srcmatch allows more than one EPIC source list to be merged or cross-correlated with the summary source list. In order to keep track of the original source identifications of task emldetect we allow the columns EP_BOX_ID and EP_ML_ID and the columns (PN_ML_ID_SRC, M1_ML_ID_SRC,..,PN_BOX_ID_SRC, etc.) to have more than one entry. Additionally, we use this scheme (but now with floating point data) for columns with maximum likelihood values (PN_DET_ML, etc, and PN_EXT_ML, etc.). The number of entries depends on the number of datasets, which will be stored using the keywords NSETS (for EP_BOX_ID, EP_ML_ID and the maximum likelihood parameters) and NCROSSID (for the other ID columns). The number of entries per row for a given column can also be retrieved from the FITS keyword TDIMi (i being the column index).
srcmatch will pass the columns RA_CORR and DEC_CORR (generated by eposcorr) from the input X-ray source list to the summary output list, only if the number of input source lists is one. The reason for this restriction is that coordinates determined by different tasks should not be merged / averaged. The keywords generated by eposcorr will also be passed on. In case the input file was not modified by eposcorr, the above mentioned columns and associated keywords will not be written to the output set.
The names of the merged datasets and their order can be retrieved from the header keywords EP_SET1, EP_SET2, ... with a maximum number stored in the internal parameter max_nsets. The datasets used for cross-identification can be retrieved from the CROSSID1, CROSSID2,..., keywords. The following additional header keywords are present (they correspond to the input parameters with the same names): SYSTERR, MAXERR and OMDIST.
Whether the input X-ray source list(s) was/were cross correlated with an OM source list, can be verified with the (logical) keyword OMCROSS. OMCROSS = T means it was cross correlated. The maximum number of associated optical sources (counterparts) from the OM source list is set to five (corresponding to five additional columns of the EPIC observation source list). If more correlations are found, the five counterparts with the smallest distance to the EPIC position will be listed and a label (column OMFLAG, see below) will be set accordingly.
In addition to the five columns giving the OM source numbers, a numeric column giving the total number of OM counterparts found for each EPIC source (which may be more than five) and a label column OMFLAG is added to the observation list.
The label OMFLAG can take the following string values: