How to evaluate and test pile-up in an EPIC source
This thread illustrates how to assess whether an observation is
affected by pile-up. Pile-up occurs whenever a X-ray source is too bright for the selected
read-out mode, thus in practice is like an overexposure effect.
During a single read-out cycle more than one photon is detected in two
or more adjacent pixels. During the read-out, the electronics cannot
distinguish whether signals in adjacent pixels originate from one or more
photons, thus two or more photons are erroneously combined to an
individual event of higher pattern type, e.g. two adjacent individual
photons are erroneously combined to a double event whose
energy is equal to the sum of the individual energies of the incoming
During a single read-out cycle more than one photon hits the same
pixel. During the read-out, the electronics cannot distinguish
whether the signal in a pixel originates from one or more photons, thus
a single event of erroneous energy is read whose energy is equal to
the sum of the individual energies of the incoming photons.
The effect of pile-up on the spectra is therefore three-fold:
Photon loss, either due to the fact that one photon is "read"
instead of several, or to the fact that the summed energy may assume
values beyond the upper energy on-board threshold. Also, an invalid
pattern could be produced and hence the photons lost.
Energy distortion, whereby photons are moved
to higher X-ray regions of the spectrum.
Pattern migration, where the expected
pattern distribution is distorted.
This thread provides a step-by-step description on how to assess
pile-up using the pattern distribution and provides one method
to minimize its effects and another one to correct for it.
Also, this threads explains how to deal with X-ray loading which
accompanies pile-up observations. X-ray loading occurs when the count rate is
sufficiently high that the offset map, created on-board, is distorted by
X-ray photons. The count rate at which this becomes important
is discussed in XMM-SOC-CAL-TN-0050.
The net effect of X-ray loading is to artificially reduce the energy of
events by over-subtracting the dark current in each pixel. The X-ray loading
correction is only necessary for EPIC-pn imaging modes. For Timing
mode, this is dealt with by SAS automatically, while for MOS, the correction is not
The EPIC-pn offset map has been designed to correct events for optical
loading. For this reason it is important NOT to run X-ray loading
correction if the source is optically bright.
Magnitude limits are given in
In other words, if the source exceeds the optical and the X-ray limits for the
observation mode and transmission filter combination, then the
application of the automatic correction for X-ray loading is not possible.
Plot (PS-file) comparing the observed versus the expected pattern
distribution within a source extraction region. If both agree, pile-up
is not a problem for this source.
A response file to be used for correcting the spectra for the flux loss
and energy distortion caused by the pile-up of photons. As of SAS v13,
this method can only be applied to the EPIC-pn camera in imaging mode,
i.e. ExtendedFullFrame, FullFrame, LargeWindow and SmallWindow modes.
Create EPIC calibrated event list, a corresponding GTI file and
identify the centroid position and extraction radius of your source,
using the procedure explained in the
pn spectrum extraction thread.
Extract a filtered event list, including only photons within the
source region, without any filtering in PATTERN or quality FLAG.
Assuming that the region has a centroid position (27600,27900)
and radius (800) in sky coordinates (X/Y), and that the original event
list file name is pn_evt.fits and the GTI file is
The file pn_filtered_pat.ps contains a plot like the following:
Fig.1: epatplot output
file, which in this case indicates the presence of pile-up.
This plot contains two panels:
The upper panel shows a spectrum (distribution of
counts as a function of the PI channel: be aware that the
counts are not folded with the detector response) for
each event pattern class.
The lower panel shows the expected pattern
distribution functions (smooth solid lines)
superimposed on the observed ones (histogram).
In this panel, also displayed is the observed-to-model fractions for
single and double events within a certain energy range.
In all plots, the distributions
corresponding to the different pattern classes
are recognizable through colors: red
for single, blue for double, green for triple, and
turquoise for quadruple events.
Your source spectrum will be affected by pile-up if - as in the case shown
in Fig.1 - the expected distributions are significantly discrepant from
the observed ones. epatplot calculates two
diagnostic numbers which may be used to assess the presence of pile-up:
In the absence of pile-up, the 0.5 - 2.0 keV observed-to-model singles
and doubles pattern fractions ratios should both be consistent with 1.0
within statistical errors (1 sigma errors are given). If pile-up is
present, the singles ratio will be smaller than 1.0 and the doubles
ratio will be larger than 1.0. These ratios are both printed
to the console and displayed on the plot. They are also added to
the header of the input event set as attributes SNGL_OTM and DBLE_OTM (1 sigma
errors: ESGL_OTM and EDBL_OTM).
