XMM-Newton Science Analysis System


evselect (evselect-3.71.2) [xmmsas_20230412_1735-21.0.0]

Filtering stage (selector)

The filtering process is carried out on an event-by-event basis controlled by user-specified selection criteria which take the form of a single boolean expression. The expression can contain symbolic names of “intrinsic” event attributes, and numeric constants combined with logical and arithmetic operators and functions. Events for which the expression does not evaluate to true will be marked as invalid or discarded from the data set and shall not be considered in the product extraction.

evselect supports selections based on intrinsic event attributes. These attributes are present in the input event list table as corresponding columns, the names of which are to be used in the selection expression. This group further falls into the following sub-categories:

1. spatial selections in raw pixel (RAWX/RAWY), physical detector (DETX/DETY) or sky pixel (X/Y) space, or indeed in any spatial coordinate system (e.g. TELX/TELY). Special pre-defined region shapes exists such as CIRCLE, ELLIPSE, ANNULUS, BOX, POLYGON. Spatial filtering with mask images is possible as well.

2. Celestial coordinates (RA/DEC) are available for the CIRCLE, BOX and ANNULUS shapes,

             e.g. (RA,DEC) in CIRCLE(253.231,-2.052,0.0166)
             or (RA,DEC) in BOX(121.234,23.456,0.125,0.125,0)
   

   where all values are in units of degrees. These values are pre-processed into sky pixels.

3. A special construct IMAGE may be used to extract data from a spatial region which is defined by non-zero pixels in a supplied FITS image. This image may have axes in DETX/DETY coordinates or in celestial coordinates. Any celestial image with standard WCS header keywords and integer or boolean values is acceptable. Note that the coordinate system should be epoch 2000 (FK5), earlier 1950 epoch (FK4) images will give a significant offset.

             e.g. expression='IMAGE(myHSTimage.fits)'
   

   This expression is pre-processed into a BOX, encompassing the whole image logically ANDed with a MASK, e.g.

             '(X,Y) in BOX(25000,25000,15000,15000,0) && MASK(myHSTimage.fits)'
   

4. energy selections in the form of one or more interval specifications in either PHA or PI space.

5. time selections in the form of
   1. one or more interval specifications
   2. one or more Good Time Interval (GTI) files created with tabgtigen or any other SAS tasks which creates GTI files as output
6. any other event attributes which are present (e.g. PATTERNID, or CCDID, or WAVELENGTH in case of RGS)