Calculate Source Count Rates

 

Count rates can be calculated for a source list either by evaluating the exposure image at the source positions or by evaluating the TIM_INT   and the LIF_TIM descriptors of the background image. The first method makes it necessary to use the command

READ/EXPOSURE_OF_SOURCES sou_tab exp_ima [lab_rad] [sel_reg]

 

• sou_tab is the name of the source table containing the columns :X_SKY, :Y_SKY. Attention, the sky pixel positions must agree with the world coordinate system of the exposure image. Use the command TRANSFORM/RADEC_TO_PIXEL with the exposure image as the reference image in order to get the proper coordinates.
• exp_ima is the name of the exposure image.
• lab_rad is the column label of the table sou_tab specifying the radius of the region from which a mean exposure will be calculated. it has to be specified as :LABEL. The unit must be the same as the unit of the position specified in sou_tab. The default is no label. The exposure will be the value of the exposure image at the position specified in sou_tab.
• sel_reg is the spatial select region, BOX or CIRCLE. For sel_reg=BOX the radius read from lab_rad specifies half of the box side and for sel_reg=CIRCLE the radius read from lab_rad specifies the radius of the circle.

Note:For sel_reg=CIRCLE slightly different values for the mean exposure are determined compared to the result from INTEGRATE/RING as slightly different methods are used.

The exposure is written to a column :EXPOSURE. The exposure image in general contains the exposure time the vignetting. The source count rates and the error in the source count rates are calculated by the table column operations:

:RATE  = :CTS / :EXPOSURE
:ERATE = :CERR / :EXPOSURE

The second method needs no exposure map. It is automatically applied in the maximum likelihood and upper limit counts calculation command in case the command COMPUTE /LIFE_TIME has been applied to the background image

COMPUTE/LIFE_TIME image1 eventrates bacmp1

  The image image1.bdf (in this example) is used to get the accepted time intervals and the eventrates table eventrates.tbl to determine   the life time in these time intervals.
  The source count rates :RATE and the errors in the source count rates :ERATE have been calculated in the program as

:RATE = :CTS :VIG_COR / TIM_INT / LIF_TIM
:ERATE = :CERR :VIG_COR / TIM_INT / LIF_TIM

TIM_INT and LIF_TIM are descriptor values retrieved from the background image.

Note:Choose this second method if no exposure map is available. The values of :CTS and :CERR depend on the chosen radius of the extract circle. Such an extract circle radius is specified in the parameter file for the commands DETECT/MAXLIK and COMPUTE/UPPER_LIMITS in the parameter CUT_RADIUS. As no point spread function correction is applied to the counts determined   by the maximum likelihood algorithm, the count rates may be underestimated. It is not at all recommendable to use a much larger value for CUT_RADIUS than the default value as then close by sources may be contributing and give unreliable results. Sometimes it may even be appropriate to choose a CUT_RADIUS of about 1.0 (in the pointing case) to get reliable source positions. A second run of the command DETECT/MAXLIK or COMPUTE/UPPER_LIMITS with the parameter settings FIT_POSITION=fixed and CUT_RADIUS=2.5 may then be recommendable in order to get reliable counts.