\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} 

