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How can I calculate upper limits at a given source position?


Generally you can download a given image from the archive and use ROSAT data reduction software (PROS, EXSAS or XSELECT). Probably the quickest way to determine upper limits is to use the " SOSTA" (source statistics) tool available in the XIMAGE image processing package.

To do this:

  1. start XIMAGE and read in the image and exposure maps
    
    
    % ximage
    ******************************************************************************
    *                              Welcome to XIMAGE                             *
    ******************************************************************************
    
                                                              Type "help" for help
    
    
                                               VERSION 4.5.1  13:28:15 19-Oct-2011
    
     No of detectors read in:   30
    [XIMAGE> 
    [XIMAGE> read 
     Enter file name:rp200108n00_im1.fits.Z
     Telescope ROSAT PSPCB
    WARNING:  assuming DRPIX based on mission info
     Image size = 512 x 512 pixels
     Image rebin = 30.00
     Image center = 7680.5, 7680.5
     Reading an image
      Image level, min = 0.0000000 max = 48.000000
     Copied MAP1 to MAP9
    [XIMAGE> read exposure
     Enter file name:rp200108n00_mex.fits.Z
     Telescope ROSAT PSPCB
    WARNING:  assuming DRPIX based on mission info
     Image size = 512 x 512 pixels
     Image rebin = 30.00
     Image center = 7680.5, 7680.5
     Reading an image
      Image level, min = 0.0000000 max = 1601.5461
     
  2. Next, display the image using the "xtk" device
    [XIMAGE> cpd /xtk
    [XIMAGE> display
     Plotting image
      Min =   0.0000000      Max =    48.000000
    
    sample SOSTA run
    Display of zoomed in region of rp200108n00_im1.fits. Boxes show the regions used for SOSTA, chosen interactively via the cursor.

  3. Then use SOSTA to interactively select the region for which you want to calculate the upper limit
    [XIMAGE> sosta
     Using MAP1
     Using a locally computed background
     Select the center of source box (Right button exits)
    
  4. And here's the output from SOSTA, including the 3-sigma upper limit:
                        X = 5568.6411    Y = 8672.2266
      Using average energy for PSF:    1.0000000
     Source half-box for 0.66 EEF is  103.1 pixels
            Half-box for 0.90 EEF is  170.1 pixels
     Total # of counts 6.0000000 (in 37800 elemental sq pixels)
     Background inner radius:  179.0 pixels; outer radius:  340.1 pixels
     Innerbox counts 19.000000 in 129600 sq or pixels
     Outerbox counts 63.000000 in 455400 sq or pixels
     Background counts 44.000000 in 325800 sq pixels
     Vignetting is part of exposure map
     Background/elemental sq pixel :                1.351E-04 +/- 2.0E-05
     Background/elemental sq pixel/sec :            1.725E-07 +/- 2.6E-08
    
     Source counts :                                8.950E-01 +/- 2.6E+00
     s.c. corrected for PSF + sampling dead time
                                    + vignetting    1.328E+00 +/- 3.8E+00
     Source intensity :                             1.143E-03 +/- 3.3E-03 c/sec
     s.i. corrected for PSF + sampling dead time
                                    + vignetting -> 1.696E-03 +/- 4.9E-03 c/sec 
     Signal to Noise Ratio             :            3.486E-01
                                                     Poisson    Gauss
     Pr. that source is a fluctuation of back. :    4.02E-01   3.46E-01
    
    
        Vignetting corrected exposure :        782.847 s
        Sampling dead time correction :      0.968
        PSF correction                :      1.532
    
          Three sigma upper limit : 2.37E-02 cts/s
        Optimum half box size is      : 106.50000 orig pixels
         Error radius :   20. arcsecs
    

    Page Author: Michael F. Corcoran

    Last modified Wednesday, 19-Oct-2011 14:43:50 EDT


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    This file was last modified on Wednesday, 19-Oct-2011 14:43:50 EDT

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