The parameter nsigma (default value=2 controls the minimum detection threshold, ie all pixels with a value greater than or equal to background value + nsigma background noise will be considered to be part of a possible source. For values greater than 2, the part of the algorithm that searches for very "faint" sources is bypassed, and when you are only interested in the relativley bright sources then it is a good idea to set this parameter to 2.01, which will significantly decrease the execution time (this will cause the algorithms that specifically look for very faint sources to be bypassed).
The parameter minsignificance (default value=2) controls what sources are kept in the source-list- all detected sources with a computed significance less than or equal to minsignificance are discarded. If this parameter is set to 0 then all the detected sources will be kept in the source-list. Of course, the lower the value of minsignificance the more spurious sources may be in the source-list. If it is thought that there are too many spurious sources, then it is a good idea to re-run omdetect with a minsignificance value of 3, or maybe even 4.
The parameter detectextended determines if the algorithm is to search for “large” extended sources (true), or not (false). If you are not interested in such sources, a considerable amount of computing time can be saved if this parameter is set to false.
Setting the parameter psfphotometryenabled to yes would make this task to PSF-fit the 2D-image profiles of those sources that have close neighbours (which mean that the point-spread functions of these neighbours overlap). The photometry of these sources would then be based on the fitting parameters. With this feature enabled, an additional source-list table will be generated, contaning sources with their photometry based on PSF-fitting. Such a fitting, however, consumes a large amount of computing time, so the use of the parameter psfphotometryenabled must be taken with certain care.
XMM-Newton SOC/SSC -- 2019-06-02