Subject: Source extraction update

Dear SDAST,

Below is an updated description of the two precisionLevel options
in source extraction. I have also included a new recipe for how
to make the lightcurve files readable by xronos (the shell scripts
and input file for that is also in the attachment).

Regards,

Stefan

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Extraction of source spectra and lightcurves use the same
basic algorithms. The extraction is made by analysing the
flux difference between the detector elements that are
illuminated by a source and those that are not. For a
given source position those areas are deduced by ray tracing
through the instrument. The contrast between the illuminated and
non-illuminated detector area is reduced by the limited resolution
of the detector. This "leakage" of flux has to be corrected for
in the analysis. Other details of the instrument characteristics
that add complexity to the data reduction include dead anodes
and differences in pixel size and shape across the detector.
In the present version of source extraction two alternative 
extraction procedures have been implemented. They are selected
with parameter precisionLevel (0 or 20). 

When the precisionLevel is set to 0 the coded mask is projected onto the
detector as seen from the direction of a source. The position of 
each detected event is then compared to the open and closed areas
of the projected mask. An energy dependent "leakage"-correction
is applied to the open and closed area fluxes. The correction factors
are taken from a parametrization which is based on a numerical model
calculation of the leakage effect. The collimator is not included in
the ray tracing but included as a vignetting correction factor which
is applied to the extracted fluxes.

A more detailed approach to the source extraction is provided by
the default precisionLevel value (20). In this case the detector
event distribution expected from the illumination by the different
sources in the source list is computed in much more detail. 
For each source a pixel illumination map of the detector is
constructed. Pixels with an illumination fraction (PIF) larger than a
cut value will be regarded as open elements and those with PIFs
below a lower cut value will be regarded as closed elements and
used for the background estimate.
Each detector pixel, with proper size and shape is devided into
16 sub pixels. For each sub pixel the line of sight towards the
sources is followed through the collimator and coded mask. When
a source is illuminating a sub pixel a probability distribution
for event detection is added to the nearby pixels. By scanning
over the full detector area, excluding non-used areas (e.g.
dead anodes) a detector illumination pattern is built up for
each source. Source extraction then proceds by counting the
number of events in the pixels with PIF values above and below the cut
levels. After area normalization, background subtraction and
leakage correction, the extracted count rate per time and energy
bin is stored as source light curves and spectra.
In precisionLevel 20, the energy dependence of the detector
resolution is not yet taken into account. For intermediate
energies (10-20 keV) this has little effect but for lower
and higher energies it does causes a slight underestimate of 
the flux. 

Error estimates for both extraction methods are based on the
assumption of poisson statistics and many counts per bin. For
lightcurves with short time bins and spectra computed
with integration times << a typical science window length
the number of counts per bin can be small and these error
estimates are then no longer valid. Sources in the outer field
of view are more sensitive to this since they only illuminate
a small fraction of the detector. One should be cautious
when analysing sources more than 3 - 4 degrees off-axis.




TO USE JEMX LIGHTCURVES IN XRONOS.

Three keywords (ONTIME, DEADC, VIGNET) are presently not set by
the source extraction program, but are required by XRONOS.

Example.

If jmx2_src_lc.fits contain lightcurves in extensions 2 to 6 the
keywords can be set by running the following a script as follows.


Command to run the script:    j2_to_xronos 2 3 4 5 6

Script "j2_to_xronos":


#!/bin/sh -f

for arg
do
 echo "Updating extension " $arg
 fmodhead jmx2_src_lc.fits+$arg j_xronos_key
done



File "j_xronos_key":

ONTIME=1.0
DEADC=1.0
VIGNET=1.0