icl> xrtsort ! Sort an image
icl> iload image xwin ! Start the image processing system
icl> idisplay ! Display the image
icl> icircstats ! Find the counts in a source
To determine accurate source counts, the source searching routine, pss should be used. This routine provides a sensitive means of finding the sources in an XRT observation. Full details are given in the help and in the document, USER_004. It has a large number of operational modes but the following method works well for finding sources and obtaining count rates from a PSPC image.
1. Produce a background model image using xrtsub. This program creates a model of background by scaling the vignetting function as a function of energy using a source free portion of the input data and adding an estimate of the particle background (which is not vignetted). The simplest way to do this is to create a spectrum of a blank region using xrtsort. The lower and upper bounds in channels must be exactly the same as used to create the image to be searched by pss, and the spectrum should contain between 4 and 10 bins. For instance if the image was sorted over the corrected PH range 8-247, then the spectrum must be produced over the same 240 channels and may consist of 4,6,8, or 10 bins.
Invoke xrtsub giving it the image you want to search at the SOURCE prompt, your background spectrum at the BCKGND prompt, and supply it with a file name for the background model using the BGMODEL parameter. It will also output a background subtracted copy of the SOURCE image using the OUT parameter. pss can accept either the raw image or the background subtracted image.
2. Run pss using the XRT_PSPC VARPROFILE option in POLAR mode as the PSF and giving the background image generated above as the background (BGND) option. See Example 3. pss produces two outputs, a significance map and source list. The first of these is optional but useful in diagnosing problems. It is a map over the image region searched of the likelihood ratio of the two models background only and background + point source. pss searches for points in this map above a user supplied threshold, identifies these as sources and finds confidence intervals on the parameters describing the source, ie. flux, X and Y. The flux output is in raw counts and takes into account the fact that not all of the psf may be included in the search box.
3. The source list from pss may be displayed to the screen, file or printer using the ssdump command.
4. The source counts in the list produced by pss may be exposure corrected using xpsscorr.
xpsscorr can be used to convert the source counts calculated by pss into a count rate. This is the count rate which would have been observed had the source been on axis. It does this by dividing by the exposure time and by using calibration files to determine the vignetting. By default it also multiplies by a factor 1.0/0.79 to correct for the absorption by the fine wires in the support structure.
Alternatively the merged exposure map which is provided with each observation may be used to calculate the exposure time. This latter approach has the advantage of coping with sources which are partially obscured by the window support structure, however, it is only valid for images produced over the full time range and over the full pulse height channel range. Any subset of these quantities will invalidate the use of the exposure map.
With either method, the wire correction may be turned off by specifying "wires=no" on the command line. If the count rates are going to be subsequently converted into a flux using the effective area files or used within spectral fitting, the wire correction should NOT be applied as it is already included in the detector matrix.
5. Source positions can be marked on the displayed image using imark, or added as permanent annotations in the presentation graphics using ssanot.
6. Time series or spectra can be extracted for every source in a pss list using the xrtmtime and xrtmspec procedures respectively.
7. The whole process can be performed iteratively. To refine your background estimate those regions of the image input to xrtsub containing sources can be masked out using iregion and iexclude or Isys. This produces a background unbiased by the presence of point sources which can then be used in pss again.