Below, we provide a way to minimize the effects of pile up and a way to
correct for pile up. The pile-up correction method was
introduced in SASv 13. While it has been proved to correctly recover flux
and spectral shapes for moderately (~a few percent) piled-up sources
(see, e.g., the
SAS Scientific Validation Reports), its performances for sources affected
by a higher level of pile-up have still to be fully assessed. It is then
recommended to try both methods and check the consistency of the
results. If the results are consistent, pile-up correction should be
employed since no photons are discarded by this method.
The way to reduce pile-up in a given data set is to excise the
core of the PSF, up to a radius where the pile-up fraction becomes
negligible. In order to apply this method, extract filtered event lists of annular regions around
the centroid position excising more and more of the core of the PSF
until the observed pattern distributions match the expected ones, as
presented in Fig.2 below.
The following commands excise a radius of 150 from the core of
the PSF used in the previous example and creates another pattern plot
for this annular region:
evselect table=pn_filtered.evt withfilteredset=yes filteredset=pn_filtered_annulus.evt \
keepfilteroutput=yes expression="((X,Y) in ANNULUS(27600,27900,150,800))"
The file pn_filtered_annulus_pat.ps contains a plot like the
Fig.2: epatplot output
file, which in this case indicates a negligible pile-up fraction.
As of SAS v13, rmfgen
includes an option to correct for the flux
loss and energy distortion caused by the pile-up of photons within a
single frame. It does this by generating a redistribution matrix that
attempts to compensate for the pile-up effects, which are calculated
from the frequency and spectrum of the incoming photons.
Before the correction can be applied, we need to produce a set of spectral
files following the standard procedure. To produce the set of spectral files follow the
How to extract PN spectra of a point-like source and associated
matrices Thread. The set of files generated in this way
myarf.arf and myresp.rmf), will correspond to a non
pile-up corrected spectrum.
Generate a response file which will include the pile-up correction.
the file spectrum.ds corresponds to the spectral file generated
in the previous step.
The task rmfgen
needs to read a raw event file (raweventfile=events04.dat) to
make the pile-up correction. This should contain all the charges which
landed on the CCD where the source is centred (in this example CCD#4).
This raw event file is not created by default by the SAS processing. It
has to be produced in one of the following ways:
which produces a raw event file for each CCD with the name
which produces a raw event file for each CCD with the name
The pile-up correction technique used is very compute-intensive. The execution
time is of the order of hours for typical observations.
We should consider at this point as well correcting for X-ray loading. For this purpose,
run the EPIC reduction meta-task epproc
and select the X-ray loading correction option at the same time as pile up correction.
This produces an output event file with corrected energies.
Once the response file, myresp_pileupcorr.rmf, including
pile-up correction is available, we can rebin the spectrum and link all
the associated files. In the following example the spectrum is rebinned
in order to have at least 25 counts for each background-subtracted
spectral channel and not to oversample the intrinsic energy resolution
by a factor larger than 3.
where the files spectrum.ds, background_spectrum.ds
and myarf.arf correspond to the spectral files generated
in the first step, and spectrum_grp.fits is the grouped spectrum.
The rebinning options shown in the example above, are by no means the
only possible binning available, or the recommended one, more binning
options are available in the description of the task specgroup.
We advise to check for pile-up using
even if the count rate of the observation is below the count
rate limits of the corresponding mode provided in the
The calculated observed-to-model fractions for single and
double events within a certain energy range provided in the
lower panel of the plot can be incorrect. The
important factor is the graphical agreement of the observed
(histogram) and expected (solid line) pattern distribution.
If the statistical quality of the observed pattern
distributions (histograms) become too low, the statistical
significance used by
can be reduced using the sigma keyword. The following
example reduces the default statistical significance of
sigma=3 (default) to sigma=2:
The energy range of the observed-to-model fraction
calculation can be adapted via the keyword
pileupnumberenergyrange. The following example
switches the default energy range (0.5-2.0 keV) into the energy range used
as in Fig.1 and Fig.2:
Excising the core of the PSF on scales too small with respect to the
instrumental pixel size (1.1" for the MOS cameras, 4.1" for the PN
camera) may introduce systematic inaccuracies in the calculation of the
source flux. Simulations show that these systematics are lower than 1%
if the radius of the excised core is larger then 5 times the
instrumental pixel half-size. For lower sizes these systematics are
never larger than 4%. An enhancement of the PSF handling in the SAS is
under development to reduce the impact of this effect. In the meantime,
users should carefully evaluate the size of the excised core depending
on their scientific requirements, and of the source pile-up level. It
goes without saying that excising a core, whose size is smaller than the
instrumental pixels size, shall be absolutely avoided.