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Дата изменения: Tue Aug 22 17:54:07 2006
Дата индексирования: Sat Dec 22 14:51:57 2007
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Поисковые слова: photosphere
From 0 to <tt>Xspec</tt> in a few clicks

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From 0 to Xspec in a few clicks

The SAS task especget has been designed to allow extraction of source+background and background spectra, redistribution matrices, and effective area vectors from an EPIC event list with a single command.  especget can be launched from the main xmmselect window, in which case an extraction region size optimization algorithm is also ran, which maximizes the signal-to-noise of the net source spectrum. From xmmselect the user is guided by the GUI through the especget steps. This thread illustrates how to use this functionality. In the following, PN.evt is the pn event lists employed to extract spectral products.

  1. set up your SAS environment (following the SAS start-up thread)

  2. start xmmselect
    xmmselect table=PN.evt &

    The xmmselect call pops-up a window similar to that shown in Fig.1.


    Fig.1: The main xmmselect window
     

    In this window, we identify:

  3. extract a single event (i.e. pattern zero only), high energy (E > 10 keV) light curve, to identify intervals of flaring particle background
  4. . This is done by:

    This will pop-up another window: the evselect parameter user interface (see Fig.2).


    Fig.2: The main evselect window
     

    On this multi-panel window, one should at least:


    Fig.3: The Lightcurve panel in the evselect window
     

    As customary for SAS task, each widget, button or menu in the evselect window corresponds to a task parameter. The whole list of available evselect parameters, with their description, is available at the evselect task description.

    Once Run is clicked, dsplot is silently run on the created light curve, and the corresponding grace window appears (see Fig.4)

  5. define a "low-background" interval on the light curve, using the following steps in the grace window:


    Fig.4: A grace window, displaying a light curve, and one interval created as explained in text
     

    Be aware that grace allows you to define several different types of intervals. Among the normally most useful: Above/below line, in Vert. range,Out of Horiz/Vert. range, Left/Right of line

    When you are happy with your definition, click the button 1D region in the xmmselect window. The selection region will be automatically transferred into the data selection widget of the xmmselect window, and properly translated into a selectlib expression

  6. extract an image (sky coordinates in this example; extraction in detector - DET[XY] - coordinates is possible as well, and may be preferable for some specific scientific needs)
  7. . This is accomplished by:


    Fig.5: The Image panel in the evselect window
     

    xmmselect will automatically launch a ds9 window on the created image

  8. select the region, from which the source+background spectrum shall be accumulated, using the Region/Circle menu in ds9
  9. . Properties shall be set to Source (the green circle on the ds9 window will appear as a solid line)

  10. select the region, from which the background spectrum shall be accumulated, using the Region/Circle menu in ds9
  11. . Properties shall be set to Background (the green circle on the ds9 window will appear as a dashed line)

  12. click the function button OGIP Spectral Products in the main xmmselect window

  13. a window pops-up, asking the user for the preferred method to determine the source+background extraction region:


    Fig.6: Spectrum extraction region optimization window in especget
     

  14. a 7.0.0 window pops-up, asking for confirmation of the input parameters (see Fig.7)


    Fig.7: Main especget window
     

    It is recommended to change the filestem parameter to a string of user's choice. Be reminded that xmmselect silently overwrites files with the same name. Given the choices in Fig.7, the following files will be created:

  15. prepare the source+background spectrum and link associated files
  16. FTOOL: grppha: PHA filename: mypn_src.ds
                   output filename: PNsource_spectrum.grp
                   group min 25 ! as an example
                   exit
  17. fit the spectrum

  18. NOTE ON PATTERN SELECTION IN PN SPECTRA:

  19. For bright sources and sources with narrow lines it might be better to extract two spectra and corresponding backgrounds, response and ancillary files: one set for single pixel events (PATTERN==0) and another set for doubles (PATTERN IN [1:4]).
    Fitting these two spectra simultaneously will show if there are any problems with pile-up (see also SAS task epatplot) and - as the energy resolution for singles is slightly better than the one for doubles - will show the line features at highest energy resolution in the single events spectra.
    However, in case of TIMING mode observations (where the rate of single to double events depends on the source position) one should always create and fit a spectrum of the combined single and double events.