XMM-Newton at MPE - Barycentric correction

The arrival time listed in the `original' photon event table is the local frame time of the satellite counted as the XMM Mission Elapsed Times with the starting time 1998-01-01 00:00:00. However, the satellite is moving, so a) the distance between the source and the satellite is changing and b) the satellite is moving towards or away from the source. This causes different absolute arrival times of photons at the location of the satellite. If you are interested in very short time periods, such as they appear in pulsars of cataclysmic variables, you have to perform a a barycentric correction. This means that the arrival time of a photon is shifted as is it would have been detected at the barycenter of the solar system (the center of mass) instead at the position of the satellite. In this way, the data are comparable.

The XMMSAS provides a task called barycen to perform the barycentric correction on an input event file. Besides the input event file, barycen also need the orbit file from the ODF. So, before you run barycen, you have to set the SAS_ODF parameter to the ODF directory, e.g. export SAS_ODF=your-odf-directory.

Be aware!: The task barycen will change the input file and overwrites it. If you do not want this you should copy your original event file before you apply barycen.

The syntax of the task barycen is:
    barycen table=events.fits
The task will first check the keyword TIMEREF in the header of the input event file. This should be set on LOCAL. If a barycentric correction has been performed already on that event file by barycen it is set to SOLARSYSTEM. In this case barycen stops immediately. If TIMEREF is set to LOCAL, barycen starts and converts the arrival times from the local satellite time system to the Barycentric Dynamical Time. It also changes the keywords TIMEREF, TSTART, TSTOP, and TELAPSE.
With the corrected event file you can now perform you timing analysis.