How to Produce an Vignetting-Corrected XIS Image using XISSIM
For the study of extended sources with Suzaku XIS, it is necessary to know the exposure times as well as vignetting at various sky locations within the XIS image. This document describes methods for generating appropriate maps.
Prerequisits
We assume that the users have installed the latest version of Suzaku FTOLLS (Version 6 or later).
We also assume that the users have obtained data that have been processed with the Version 2 processing pipeline or that the users have reprocessed their own data using Version 6 software. This reduces the offset between the observed XIS image and the calculated exposure map.
Types of Exposure Maps
One type of exposure maps can be created by simply considering the detector field of view and the spacecraft attitude, the result being the actual exposure time per sky pixel. Such exposure maps can be created by using xisexpmapgen, which allows users to exclude unused pixels such as bad columns, hot/flickering pixels, SCI rows, and the 55Fe calibration source area. See section 3.1 below as well as the help file of xisexpmapgen for further details.
In the other type, the effective exposure times per sky pixel are calculated, taking into account the vignetting of the XRT. Below, we describe how to use xissim to simulate a "flat field" image for this purpose.
1. Running xissim
As an example, we show how to simulate an XIS0 flat field image at 2.45 keV of the observation sequence 102002010. The attitude wobbles during this observation are included in the simulation by supplying the attitude file and a GTI table. We assume the use of Version 2.x processed data, which includes attitude files that are significantly more accurate than those found in Verion 1.X processed data.
> xissim instrume=XIS0 enable_photongen=yes photon_flux=1 flux_emin=1.0 \\
flux_emax=10.0 spec_mode=1 image_mode=2 time_mode=0 limit_mode=1 energy=2.45 \\
ra=16.0083 dec=-72.0313 sky_r_min=0 sky_r_max=20 exposure=15825.09 \\
pointing=AUTO gtifile=cleaned.evt\[GTI\] attitude=ae102002010.att \\
ea1=16.007012398071 ea2=162.031577674707 ea3=29.330729822566 \\
xis_rmffile={CALDB_directory}/data/suzaku/xis/cpf/ae_xi0_20060213.rmf \\
outfile=sim_x0.fits phafile=allarea.pi
- Notes
-
- In this example, we supply the name of the event file after screening (cleaned.evt) as the "gtifile" parameter value, and use a spectral file made from cleaned.evt (allarea.pi) as the "phafile" value (this is used by xissim to determine the observation mode, such as the window and the spaced charge injection options; cleaned.evt would also work).
- The Euler angles (ea1, ea2 and ea3 parameters) are used if, any time during the specified good time intervals, the attitude file does not have data. These can be obtained from the header keywords MEAN_EA1, MEAN_EA2, and MEAN_EA3 in the event file.
- The value of the "exposure" parameter should be equal to (or an integer multiple of) the actual exposure time of the observation, to consider the effect of the attitude wobbles correctly. Increase the value of the "photon_flux" if more photons are needed.
- In the above example, simulation is carried out for a single energy (spec_mode=1) of 2.45 keV (energy=2.45). To consider a range of photon energies, change spec_mode to 0, and supply a QDP file of the spectral model ("qdp_spec_model") - see help file on mkphlist for details.
Note that the output file has only ~10% of the seed photons. This is because most of the photons end up outside the XIS field of view.
2. Extracting a flat field image using xselect
The simulated events created by xissim have the STATUS information, which describes the quality of each simulated photon. Thus the simulated event files should be screened using the same STATUS criteria as was used for the observed events.
Then the flat field image can be extracted in xselect, making sure that the same XY binning is used.
3. Smoothing the flat field image (optional)
It is difficult to avoid statistical fluctuation in a simulated flat field map, so it is often desirable to smooth the map using, e.g., ximage or ds9. We assume that the flat field map has been smoothed, with the file name "flatfield_smo.img."
A smoothed map generally has rough edges, so it is useful to trim such a map
with a masking image. The masking image should have 1s where good data were
obtained, and 0s where exposures were too short. Such an image can be
generated as follows, first using xisexpmapgen.
Here we assume "ae102002010.att" to be the name of the
attitude file, and use the cleaned event file as the value of the
"phafile" parameter to supply XIS mode (such as the window option).
The output file ("expmap.img") contains two maps; a mask image in the
detector coordinate as the primary image and an exposure map in the sky
coordinate in the 1st extension. Here, we generate a mask image in the
sky coordinate, and so use the image in the 1st extension.
Next, we apply the same binning to this as used in the exposure map
(by default, xselect bins Suzaku images by a factor of 8).
This produces a masking image, called "skymaskmap.img," which
only contains 0s and 1s. This can be multiplied with the smoothed flatfield
image.
The above produces a vignetting corrected image. The flat field image
can be scaled to make it a true effective exposure time map, although
the normalization depends on the purpose of such an operation.
Depending on the scientific objectives, it may well be desirable to
subtract particle, cosmic X-ray, or Galactic X-ray background from
the observed image before dividing by the flat field.
3.1 Trimming the flat field map (optional)
> xisexpmapgen expmap.img cleaned.evt ae102002010.att
> fimgbin expmap.img\[1\] expmap_8bin.img 8
The output file (expmap_8bin.img) will have the original (unbinned)
primary image, plus the binned 1st extension. By displaying the 1st
extension, one can empirically determine a good threshold for masking.
Typical pixel values at the center of the rebinned 1st extension
should be 8x8=64 times the exposure of your observation.
For a threshold of 320,000 (s x 64), use:
> fimgtrim infile=expmap_8bin.img\[1\] threshlo=320000 threshup=320000 const_lo=0 const_up=1 outfile=skymaskmap.img
> farith flatfield_smo.img skymaskmap_8bin.img flatfield_smo_trim_8bin.img "*"
4. Applying the flat field image
> farith input.img\[0\] flatfield_smo_trim_8bin.img input_vigcor.img "/"
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