sosta - source statistics

  sosta[/qualifiers]

Detailed statistical analysis of sources.

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 /LOOP qualifier can be used to repeatedly enter values using the cursor, or by typing in the values. To exit the loop click outside the image.

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.

SOSTA will estimate the optimum box size to maximize the signal to noise ratio. This is given at the end of each run. By using the option /OPTIMIZE then SOSTA will go around a second time, and use the optimum box size to calculate the optimum source statistics.

sosta/x_source

The source position in units of elemental x pixels. See also /y_source.

sosta/y_source

The source position in units of elemental y pixels. See also /x_source.

sosta/source_box_radius

This specifies the half-size of the box where the source intensity is estimated.

sosta/optimize

SOSTA makes an estimate of the optimum box size to maximize the signal to noise ratio. The /OPTIMIZE qualifier will make SOSTA go around a second time so that it uses the optimum box size to estimate the source statistics.

sosta/local_background

This is the default, and specifies that the background intensity must be calculated within two boxes centered on the source and of half-size equal to inner_radius and outer_radius respectively. The default starts the inner box at the 98% EEF and includes an area approx twice that of the 98% EEF area.

sosta/inner_radius=size

The radius of inner box (centered on the source) where local background is calculated. The background intensity is calculated within the inner and outer boxes and rescaled to the position of the source. Must be used in combination with /OUTER_RADIUS.

sosta/outer_radius=size

The radius of outer box (centered on the source) where local background is calculated. The background intensity is calculated within the inner and outer boxes and rescaled to the position of the source. Must be used in combination with /INNER_RADIUS.

sosta/box_background=size

This specifies that the background is estimated in user-defined boxes. The user is then prompted for the position and size of each box. If the /SIZE_OF_BOXES qualifier is used, than all the boxes have the same specified size. The /CURSOR can be used to select the corners of the boxes using the cursor.

sosta/no_of_boxes=n

The number of square boxes where the background is to be determined. the user is prompted for position and size for each box, or can use the /CURSOR option.

sosta/size_of_boxes=m

The half size of the boxes used to determine the background. All the boxes are then defined with the same size. If this qualifier is not used, the user is prompted for a size for each box.

sosta/background_level=counts

The background intensity is provided in units of counts/elemental pixel.

sosta/cursor

The image inputs (positions) are given from cursor.

sosta/loop

Enter multiple positions, without exiting sosta. Click outside the the image are to exit.

sosta/detect_sources

Run sosta on the sources found in the previous run of DETECT.

sosta/eef_source=frac

The fractional EEF used to determine the source box size. The default is 0.66.

sosta/eef_background=frac

The fractional EEF used to determine where the inner background box size begins. The default is 0.98.

Examples:

sosta/cur               ! The source position is specified using the cursor.
                        ! All the defaults are used. The background is
                        ! determined from a region surrounding the source.
 
sosta/loop/cur          ! keep pointing and clicking, over and over.
 
sosta/cur/opt           ! The optimum source box size is determined.
 
sosta/cur/box           ! The background is to be determined from a box
                        ! specified by the user.
 
sosta/cur/in=20/out=40  ! The background is determined from the difference
                        ! between the two boxes with half sizes 20 and 40
                        ! elemental radii.
 
sosta/cur/source=10     ! The source box size is set to 10 pixels.
 
sosta/cur/eef_s=0.5     ! The source box size is set to be at the 50% EEF.
 
sosta/cur/eef_b=0.5     ! The inner background box size is set to be at the
                        ! 50% source EEF.