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Introduction
This thread describes how to filter an EPIC event list for periods of
high background flaring activity.
Expected Outcome
The outcome of this thread is an EPIC filtered event list clean of time
intervals of high background activity, and a GTI file
containing the definition of Good Time Intervals for a given observation.
SAS Tasks to be Used
Prerequisites
It is assumed that the processed eventlists, like the ones produced by
the SAS tasks epproc
and emproc,
and corresponding attitude and summary files as well as the ccf.cif file are available.
Calibrated event lists may also be obtained from the archive.
Useful Links
In the following links, a full description is given of the different background components of the EPIC
cameras, including the external flaring background component which is treated in
this thread.
Caveats
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Before proceeding with this thread, the following steps are mandatory.
- Follow the SAS Startup thread and define the enviroment variables
SAS_ODF and SAS_CCF.
- Follow the Produce EPIC Event Lists thread and produce EPIC-pn and
EPIC-MOS event files.
Below you can find the step-by-step verion of the thread or the GUI
version which makes use of the user-friendly, graphical interface to
SAS, both producing the same end result. The procedure can be applied to
both EPIC-pn and EPIC-MOS instruments, with some changes to the value of some
parameters, which will be highlighted. For simplicity, to make the name
convention valid for both instruments, file names use the word
EPIC. With this naming convention it is assumed that
EPIC.fits is the name of the unfiltered event file.
Procedure
- Step-by-step version
-
Extract a single event (i.e. pattern zero only), high energy
light curve, from the event file to identify intervals of flaring particle background:
evselect table=EPIC.fits withrateset=Y rateset=rateEPIC.fits \
maketimecolumn=Y timebinsize=100 makeratecolumn=Y \
expression='Selection_Expression'
where Selection_Expression is:
#XMMEA_EM &&
(PI>10000) && (PATTERN==0) for EPIC-MOS
#XMMEA_EP &&
(PI>10000&&PI<12000) && (PATTERN==0) for EPIC-pn
Notice how the EPIC-pn energy range selected for producing the
background light curve includes events only up to 12 keV. The reason for
this is to avoid hot pixels being miss-identified as very hig
energy events. One can look at the produced light curve by running:
dsplot table=rateEPIC.fits x=TIME y=RATE
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Determine a threshold on the light curve counts, defining "low background"
intervals, to create the corresponding GTI file:
tabgtigen table=rateEPIC.fits
expression='RATE<=Rate_Expression' gtiset=EPICgti.fits
where Rate_Expression is:
RATE<=0.35 for EPIC-MOS
RATE<=0.4 for EPIC-pn
and use it to filter the event list:
evselect table=EPIC.fits withfilteredset=Y filteredset=EPICclean.fits \
destruct=Y keepfilteroutput=T \
expression='Selection_Expression'
where Selection_Expression is:
#XMMEA_EM && gti(EPICgti.fits,TIME) && (PI>150) for EPIC-MOS
#XMMEA_EP && gti(EPICgti.fits,TIME) && (PI>150) for EPIC-pn
- The file EPICclean.fits contains the filtered EPIC event
list and can be now used to produce scientific products. The file
EPICgti.fits contains the definition of Good Time Intervals.
- GUI version
- Start xmmselect
xmmselect table=EPIC.fits &
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 expression defined in a
grace
or ds9
window, respectively, into proper
selectlib
expressions
- "action" buttons (bottom)
- Extract a single event (i.e. pattern zero only), high
energy light curve, to identify intervals of flaring particle background.
This is done by:
- writing the following selection expression in the data screening
widget: '#XMMEA_EM && (PI>10000) &&
(PATTERN==0)' for EPIC-MOS and '#XMMEA_EP && (PI>10000&&PI<12000) &&
(PATTERN==0)' for EPIC-pn
- clicking on the radio button close to the TIME column
in the data column panel
- clicking on OGIP rate curve
This will pop-up another window: the
evselect
parameter user interface (see Fig.2).
Fig.2: The main
evselect
window. This particular GUI shows the case for EPIC-MOS.
On this multi-panel window, one should at least:
- click on the Lightcurve menu (and this
leads you to the sub-panel shown in Fig.3)
- define the output file name in the rateset widget
(e.g.: rateEPIC.fits)
- define the time bin size (in seconds) in the timebinsize
widget
- click Run
Fig.3: The Lightcurve panel in the
evselect
window. This particular GUI shows the case for EPIC-MOS.
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).
- Define a "low-background" interval on the light curve, using the
following steps in the
grace
window:
- go to the item Edit/Regions/Define
- define region type (e.g.: in Horiz. Range)
- click Define
- click once with the left mouse on the
grace
window to define the starting time of the first interval,
and once to define the end time.
- repeat this as many time as needed. Just be aware that
the counter Define region: needs to be updated, if
you want to define everal regions (i.e., not to overwrite the
old ones)
Fig.4: A
grace
window, displaying a light curve, and one selection 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.
- Produce the filter event file by clicking Filtered Table
in the main xmmselect
window. This will bring up again the
evselect
window (see Fig.2). Here:
- Mark the keepfilteredoutput option
- Define the output file name in the filteredset widget
(e.g.: EPICclean.fits)
- click Run
- The file EPICclean.fits contains the filtered EPIC event
list and can now be used to produce scientific products. The
procedure described using the xmmselect GUI,
does not produce the EPICgti.fits. If one wants
to profuce this file for further use, simply run the command:
tabgtigen table=rateEPIC.fits
expression='Selection_Expression' gtiset=EPICgti.fits
where the Selection_Expression is the one used to create
the file EPICclean.fits, and can be taken from
the Filtering panel of the evselect
window (see Fig.2).
An alternative way to produce an EPIC filtered event list
and corresponding GTI file
The SAS task espfilt,
one of the tools within the XMM-ESAS package (available as of SAS v9.0),
is able to perform GTI filtering of EPIC event files. The task generates soft
proton contamination-filtered products, including clean event lists and
GTI files (refer to the espfilt
documentation for a description of all the products generated and
information on the task input parameters).
In its simplest form, the task can be ran by providing the name of the EPIC event list in the
following way:
espfilt
eventset=EPIC.fits
Relevant to this thread, the two files produced containing the clean event list
and GTI file used the following naming convention:
****-gti.FIT, contains the definition of GTI
****-objevlifilt.FIT, contains the EPIC clean event list
where **** is a combination of the observation and exposure IDs.
Last Updated: 16 April 2010
