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Дата изменения: Tue Aug 16 03:26:38 2005
Дата индексирования: Sat Dec 22 07:59:11 2007
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Поисковые слова: m 11

XMM-Newton Science Analysis System


eident (eident-1.6) [xmmsas_20050815_1803-6.5.0]

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Description

The eident tasks reads two or three source lists, one for each instrument (PN,MOS1, MOS2) and looks for misalignments between the two or three instruments, which may be present during the initial phase of the mission. Additionally, the offsets found will be used to make a merged source list, which maybe of use for non-standard setups of the detection pipeline. The resulting output set will contain offset keywords and a merged source list compliant with the format of the emldetect output set. The task also works if only two source lists (say MOS1 and MOS2) are present.

The offset optimization routine is similar to the routine in used by the task eposcorr, which optimizes the statistic $S = \sum_{i,j} exp( -\frac{1}{2} (\frac{d_{i,j}}{\sigma_{i,j}}))$, where $i, j$ represent the source numbers of two different sets, $d_{i,j}$ is the distance between two sources and $\sigma_{i,j}$ the distance error. A basic difference between the two routines is that eposcorr uses sky coordinates, whereas eident uses pixel coordinates. Unlike for the task eposcorr we do not have to worry here about chance alignments, since most of the sources are likely to be detected by all three instruments. Since after a number of observations the positional offsets between the detector systems should be known, the offsets can also be set using input parameters. The error in the offsets is estimated using the second moment in the distribution of the statistic $S$ around the maximum value and divided by $\sqrt{N}$, with $N$ the number of matching sources.

For the merging of the three input source lists (one for each instrument) the following scheme was used. The tasks uses a data structure containing the relevant parameters for each source. Additionally there are two extra fields called cross_ref and dist, both are arrays with dimension 2. The cross_ref elements are initiliazed to zero, but when two sources from different data sets are near each other the cross_ref fields will point to each other in the following way:

• PN set: cross_ref[1] refers to a source in the MOS1 set.
• PN set: cross_ref[2] refers to a source in the MOS2 set.
• MOS 1 set: cross_ref[1] refers to a source in the PN set.
• MOS 1 set: cross_ref[2] refers to a source in the MOS2 set.
• MOS 2 set: cross_ref[1] refers to a source in the PN set.
• MOS 2 set: cross_ref[2] refers to a source in the MOS1 set.

The dist field contains the associated distance between two sources linked by the cross_ref fields. Sources in two lists are taken to be one source if their distance $d_{i,j}/\sigma{i,j} < {\it maxerror}$, maxerror being an optional parameter. If two possible counterparts are found, the closest one is considered to be the actual counterpart. If the MOS1 to MOS2 reference is not consistent with the MOS1 to PN resp. MOS2 to PN reference, preference will be given to the MOS1/2 to PN references (this preference is determined in the routine write_set).

The pixel coordinates (called in accordance with other detection tasks X_IMA/Y_IMA) are not updated, but instead the estimated offsets between the detectors are listed in the header. The sky coordinates and other derived quantities will be averaged. The output set will be used by eboxdetect/emldetect to correct offsets between the instruments and make a combined detection run. However, if for some reason the combined detection using all three instruments together does not work satisfactory, eident can be used to merge the three individual source lists (i.e. one for each instrument) into one, which can be read by the pipeline task srcmatch to correlate with the OM source list and produce the final summary source list.