Cross-correlating EPIC source lists for either merging or for correlating earlier pipeline source lists is done in two stages. The first stage is similar for both the merging of source lists and cross-identifying sources with earlier pipeline source lists. In this stage (subroutine compare_srcs) correlations are investigated and pointers between matching sources are set. Always the closest companion (in terms of sigmas) is chosen. Only sources within 3 to 5 sigmas will be correlated (the exact number can be set using input parameter maxerr). In order to avoid problems due to boresight effects an input parameter systerr (for systematic error) has been introduced.
In the second stage, the cross identifications are either used to get the pointers to the source numbers from previous detection stages (e.g. the PN_ML_ID_SRC field), or the two lists are merged. The first option works for the data sets provided with the crossidsets command line parameter, whereas merging is done if more than one set is provided using the inputlistsets parameter. Both options can be used simultaneously.
Merging means that new sources will be added to the list and parameters of cross-identified sources will be averaged using weights based upon the statistical errors. The maximum likelihood parameters will be all listed separately using columns with dimension larger or equal to one, in the same manner as the cross-identification columns. The reason is that maximum likelihood values cannot be simply averaged or added without losing its meaning as a probability related parameter.
Both for merging and for cross-identification, the source lists are read one by one and are correlated separately. This ensures that not more than 2 source lists will be stored in memory. Upon execution the number of sources that can be stored in the summary source list is three times the number of sources in the first input set. If during the execution this turns out not to be enough, the source list will be copied to a new array with a three times larger capacity.
The output EPIC observation source lists are provided both in FITS and in HTML format, where the HTML list is intended to give a quick overview for each source detected, but does not provide instrument-specific data.
Combined values for all EPIC instrument and per individual EPIC instrument can be determined for several source parameters (described below). They are calculated if the parameter allepicmode is true and set to NULL otherwise.
Fluxes and flux errors from the input source lists are copied to the columns
inst_b_FLUX and ERR_inst_b_FLUX
(inst=[PN,M1,M2], b=[1,2,3,4,5,TOT,XID]). Combined instrument
fluxes and errors inst_TOT_FLUX,
ERR_inst_TOT_FLUX are the sums of the fluxes and errors over all energy bands
of the instrument. The columns EP_b_FLUX
and ERR_EP_b_FLUX contain all-EPIC flux values and errors,
calculated from the available single camera values using error-weighted
averages.
As of version 3.10.2, the output FITS table contains the columns inst_b, inst_b_ERR, inst_EXP, with inst=[PN,M1,M2] and b=[1,2,3,4,5,TOT,XID]. These columns contain count rates, (inst_b), count rate errors (inst_b_ERR), and exposure map values (inst_n_EXP, n=[1..5]) from the individual instruments and up to 5 energy bands. Total count rates and errors per instrument inst_TOT, inst_TOT_ERR and total EPIC count rates and errors EP_TOT, EP_TOT_ERR are the sum of the count rates in the individual energy bands. In the case of several input source lists from one instrument, count rates are averaged and exposure map values are added.
The hardness ratios and errors from the input source lists are copied to
output columns inst_HRn and inst_HRn_ERR (inst=[PN,M1,M2], n=[1,2,3,4]). In addition, weighted
averages of the single camera hardness ratios can be written to
columns EP_HRn, EP_HRn_ERR, again controlled
by the parameter allepicmode.
Note: Since the EPIC cameras have different energy dependent effective
areas, the averaged HR values depend on the relative exposures and/or presence
of the (sources in the) individual cameras.
As of version 3.10.5, the strings from the input columns VAL_FLAG, VER_FLAG, VER_COMM (set by task dpssflag) are copied into the columns of the output file inst_VAL_FLAG, inst_VER_FLAG, inst_VER_COMM (inst=[PN,M1,M2]).