  This paper describes the VARTOOLS program, which is an open-source
command-line utility, written in C, for analyzing astronomical time-series
data, especially light curves. The program provides a general-purpose set of
tools for processing light curves including signal identification, filtering,
light curve manipulation, time conversions, and modeling and simulating light
curves. Some of the routines implemented include the Generalized Lomb-Scargle
periodogram, the Box-Least Squares transit search routine, the Analysis of
Variance periodogram, the Discrete Fourier Transform including the CLEAN
algorithm, the Weighted Wavelet Z-Transform, light curve arithmetic, linear and
non-linear optimization of analytic functions including support for Markov
Chain Monte Carlo analyses with non-trivial covariances, characterizing and/or
simulating time-correlated noise, and the TFA and SYSREM filtering algorithms,
among others. A mechanism is also provided for incorporating a user's own
compiled processing routines into the program. VARTOOLS is designed especially
for batch processing of light curves, including built-in support for parallel
processing, making it useful for large time-domain surveys such as searches for
transiting planets. Several examples are provided to illustrate the use of the
program.
