Extraction and correction of an X-ray light curve for a point-like source
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.
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.
Select the region, from which the light curve shall be accumulated, using the
Region/Circle in ds9 (see Fig.1)
Fig.1: ds9 main window. A circular region (green circle)
has been defined using the highlighted menu.
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
Centre (25910.5, 25870.5) and the Radius(400).
Fig.2: Selection region properties window, popped-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.
Now you can extract a source+background light curve,
using all the selection region and including a quality selection
appropriate for a light curve extraction.
For PN, taking good events, singles and doubles with an energy
range between 200 and 10000 eV
(#XMMEA_EP && (PATTERN<=4) && (PI in [200:10000])).
For MOS, taking good events, singles, doubles, triples and
quadtriples with an energy range between 200 and 10000 eV
(#XMMEA_EM && (PATTERN<=12) && (PI in [200:10000])).
In the example, the bin size is 100 seconds.
evselect table=PN_evt.fits energycolumn=PI expression='#XMMEA_EP&&(PATTERN<=4)&& \
((X,Y) IN circle(25910.5,25870.5,400))&&(PI in [200:10000])' \
withrateset=yes rateset="PN_source_lightcurve_raw.lc" timebinsize=100 \
maketimecolumn=yes makeratecolumn=yes
The parameter makeratecolumn=yes produces a light curve in count
rates (with errors). Otherwise the light curve is produced in counts
(with errors).
Repeat steps #4 to #6 above to determine the region, from which
the background light curve is to be extracted. We will assume
in the following that the extraction region corresponds to an
annulus, centred in (25910.5,25870.5) and with inner
and outer radii 1000 and 2000 pixels,
respectively.
Extract a background light curve, using all the selection
expressions defined so far, and the same bin size (100
seconds) and energy range as for the source+background
light curve
evselect table=PN_evt.fits energycolumn=PI expression='#XMMEA_EP&&(PATTERN<=4)&& \
((X,Y) IN annulus(25910.5,25870.5,1000,2000))&&(PI in [200:10000])' withrateset=yes \
rateset="PN_light_curve_background_raw.lc" timebinsize=100 \
maketimecolumn=yes makeratecolumn=yes
The light curves are OGIP-complaint, and therefore can be
processed by standard XRONOS-like LHEASOFT packages.
However, light curves obtained in such a way should be corrected for
various 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. A simple
command line call:
This command will launch the following xmgrace window
Fig.3: xmgrace window, containing the background-subtracted exposure-corrected light curve
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
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.
