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Suggested methods for dealing with data contaminated by solar X-rays

If the angle between the LAC pointing direction and the Sun is less than 90° then the data may be contaminated with solar X-rays. In this case there are two possible ways to proceed with the analysis:

  1. Sort the raw data as normal, using sortac. Then run select to extract only the data with the SUNSHINE flag set to zero (the lower and upper limit on SUNSHINE are set to cover zero but not one). This will restrict the observational intervals to those in the Earth's shadow. Whilst this may lead to a large reduction in the useful length of an observation, it does ensure that the end result is free of solar contamination. An example of this approach is illustrated below:

    qcl>select
    
    ** SELECT Version 2.4 Tue Jun 17 15:08:21 1997 **
     
    SEL_DATIN  Input data filename =mkn335_91a.cube
    SEL_OPT  Extract  a TIME / PHAD / CUBE  file? = PHAD =cube
    SEL_DATOUT  Output data filename = mkn335_91a.cube =mkn335_91a.cube_ex
     
    SEL_NPHA  How many PHA ranges? = 48 =
    SEL_NUM_SEL  Number of HK parameters to select on  (-1 = default list)
    = 1 =
     
    SEL_HK_NAME  Name of HK parameter =sunshine
    SEL_WINDOW  Normal or inverse  window = Norm =
    SEL_PAR_LO  Lower limit on SUNSHINE = 0.0 =-0.2
    SEL_PAR_HI  Upper limit on SUNSHINE = 0.0 =0.02
    
    SEL_NUM_PER  Number of time windows = 1 =
    SEL_ST_PER001  Start time of window - secs from begining = 0.0 =
    SEL_END_PER001  End time of window - secs from begining = 50304.0 =
     
         DATIN  mkn335_91a.cube_cl
         Shape :      393 x    48   x     2  =    37728
                   TIME    PHA     IDENT   
     
         Time binning is 128.0000 secs
         ... searching    393 time bins from range of    393
     
          ID_MASK( 2)
         0101010101010101
         1010101010101010
     
     
         OUTPUT  mkn335_91a.cube_ex
         Shape :      393 x    48   x     2  =    37728
                   TIME    PHA     IDENT   
     
             11 time bins are occupied 
    qcl>

  2. An alternative approach, that works for MPC1 data, makes use of the fact that certain LAC detectors may be shielded from the Sun by the body of the spacecraft and will therefore not be sensitive to solar X-rays. To see if this is the case, the user may proceed as follows:

    1. Run sortac with the parameter GNG_SQUISH set to "NO". This will keep the data from the eight different detectors separate.
    2. Run select as above to restrict the observational intervals to those in (a) the Earth's shadow (SUNSHINE = 0) and (b) the Sun (SUNSHINE = 1). This produces two spectral files with full detector information. Setting SEL_WINDOW = Inv selects data when the sunshine flag is one rather than zero.
    3. Subtract the two spectral files using subcb:
    4. qcl>subcb
      
      ** SUBCB Version 1.0 Tue Jun 17 15:33:37 1997 **
       
      SCB_DATIN1 First input data file =  =mkn335_91a_sun.phad
      SCB_DATIN2 Second input data file =  =mkn335_91a_nosun.phad
      SCB_DATOUT File containing FILE1-FILE2 = mkn335_91a.phad
       
           SCB_DATIN1: mkn335_91a_sun.phad
            Shape :      1 x    48 x    16   =    768
                    TIME    PHA     IDENT   
       
           IDENT masks
             1  1000000000000000
             2  0100000000000000
             3  0010000000000000
             4  0001000000000000
             5  0000100000000000
             6  0000010000000000
             7  0000001000000000
             8  0000000100000000
             9  0000000010000000
            10  0000000001000000
            11  0000000000100000
            12  0000000000010000
            13  0000000000001000
            14  0000000000000100
            15  0000000000000010
            16  0000000000000001
       
       
           SCB_DATIN2: mkn335_91a_nosun.phad
            Shape :      1 x    48 x    16   =    768
                    TIME    PHA     IDENT   
       
           IDENT masks
             1  1000000000000000
             2  0100000000000000
             3  0010000000000000
             4  0001000000000000
             5  0000100000000000
             6  0000010000000000
             7  0000001000000000
             8  0000000100000000
             9  0000000010000000
            10  0000000001000000
            11  0000000000100000
            12  0000000000010000
            13  0000000000001000
            14  0000000000000100
            15  0000000000000010
            16  0000000000000001
       
       DATIN2 is a PHAD. Subtracting DATIN2 from each PHAD in DATIN1.
      qcl>

    5. Plot the Sun minus Earth shadow spectrum (using the qcl procedure pl_phad.qcl; see section  6.2). If solar contamination is significant in a particular detector then there will be excess counts in the subtracted spectrum below 5 keV. Identify detectors with no excess. Note that only even detectors need be plotted as they refer to the top-layer; the mid-layer is insensitive to genuine X-rays (and hence solar X-rays) below ~5  keV.
    6. Run hyper on the original, uncompressed cube (i.e. the cube with no sunshine selection) to select only the detectors with no soft excess in the sunlit data:

      qcl>hyper
      
      ** HYPER Version 1.2 Tue Jun 17 15:49:51 1997 **
       
      HYP_DATIN data file name =mkn335_91a.cube
      HYP_DATOUT new data file name = mkn335_91a.cube_sq =
       
            Shape :    393 x    48 x    16   =   301824
                    TIME    PHA     IDENT   
       
            IDENT masks
             1  1000000000000000
             2  0100000000000000
             3  0010000000000000
             4  0001000000000000
             5  0000100000000000
             6  0000010000000000
             7  0000001000000000
             8  0000000100000000
             9  0000000010000000
            10  0000000001000000
            11  0000000000100000
            12  0000000000010000
            13  0000000000001000
            14  0000000000000100
            15  0000000000000010
            16  0000000000000001
       
      HYP_OPTION TIME, PHA, or IDENT dim. =ident
       
      HYP_NCHOUT no.of output ident chans = 1 =2
      HYP_COMB001 chan combination = 1-16 =2,4,6,8,10,12,14,16
      HYP_COMB002 chan combination =1,3,5,7,9,11,13,15
       
            New ID channel  1 : ID chans 2,4,6,8,10,12,14,16 
       
       
            New ID channel  2 : ID chans 1,3,5,7,9,11,13,15  
       
            Squished file 
       
            Shape :    393 x    48 x     2   =    37728
                    TIME    PHA     IDENT   
       
      qcl>

      HYP_NCHOUT and HYP_COMB are the number of output detector idents and ident combination respectively. Even and odd idents refer to the top- and mid-layers respectively.

    It will not usually be possible to perform an universal background subtraction (see section  3) on the resulting cube as the background model coefficient files generally assume that all 8 LAC detectors are present (the LAC background is not identical in every detector). It will be necessary to create a local background cube using the same detector combination selected for the source cube.

    Further details regarding the programs select and hyper are given in section  5.


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