XMM-Newton Science Analysis System


ommergelists (ommergelists-1.2) [xmmsas_20230412_1735-21.0.0]

Source-matching algorithm

The matching algorithm takes into account that more than one source in source-list 1 may have a match with the same source in source-list 2. This can become important in crowded-fields. For example, sources 1 and 2 in source-list 1 may be matched with source 10 in source-list 2, with separations of 2 and 1 arcsecs, respectively. In a simple matching algorithm, source 1 could be matched with source 10, when in fact source 2 of source-list 1 is better matched with it..

In the following, a list is a c++ vector that can grow in size as necessary, and each element of the vector is also a vector which can grow in size.

The algorithm works as follows-

1. Create an empty list, matchedindex1, that will record the row-number of a source in source-list 1 that has a match with a source in source-list 2.
2. Create an empty list, matchedindex2, that will record the corresponding row-number of a source in source-list 2 that has been matched with a source in source-list 1.
3. Create an empty list, matcheslist, of size equal to the number of rows in source-list 1, that will store the row-numbers and distances of the sources in source-list 2 that are no greater than the distance tolerance from each source in source-list 1.

4. Loop through from i=1 to the number of rows in source-list 1 and for each one:
   • Find all the sources in source-list 2 that are no more than a distance tolerance arcsecs from it, and add the source-list 2 row-numbers and distances to matchesList[i].

5. Loop through matchesList and for each element sort the list of sources in source-list 2 that have 2 or more matches with a source in source-list 1 into increasing distance order, preserving the correct source-list 2 row numbers.
6. Loop through from i=1 to i=size of source-list 1 and
   • If source i has no matches move on to the next i.
   • Loop through from j=1 to j=size of matchesList[i] and
     • Set l to the index of matchesList[i][j]
     • Check to see if this source in source-list 2 has been matched with any other source in source-list 1
     • IF NOT then increase the size of matchedIndex1 and matchedIndex2 by 1, and set the former to i and the latter to l. Move on to the next i.
     • If YES Check to see if the distance in matchesList[i][j] is no greater than any of the other matched distances for this source in source-list 2- if not then increase the size of matchedIndex1 and matchedIndex2 by 1, and set the former to i and the latter to l. Move on to the next i.

7. Using the refined matched-list of sources, compute the mean differences in RAxcos(Dec) and Dec.
8. If either of these offsets are more than 0.1 arcsecs then correct the coordinates of the second source-list and repeat the matching process (a maximum of 3 iterations).