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Дата изменения: Thu Jun 29 00:17:03 2006
Дата индексирования: Sat Dec 22 10:51:54 2007
Кодировка:
Поисковые слова: южная атлантическая аномалия

XMM-Newton Science Analysis System


emldetect (emldetect-4.59.1) [xmmsas_20060628_1801-7.0.0]

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Algorithm

subroutine emldetect

Read in EBOXDETECT source list (map detect) and 
  sort by source count rate
  
  Loop over sorted source list (begin with brightest source):

    Selection of sources for simultaneous multi-source fitting:  
      1) Search close neighbours within source cut radius of 
         current source. Don't consider sources which have already
         been processed; mark selected sources as processed.         
      2) Repeat (1) for each close neighbour until the maximum
         number of sources for simultaneous fitting (max. 8; specified
         in parameter file) is reached (i.e., the selection of additional
         sources for the multi-source fitting terminates when either the 
         maximum number of sources (parameter max\_fit) is reached or 
         when no additional sources fulfill the distance criterium 
         (parameter scut). 
                     
    Determine data area (2d-mask array) to be used for multi-source fitting:
      Loop over mask array: Set to 1 if pixel is (a) within event cut 
      radius of selected source and (b) within the area marked in the
      detection mask; set to 0 otherwise.     
            
    Read in data:
    Fill (x, y, count) data records: 
       Binned mode: 
          x,y:   image pixel coordinates
          count: number of events in pixel               
       Single photon mode:
          x,y:   event coordinates
          count:=1

    Feed data records, background maps, and exposure maps
      into Maximum Likelihood PSF fitting algorithm:  
      
    Use Marquarth-algorithm to minimize likelihood function of 
      multi-source PSF fit.
 
    Remove sources from fit which do not significantly improve 
      goodness of fit (required improvement in likelihood specified 
      by parameter).

    Set source extent to 0 if extent does not significantly improve
      goodness of fit (required improvement in likelihood specified
      by parameter).

    Add best-fit source models (PSF + source extent) to background 
      maps (i.e., sources which have already been fitted by the program 
      are treated as background for the remaining sources. Note that
      the sources are processed in the order of decreasing count rate
      such that all the bright sources will have been modeled into the
      background map once the weak sources are processed. 

    If likelihood of detection exceeds threshold THEN
      Write source parameters to EMLDETECT source list. 
      Add fluxes and hardness ratios.
    END IF

  END Loop  

end subroutine emldetect

Code segment describing the hardcoded analytical representation of the PSF (used if parameter usecalpsf is set to false).

! eps:    off-axis angle in arcmin
! sig:    source extent in arcsec
! psfoff: surface brightness of point spread function,
!         normalized, such that  Integral 2*PI*r*dr*psfoff
!         from 0 to infinity = 1 [1/arcsec2]

    data norm  /0.4169,0.3470,0.1279,0.1082/, &
         sigma /4.1450,8.6218,22.372,66.698/, &
         offsg /0.0324/

! off-axis component of telescope
    sigtel=offsg*eps*eps  

    sum=0.
    do i=1,4
      sig2=0.5/(sigma(i)**2+sigtel**2+sig**2)
      rnorm=norm(i)*sig2/pi
      sum=sum+rnorm*exp(-sig2*(x)**2)
    end do

    psfoff = sum