barycorr is a multi-mission tool for applying barycenter corrections to X-ray timing data. The tool is designed to apply to data from RXTE, Swift, Chandra/AXAF, NuSTAR and NICER.
barycorr recognizes time-related columns and keywords within X-ray FITS files, and corrects these time values to the equivalent time at the solar system barycenter (SSB), expressed in TDB. barycorr applies geometric light-time calculations, relativistic corrections, and mission specific clock offset corrections. If a photon arrived at the given observatory timestamp, barycorr calculates the time that the same photon would have arrived at the solar system barycenter from the target.
barycorr uses the observatory orbit ephemeris file orbitFile, target position (ra, dec), and refframe "FK5" or "ICRS." When RA and Dec are not provided, an effort is made to retrieve the target information from the FITS file.
barycorr recognizes the mission-specific components of a FITS input file according by reading the MISSION keyword.
In principle, it is possible to modify an input file "in place" by specifying the same file name for infile and outfile. This is not recommended however, since the original file is lost. barycorr will create temporary files in the directory where the output file is intended to reside, and not move the output file into place until the barycenter correction operation is successful.
By default, barycorr will choose JPL planetary ephemeris DE-200 if refframe="FK5" and DE-405 if refframe="ICRS". The user can override this choice with another more recent ephemeris, and must specify refframe="ICRS" in that case. The ephemeris file must be placed in $LHEA_DATA/JPLEPH.NNN where NNN is the ephemeris version, and set ephem="JPLEPH.NNN". As of June 2020, the following ephemerides are available in the standard HEASoft release. The required keyword parameters to barycorr are listed in parentheses to select the indicated ephemeris.
The "barytime = no" option (default) will replace the values in the original TIME (case-insensitive) column from the input file with the barycenter-corrected times and update all time-related keywords and GTI values appropriately. Use of this option on binned (ie, light curve) data files is discouraged as it will preclude subsequent application of tools that expect evenly spaced time intervals to the modified datafile.
The "barytime = yes" option will simply append a new column, "BARYTIME", to a copy of the original file. No changes will be made to the time-related keywords or GTI table(s) in this case. Thus, barytime=yes is in a sense non-destructive, but will not allow one to fully utilize downstream software that requires time-related keywords or GTIs. For that reason, it is recommended to use "barytime=no" but not overwrite the original input file.
Do not ever use DE405 on a FITS file that has:
TIMESYS="TDB"
but not:
RADECSYS="ICRS" or PLEPHEM="JPL-DE405"
TIMESYS="TDB" _with_ RADECSYS="ICRS" and/or PLEPHEM="JPL-DE405"
should use ephem=JPLEPH.405.
TIMESYS="TDB" _without_ RADECSYS="ICRS" or PLEPHEM="JPL-DE405"
should use ephem=JPLEPH.200.
DE405 may be used in conjunction with FK5 spacecraft orbit ephemeris. The maximum error is 2 ns for each earth radius that the spacecraft is removed from the geocenter. All positions provided by Tempo are on the DE200 reference frame, however - closer to FK5. This could introduce errors of up to 0.02 ms. A function, c200to405, is provided to convert DE200 positions to ICRS. Even so, it is recommended that DE200 is used for Tempo solutions that are based on DE200; apparently, efforts are underway to make Tempo support DE405.
Performing only geocenter-to-barycenter correction. If spacecraft orbit files are not available, it is still possible to perform correction from the geocenter to the solar system barycenter, by setting orbitfiles=GEOCENTER. In that case, all input timestamps will be considered measured at the geocenter, and only the correction from the geocenter to the solar system barycenter is computed. This is really only useful when the errors in doing so (about 25 msec maximum error for low earth orbit satellites) are within scientific tolerances. So for example, long period pulsars or orbits where a 25 msec error is insigificant, or long term light curves with bin sizes much longer than 25 msec, would be useful candidates for setting orbitfiles=GEOCENTER.
RXTE: The barycorr tool applies a RXTE clock offset correction, which is normally stored in the file named $LHEA_DATA/tdc.dat. The RXTE mission is now complete and no further clock offset corrections are expected.
NuSTAR: The barycorr tool applies a NuSTAR clock offset correction, if the clockfile parameter is supplied. If the parameter is not supplied (clockfile='NONE'), then the user can expect accuracies of the magnitude +/- 100 msec. As of early 2019, a new type of fine clock correction capable of reaching 20-30 usec precision is available; barycorr v2.2 or later will first try to use the fine correction and if not available, fall back to the coarser correction (with errors in the msec range).
NICER: The barycorr tool does not need to apply any special corrections since the on-board clock is synchronized to GPS.
1. run barycorr prompting for all required parameters:
barycorr
3. run barycorr specifying the RA/DEC (with required parameters prompted for):
barycorr ra=272.4338 dec=-19.7495