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Extraction of a X-ray light curve for an EPIC point-like source (GUI version)

This thread contains a step-by-step recipe to extract EPIC (pn will be used as an example) light-curves of a point-like source, starting from a calibrated, concatenated event list (either produced with epproc or available as PPS product; it will bear the name: PN.fits in this thread). The content of this thread is the same as Extraction of a X-ray light curve for a point-like source (command-line version). Here, however, the extraction of scientific products is being done using the SAS task xmmselect. This is a user-friendly, graphical interface to the SAS extractor (evselect), which allows you to create a wide range of scientific products (images and pseudo-images, time series, spectra, histograms), and screen the data prior to any product accumulations. xmmselect takes advantage of the full integration between SAS and tools such as grace and ds9, to define data selection regions on a 1-D (light curve) or 2-D (image) plane.

set up your SAS environment (following the SAS start-up thread)
start xmmselect
```
xmmselect table=PN.evt &
```
First a window pops-up, asking if you wish to visualize the "[...] selection expression [...]" corresponding to "[...] data subspace information [...]". In practise, xmmselect is asking you if you wish to see the data screening expression, which was employed to generate the event list. The answer to this question does not affect the following steps.
The xmmselect call pops-up a window as shown in Fig.1.

Fig.1: The main xmmselect window

In this window, we identify:
- a data screening widget (top)
- a data column panel (middle)
- the buttons 1D region and 2D region, which allow to translate selection expressions defined in a grace or ds9 window, respectively, into proper selectlib expressions
- "action" buttons (bottom)
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.
extract an image (sky coordinates in this example; extraction in detector - DET[XY] - coordinates is possible as well). This is accomplished by:
- clicking the square checkbox besides X and Y in the data column xmmselect panel
- click on Image. This will pop-up another window: the evselect parameter user interface (see Fig.2)
  
  Fig.2: The main evselect window
- go the the Image panel in the evselect window (see Fig.3)
  
  Fig.3: The Image panel in the evselect window
- change at least the file name in the imageset window (e.g. to PNimage.fits)
- click Run
xmmselect will automatically launch a ds9 window on the created image (Fig.4)
select the region, from which the source light curve shall be accumulated, using the Regions/Shape/Circle in ds9 (see Fig.4)

Fig.4: ds9 main window. A circular region (green circle) has been defined using the highlighted menu.

propagate the selected region into the xmmselect data screening panel, by clicking the 2D region button
extract a light curve in the spatial region defined by the previous 2 steps, restricting the accumulation to single and double events, and the energy range between 200 and 10000 eV

click the radio button close to TIME in the data column xmmselect panel
in the data screening widget:
- substitute (PATTERN==0) with (PATTERN<=4)
- substitute (PI>10000) with (PI in [200:10000])
click OGIP Rate Curve
go the the Lightcurve panel in the evselect window (see Fig.5)

Fig.5: The Lightcurve panel in the evselect window
define the parameters:
- withrateset active
- rateset=PNsource_lightcurve.fits
- timecolumn=TIME
- timebinsize=100
- maketimecolumn active
- makeratecolumn active
The last two parameters tell evselect to generate a light curve in count rates, and a column TIME in the output file
click Run

xmmselect

grace

Fig.4: the grace panel displaying the light curve

extract a background light curve (PN_lightcurve_background.fits) from a region of the PN field-of-view free from contaminating sources, using the same steps 4. to 6. above
subtract the background from the source lightcurve with lccorr
- launch lccorr -d
- in the lccorr GUI window (see Fig.7) define at least the following widgets:
  
  Fig.7: The lccorr window (two panels shown)
  - srctsset to the file name of the source light curve (PN_lightcurve.fits)
  - eventset to the file name of the event list (PN.evt)
  - withbkgtsset active
  - bkgtsset to the file name of the background light curve (PN_lightcurve_background.fits)
  - outset to the name of the output file (PN_lightcurve_bs.fits)
  - subtractbkg active
- click Run
The file PN_lightcurve_bs.fits contains the background-subtracted light curve. This file is compliant with the OGIP standards, and can be therefore analyzed with, e.g., the XRONOS package (refer to the timing analysis thread if you want to continue this way).