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

Note: if you are trying to calculate an upper limit from ROSAT all sky survey images, you should delete the DTCOR keyword from the image header before calculating the upper limit in XIMAGE, otherwise you'll get a spurious (negative) result. You can delete the DTCOR keyword using fthedit. For example:

% fthedit rs931557n00_im1.fits DTCOR delete

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
   

   
   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
   

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   Page Author: Michael F. Corcoran

   Last modified Tuesday, 19-Oct-2021 15:47:10 EDT

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