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Barycentric correction of timing data

For precise timing analysis, the detector and source should be an inertial frame. However, Swift is in orbit around the Earth, and the Earth is in orbit around the Sun; there are also general relativistic effects caused by the gravitational potential wells of the Earth and the Sun. To account for these effects, a barycentric correction should be applied to data before high time resolution analysis is performed.


barycorr is a multi-mission FTool designed to apply barycentric corrections to FITS files. In the case of Swift data, the clock correction file swclockcor20041120vNNN.fits is required. By default, barycorr will look for this file in the CALDB. Note that this file is regularly updated.

> barycorr ra=279.2375 dec=-28.927021
> Input file name: [ ] sw00033705132xwtw2po_cl.evt
> Output file name: [ ] sw00033705132xwtw2po_cl_bary.evt
> Orbit ephemeris file(s) [ ] sw00033705132sao.fits

It is best to provide the RA and Dec (in decimal degrees) of your source; if no coordinates are given, barycorr will try to determine the pointing information from the input FITS file, which may not exactly match the source position. The *sao.fits file is found in the auxil directory for the relevant dataset and contains information about the orbit ephemeris.

Further information about this tool can be found in the fhelp.

This barycentrically-corrected eventlist (the arrival time for each event has been corrected) can then be used within XSELECT to extract light-curves as required.

barycorr should also be run on the * file (created when xrtpipeline is run) and the *pat.fits file (in the auxil directory within the downloaded data). When xrtlccorr is run, these barycentrically corrected files should be used.

There was a bug in barycorr for versions of HEASoft before 6.22.

Users should also be aware that in the default eventlists (i.e. those downloaded from the archive or Quick Look site) the DATE-OBS and DATE-END keywords in the headers of the FITS files are NOT usually corrected for this clock offset (CLOCKAPP keyword in the header set to F), although the TSTART and TSTOP keywords are accurate. Once barycorr has been run on an eventlist, the DATE-OBS/DATE-END keywords are corrected, both for the clock offset and the barycentric shift, but the MJD-OBS keyword remains uncorrected for either of these.

See this page about Guide to Times in Swift FITS Files for an explanation of the time formats.