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 re-calibrated attitude file and a GTI table.
> 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.attcor \\ ea1=16.007012398071 ea2=162.031577674707 ea3=29.330729822566 \\ xis_rmffile=/FTP/caldb/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 are absorbed or blocked by mirrors or instruments.
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, which can be done using xisexpmapgen.
Here we assume "ae102002010.attcor" to be the name of the
attitude file corrected for the wobble using aeattcor, 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 in the 1st extension and an exposure map in the sky
coordinate in the 2nd extension. Here, we generate a mask image in the
sky coordinate, and so use the image in the 2nd extension.
By displaying the 2nd extension, one can empirically determine a
good threshold for masking. For a threshold of 5000 (s), use:
This produces a masking image, called "skymaskmap.img." This
may have to be rebinned to match the binning of the exposure map (by default,
xselect bins Suzaku images by a factor of 8), before
multiplying 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.attcor
> fimgtrim infile=expmap.img\[2\] threshlo=5000 threshup=5000 const_lo=0 const_up=1 outfile=skymaskmap.img
> fimgbin skymaskmap.img skymaskmap_8bin.img 8
> 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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