XMM-Newton Science Analysis System


omqualitymap (omqualitymap-1.5) [xmmsas_20230412_1735-21.0.0]

Setting of bit 3 (Diffraction-spikes)

Any object with either a raw count-rate $>$ 70 counts/sec or flagged as having a mod-8 pattern is checked to see if it might have diffraction spikes.

The algorithm works as follows:

1. Count the number of objects along position angles of 45 and 135 degrees from the centre of the star that lie within a radius of 10 and 120 pixels from the centre and a distance of no more than 5 pixels from the radius line.
2. Compute the search area from:

   SearchArea = (maxRadius - minradius) $\times$ 4 $\times$ maxDistance

   where minradius=10, maxradius=200, maxDistance=5 minradius is the minimum radius from the centre of the star maxradius is the maximum radius from the centre of the star maxDistance is the maximum distance from the centre of the star.

   These define a search area along the radius vector from the star (4 angles), 45, 135, 225, 315 degrees) for faint stars associated with a star spike.

3. Compute the number of objects within a box of width and height 200 pixels centred on the star centre, ignoring objects with a raw count-rate greater than 5 counts/sec (n1).

4. Compute the number of stars expected within the search area, again ignoring objects with a raw count-rate greater than 5 counts/sec (n2).

5. If the number of star along the spikes (n1) is $>$ 3 $\times$ n2 then conclude that there are diffraction spikes and flag sources along the spikes. All quality-image pixels within rectangular regions rotated from the vertical by $\pm 45$ and $\pm$ 135 degrees have bit 2 set. The half-width of a rectangle is 10 unbinned pixels and they extend radially, 50 to 250 unbinned pixels from the centre of the star.