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Дата изменения: Wed Jun 18 21:12:48 2008
Дата индексирования: Fri Sep 5 23:19:36 2008
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Extraction of a X-ray light curve for a point-like source

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Extraction of a X-ray light curve for a point-like source (command-line version)

This thread contains a step-by-step recipe to extract a light curve of a point-like source for all the X-ray cameras, and to start a simple analysis tool. The starting point is a calibrated, concatenated event list. All the light curves in the following threads are not-background subtracted

EPIC

As a testing case, we will consider the extraction of a light curve from a pn event list (PN.evt). A similar recipe applies for a MOS event list.
  1. set up your SAS environment (following the SAS start-up thread)

  2. extract an image (in sky coordinates in this example; extraction in detector - DET[XY] - coordinates is possible as well)

    3. evselect table=PN.evt:EVENTS imagebinning=binSize imageset=PNimage.fits withimageset=yes \
  xcolumn=X ycolumn=Y ximagebinsize=80 yimagebinsize=80
  3. display the image
    4. imgdisplay withimagefile=true imagefile=PNimage.fits
  4. select the region, from which the light curve shall be accumulated, using the Region/Circle in ds9 (see Fig.1)


    5. Fig.1: ds9 main window. A circular region (green circle) has been defined using the highlighted menu.


     
  5. double-click with the cursor on the defined region. A window pops up, showing the properties of the region (Fig.2). Write down the coordinates of the Center (25910.5, 25870.5) and the Radius(400).


    6. Fig.2: Selection region properties window, pop'd-up by double-clicking on the region in the main ds9 window


     
    Units of sky coordinates (X,Y) are 0.05 arcsec, hence the radius in our example is 20 arcsec.
     
  6. extract a source+background light curve, using all the selection expressions defined so far. In the example, the binsize is 100 seconds and the energy range is 0.2 to 10 keV. Please take into account that operating with non-synchronous time series can introduce artifacts when they are added or subtracted by programs such as the ftools "lcmath". To avoid this problem use time limits explicitely set in the command line.
    7. evselect table=PN.evt energycolumn=PI expression='#XMMEA_EP&&(PATTERN<=4)&& \
  ((X,Y) IN circle(25910.5,25870.5,400))&&(PI in [200:10000])' withrateset=yes rateset="light_curve.fits" timebinsize=100 \
maketimecolumn=yes makeratecolumn=yes timemin=126991800 timemax=130000000
    The parameter makeratecolumn=yes produces a light curve in count rates (with errors). Otherwise the light curve is produced in counts (with errors).

  7. repeat step 4. to 6. above to determine the region, from which the background light curve is to be extracted. We will assume in the following the the extraction region correspond to an annulus, centered in (25910.5,25870.5) and with inner and outer radii 1000 and 2000 pixels, respectively

  8. extract a background light curve, using all the selection expressions defined so far, and the same binsize (100 seconds) and energy range as for the source+background light curve
    9. evselect table=PN.evt energycolumn=PI expression='#XMMEA_EP&&(PATTERN<=4)&& \
  ((X,Y) IN annulus(25910.5,25870.5,1000,2000)&&(PI in [200:10000])' withrateset=yes rateset="light_curve_background.fits" timebinsize=100 \
maketimecolumn=yes makeratecolumn=yes timemin=126991800 timemax=130000000

  9. plot the resulting light curves, eg. 
     dsplot table=light_curve_background.fits withx=yes x=TIME withy=yes y=RATE
    This command will launch the following xmgrace window



    Fig.3: xmgrace window, containing the extracted light curve

    The light curves are OGIP-complaint, and therefore analyzable with standard XRONOS-like LHEASOFT packages.

    RGS

    We will assume in what follows that RGS products are available in the work directory, as according to the PPS naming convention. If the following files are available: a) RGS event list; b) RGS source list, just a single evselect command allows a user to create a light curve, corresponding to the extraction regions in the spatial and order images.

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

    2. create a 1st order, 100-seconds binned source light curve, for the n-th source in the RGS1 source list
      3. evselect table=P0133120201R1S001EVENLI0000.FTZ expression='region(P0133120201R1S001SRCLI_0000.FTZ:RGS1_SRCn_SPATIAL,BETA_CORR,XDSP_CORR) \
&®ion(P0133120201R1S001SRCLI_0000.FTZ:RGS1_SRCn_ORDER_1,BETA_CORR,PI)' withrateset=yes rateset="light_curve.fits" \
timebinsize=100 maketimecolumn=yes makeratecolumn=yes

    3. create a 2nd order, 100-seconds binned source light curve, for the n-th source in the RGS1 source list
      4. evselect table=P0133120201R1S001EVENLI0000.FTZ expression='region(P0133120201R1S001SRCLI_0000.FTZ:RGS1_SRCn_SPATIAL,BETA_CORR,XDSP_CORR) \
&®ion(P0133120201R1S001SRCLI_0000.FTZ:RGS1_SRCn_ORDER_2,BETA_CORR,PI)' withrateset=yes rateset="light_curve.fits" \ 
timebinsize=100 maketimecolumn=yes makeratecolumn=yes