srcmatch allows more than one EPIC source list to be merged or to be 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 1 entry. Additionally we use also 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 (the other ID columns). Additionally, 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 on 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 1. The reason for this restriction is that it is not obvious how the values should be averaged or what should be done if one of the input list was not modified by eposcorr. 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 datasets which were merged together and their order can be retrieved from the header keywords EP_SET1, EP_SET2, ... with a maximum up to the 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:
In order to determine the OMFLAG values NO_OM_SRCS and NO_OM_DATA an OM exposure map or detection mask would be required which at present is not included in the list of PPS data products. OMFLAG will therefore not be set in the first release software version.