The prime source is used in the pipeline by rgsangles as a reference to calculate approximate corrections for attitude variation errors in the BETA values of events. rgssources always selects one member of its output list to be the prime source and stores the INDEX column value of this source in the source-table keyword PRIMESRC.
rgssources also calculates a confusion value between every other source and the prime source (see subsection 5.2); these values are stored in the column CONFUSION.
The prime source can be designated in five ways, via a number of different parameters:
To see how these interact in detail, see the algorithm in section 7, also the examples in section 6; however the order of precedence is roughly as the entries appear in the list above.
XMM-Newton SOC -- 2021-11-30