Extraction and correction of an X-ray light curve for a point-like source (GUI version)
|
Introduction
This thread contains a step-by-step recipe to extract light curves of a
point-like source for all the X-ray cameras, subtracting the background
and correcting for exposure losses.
Expected Outcome
Corrected light curves of XMM-Newton EPIC and RGS instruments.
SAS Tasks to be Used
Prerequisites
Useful Links
Caveats
|
Procedure
EPIC
The content of this thread is the same as
Extraction and correction of an 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.
As an example case, we will consider the extraction of a light curve
from a pn event list (
PN_evt.fits). The same recipe applies for MOS.
- Set up your SAS environment (see Prerequisites for this thread at the
top of the page).
- Be aware: if you are interested in very short time periods,
such as they appear in pulsars or cataclysmic variables, you
have to perform a barycentric correction. This means that
the arrival time of a photon is shifted as is it would have been
detected at the barycentre of the solar system (the centre of
mass) instead at the position of the satellite. In this way,
the data are comparable. The SAS task
barycen performs
this correction. As
barycen overwrites the
TIME column entries, it is advisable first to copy the
original event list.
cp PN_evt.fits PN_evt_barcen.fits
barycen table=PN_evlt_barcen.fits
- Start xmmselect
xmmselect table=PN_evt.fits &
First a window pops-up, asking if you wish to visualize
the "[...] selection expression [...]" corresponding to "[...]
data subspace information [...]". In practice,
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.
- Filter the events with a quality selection appropriate for a
later light curves extraction as one shown for PN
in the xmmselect window of Fig.~1:
#XMMEA_EP && (PATTERN<=4) && (PI in [200:10000]),
taking good events, singles and doubles with an energy range
between 200 and 10000 eV (corresponding MOS selection would be
#XMMEA_EM && (PATTERN<=12) && (PI in [200:10000])).
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 check boxes beside 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.img)
- 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 circular check box close to TIME in the data column
xmmselect panel
- 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=PN_source_lightcurve_raw.lc
- 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
launches automatically a
grace
window, displaying the output light curve (as shown for
background light curve in Fig.6)
Fig.6: the
grace
panel displaying the light curve
- extract a background light curve (PN_light_curve_background_raw.lc)
from a region of the PN field-of-view free from
contaminating sources, using the same steps 4. to 6. above
- however, light curves obtained in such a way should be corrected for
diverse effects affecting the detection efficiency, such as
vignetting, bad pixels, PSF variation and quantum efficiency,
as well as for variations affecting the stability of the detection
within the exposure, like dead time and GTIs. Since all these
effects can affect in a different manner source and background
light curves, the background subtraction has to be done accordingly.
A SAS task, epiclccorr, performs all of these
corrections at once. It requires as input both light curves
(which are used to establish the binning of the final corrected
background subtracted light curve) and the event file, therefore
subtract the background from the source lightcurve
with epiclccorr
- launch epiclccorr
-d
- defining the proper parameters (Fig.7):
Fig.7: The
epiclccorr window
- srctslist to the file name of the source light curve
(PN_source_lightcurve_raw.lc)
- eventset to the file name of the event list
(PN_evt.fits)
- outset to the name of the output file
(PN_lccorr.lc)
- withbkgset active
- bkgtslist to the file name of the background light curve
(PN_light_curve_background_raw.lc)
- applyabsolutecorrections yes
- click Run
- plot the resulting light curves, eg.
dsplot table=PN_lccorr.lc withx=yes x=TIME withy=yes y=RATE
This command will launch the following xmgrace window
Fig.8: xmgrace window, containing the background-subtracted
exposure-corrected light curve
The file PN_lccorr.lc contains the background-subtracted
exposure corrected
light curve. This file is compliant with the OGIP standards, and
therefore can be processed by, e.g., the
XRONOS package
(refer to the Timing
Analysis Thread if you want to continue this way).
RGS
We will assume in what follows that RGS products are available in the working
directory: a) RGS event list; b) RGS source
list (following the
How to reduce RGS data
and extract spectra of point-like sources thread).
A call to the task
rgslccorr will perform light curve extraction, correction and
background subtraction using events from the same
selection regions used for extraction of spectra for both source and background.
- Create, for example, a combined RGS1 and RGS2, 1st and 2nd order, 100-second binned
exposure-corrected and background-subtracted light curve, for the
3-rd source in the source list
rgslccorr -d
Fig.9: the rgslccorr GUI window
- After epiclccorr has ran, plot the resulting light
curves, for example,
dsplot table=RGS_rates.ds withx=yes x=TIME withy=yes y=RATE
This command will launch the following xmgrace window
with the combined RGS light curve.
Fig.3: xmgrace window showing the background-subtracted
exposure-corrected RGS light curve
Last Updated: 16 April 2010
Caveats
If you want to compare/combine light curves of the different XMM-Newton
instruments, it is strongly recommended to explicitly set
time limits to align the time bins for all instruments.
- For EPIC, set the time limits using
the keywords timemin and timemax when you create the
source+background light curves using the evselect command.
- For RGS, using rgslccorr, set the keyword
withtimeranges=yes and set the time limits via the
keywords timemin and timemax.
For detailed scientific analysis make sure that your source
region is free of pile-up
(How
to assess pile-up).
|
