison, B., and O'Dell, S. L. 1982, in Low--Frequency Variability of Extragalactic Radio Sources, ed. W. D. Cotton and S. R. Spangler Green Bank:NRAO), 9 \end{references} \end{document}