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Can I perform the barycentric correction on OM data in Fast Mode?

Yes, you can. As it is stated in the headers (keyword TIMEREF = LOCAL), the time values in the time series files produced by processing OM data in Fast Mode (P*TIMESR*) are referred to the XMM-Newton satellite system.

This time scale can be converted into an solar system barycenter time system by applying the barycentric correction with the SAS task barycen

The task works on the final time series produced by omfchain (files P*TIMESR*.FIT):

barycen withtable=yes table=tserie.fit:RATE

where tserie.fit is the light curve file obtained from omfchain (task omlcbuild).

The barycen task overwrites the TIME column in the original file. Therefore it is recommended to copy it beforehand if one wants to keep the non-corrected timing.

It should be noted that for OM, barycen is applied to the time stamps of the light curve, and not to the arrival time of each individual photon. barycen will not work on OM Fast Mode events list files.