Sosta

The SOSTA command can be used to determine the number of counts within a given box (XIMAGE uses boxes, rather than circles, because its quicker, and avoids problems with deciding what to do with pixels that are partly in or outside a given region). SOSTA counts the number of events within a specified box, corrects those counts for vignetting, exposure and the point spread function to give the source intensity and its statistical significance. If the significance is less than 1.e-3 an upper limit is automatically calculated. The simplest way to use SOSTA is with the cursor using the /CURSOR qualifier. In its default mode SOSTA will use a box that corresponds to 66% of the encircled energy function (EEF), and will use a background from a region surrounding the source, outside the 98% EEF boundary.

The background can be estimated using several different methods, specified by the user. These are:

• The background is estimated from a region within two boxes centred on the source using the option /INNER_RADIUS and /OUTER_RADIUS to specify the half-sizes of the two boxes. This is the default.

• The background is taken from any specified part of the image using /BOX_BACKGROUND;

• The background intensity can be given using the qualifier /BACKGROUND_LEVEL. In this case the background statistical error is assumed to be zero.