Extended Source: Spectra and Light Curves

 

     

This section should be read thoroughly.

One major problem with extended objects is the spatial selection of photons. It is better to define it once and for all at the beginning of the EXSAS session, because it may require a more complex sequence of commands.

Two different methods may be followed:

As a first solution, it may be practical to select once the sublist of photons which are coming from the extended object sky area, as well as the sublist containing only background photons from another portion of the sky, and to output the resulting photon lists into new Photon Event Tables. In this way the binned datasets would be not produced immediately (i.e. with a single command line as in the examples given in the previous sections) but they would be created in a separate step by accessing the new selected sublists of photons.

As a second solution -- which has the advantage of a reduced file proliferation, higher flexibility and less execution time -- all the complex sequences of selection and bin commands may be written into an ASCII command file, which could be run at the users pleasure (this command file is not a MIDAS procedure -- filename extension .prg -- but a Projection command file -- filename extension .cmd). It would be enough to edit the command file from time to time in order to modify only the defined output binned datasets. The sequence of commands defining the complex spatial selections would be left generally unchanged.        

As an example, the content of an ASCII command file which selects the photons within a heart-shaped sky portion is shown:

    INPUT events
A = SELECT/RING : center = (-2415,-3254) radius = 2400
B = SELECT/RING : center = ( 2354,-3254) radius = 2400
C = SELECT/BOX  : (-4065, 2415) to  (4364, -3134)
    SELECT/SECTOR A+B+C: center = (-15,2355) angles = 224 to 316
    OUTPUT TO heart
    END

The command file in the example performs the complex spatial selection and writes the resulting photon list into the Photon Event Table heart.tbl. In order to execute the command file, one must use the command

Midas 001> MAKE/PROJECTION comfile

      assuming that the above select commands are contained in the ASCII file comfile.cmd. To execute this command file is anyway hardly necessary. It would take also a relatively long execution time, and the output file would be probably very large (as photon lists generally are). What the user often want, as a final result, is a binned data set (a spectrum, a light curve, an image or a profile), and not a new photon list.

At this point it is quite simple to obtain, for instance, an image of the result by simply inserting the BIN/IMAGE command just before the output definition:    

    INPUT events
A = SELECT/RING : center = (-2415,-3254) radius = 2400
B = SELECT/RING : center = ( 2354,-3254) radius = 2400
C = SELECT/BOX  : (-4065, 2415) to  (4364, -3134)
    SELECT/SECTOR A+B+C: center = (-15,2355) angles = 224 to 316
!
!   A new command line is inserted here
!
    BIN/IMAGE 30
!
    OUTPUT TO heart
    END

    Now the output will be the image heart.bdf, made out of photons which are selected in the previously defined heart-shaped sky area.

Similarly, different bin commands may close the select commands sequence in order to produce spectra and light curves, using different options. The possibilities are large.

One more example: a command file which is used to select the source photons from the already define heart-shaped area in the sky, and the background photons from the intersection of two sectors (i.e. a four sided polygon). The two virtual photon lists are taken separately and binned into two spectra (source+background and background spectra) and written into the same table spectra.tbl, which is the only file created on disk.    

    INPUT events
!
! Source + background photons are selected and binned here
!
A    = SELECT/RING : center = (-2415,-3254) radius = 2400
B    = SELECT/RING : center = ( 2354, 3254) radius = 2400
C    = SELECT/BOX  : (-4065, 2415) to  (4364, -3134)
       SELECT/SECTOR A+B+C: center = (-15,2355) angles = 224 to 316
SB   = BIN AMPL 1 
!
! Background photons are selected and binned here
!
       SELECT/SECTOR : center = (-15,2355) angles = 90 to 220
       SELECT/SECTOR  center = (-2300,2355) angles = 270 to 300
BACK = BIN AMPL 1 
!
       OUTPUT SB,BACK: to spectra
       END

               

The output spectra are written to the table spectra.tbl at the columns labeled SB and BACK.

In order to produce light curves instead, it would be enough to change in the command file the item AMPL into TIME (and of course a new TIME binsize and a new output table name should be chosen).

In order to get familiar with the projection command file syntax it is strongly suggested to read thoroughly section 4.3.