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How to create ARFs for extended sources

last updated on January 13, 2000

ARFs for extended sources

For precise ASCA spectral analysis of extended sources, characteristics of the XRT point spread function have to be taken into account. In particular, it is important to estimate effects of the scattered X-rays from outside of the selected region to extract the spectrum. In order to estimate effects of the scattering, ray-tracing simulation will be required.

ascaarf (vers 3.00 and later), the standard ARF creating FTOOL, has been updated to have a new functionality to create ARFs using ray-traced images. By combining ascaarf and ascaray (ray-tracing FTOOL), users can create precise ARFs for uniformly extended sources. The make_ascaray_images script (distributed with the FTOOLS v4.3) will provide a simple user-interface to ascaray, running ascaray sequentially to create many ray-tracing images which may be used by ascaarf.

Also we provide the output ray-traced images of make_ascaray_images for uniformly extended sources at 1 degree, 2 degree and 3 degree radius. Users may use these ready-made ray-traced images with ascaarf to create extended source ARFs, or run make_ascaray_images for different source distribution.

How to run make_ascaray_images

make_ascaray_images will spawn ascaray and create a set of ray-traced images which may be used by ascaarf. Users specify radius of the uniformly extended source, and ray-tracing is carried out by randomly distributing input photons within that extension.

It is easy to run make_ascaray_images. In the following example, 60 arcmin radius is assumed, and one million photons are input for each of the 300 mono-energetic ray-tracing runs from 0.1 keV to 12 keV. Default gold density 19.3 g/cc and Nagoya optical constants (on which official XRT response is based) are used. The output images are made with the GIS size, but they can be used to create SIS ARFs too (ascaarf normalizes the image size). The output ray-traced images will be compressed with gzip. List of the ray-traced image files is written in the ASCII file "ascaray_list". You may specify this file to the ascaarf "rayfile" parameter.

   % make_ascaray_images
   make_ascaray_images version 1.1 
   For help, enter "make_ascaray_images -h"
   Output file list name (used for ascaarf;default ascaray_list) ?

   Output file list = ascaray_list
   Number of input photons for each energy  [ 1000000 (or larger suggested) ]: 
   Extended (e) or point source (p) ?
   Size of the source extension  [ >0 (arcmin) ]: 60
   Gold density value [ 19.3 (default) ]: 
   Nagoya or Owens atomic constants  [ 1 or 2 (1 for Nagoya; 2 for Owens) ]: 1
   Create GIS (g) or SIS (s) wmap size images?
   As a default, 300 images are created from 0.1 keV to 12.0 keV.
   (20 images for 0.1-2 keV, 200 images for 2-4 keV, and 80 images for 4-12 keV)
   You may choose the energy range for the images to be created.
   Index of the lowest energy image [ 1 (corresponds to 0.1 keV) ]: 
   Index of the highest energy image [ 300 (corresponds to 12.0  keV) ]: 
   Each image is 100~300 kbytes, but the size will be about one-sixth after the compression.
   Do you like to compress the images? (Y/n): 
This session creates 300 files named as ascaray_d60_e0.10.fits.gz, ascaray_d60_e0.20.fits.gz,,,,ascaray_d60_e12.00.fits.gz, and "ascaray_list" looks like as follows:

    0.10 ascaray_d60_e0.10.fits
    0.20 ascaray_d60_e0.20.fits
    0.30 ascaray_d60_e0.30.fits
    12.00 ascaray_d60_e12.00.fits
Note that ray-tracing takes time. A single ascaray run with one million photon will take typically ~150 seconds on a Pentium 233 MHz machine.

We have already created sets of ray-traced images for 1 deg, 2deg or 3deg radius sources by running make_ascaray_images. These images are found at The list file "ascaray_?deg_list file" in each directory (? is either 1, 2 or 3) has entries of these ray-traced images in the ftp address. You may use these list files with ascaarf to read the remote files at GSFC through network (cfitsio has a capability to read remote files). Please note that make_ascaray_images normalizes ray-traced images created by ascaray for extended sources, so that unit of each pixel will become cm2*str. Because of that, the extended ARFs created with ascaarf from these ray-traced images will have units cm2*str instead of just cm2.

How to use ray-traced images with ascaarf

You should have a standard spectral file created with xselect and corresponding RMF. If your "list file" of ray-tracing images is ascaray_3deg_list file,
   ascaarf raytrace=yes rayfile=ascaray_3deg_list file
should work both for SIS and GIS. Input your spectral file and response file as prompted.

Example: GIS spectral analysis of Cosmic X-ray Background

The best example of "uniformly extended sources" will be the Cosmic X-ray Background (CXB). Already there are several precise spectral studies of CXB by ASCA team members through ray-tracing technique, and ascaray, make_ascaray_images and new ascaarf will provide essentially the same approach. Following references will be suggested for extensive accounts of these analysis techniques ; Gendreau, K. 1995, PASJ, 47L, 5; K. Gendreau's Ph-D Thesis; Y. Ishisaki's Ph-D thesis ).

Let's carry out GIS CXB spectral analysis. The cosmic background spectral file is created with mkgisbgd, and non-X-ray background is extracted from night earth observation.

  • g2_cxb_17mm_grouped.spec -- CXB spectral file created with mkgisbgd. Only the r< 17mm detector region is used. All the blank sky data untill 1995 December are used, and no GTI selection is made. To specify the detector region, a temporal spectarl file, g2_17mm.pha, was made with xselect by specifying the 17mm.reg region file, and used as the input for mkgisbgd.

    The g2_17mm.pha file is made from a 3C273 observation, but this can be anything, as only the WMAP of this file is used to create the CXB spectral file and ARF.

  • g2_17mm_night_earth_1999-1995.spec -- Night earth spectrum for r<17mm region (17mm.reg used) until 1995 December extacted from the GIS2 night earth event file. It is important to choose the same time period as the CXB observation, since there is long term increase of the GIS intrinsic background, so

  • g2_cxb_17mm.arf -- ARF file created with ascaarf using the make_ascaray_images outputs for 3 degree extension (ascaray_3deg_list) . g2_17mm.pha is used to specify the detector region, and the standard RMF gis2v4_0.rmf is used.
It has been known that within ~20 mm from the XRT optical axis, scattering lights further from ~2 deg off-axis are completely negligible (e.g., Gendreau thesis; Ishisaki thesis). Therefore, in the present case the GIS spectrum and ARF we have made are considered to be a fair sample of the CXB spectrum and corresponding response.

Using these files in xspec, the plot eff command will display the following XRT+GIS response. Note that here the unit should be "cm2 str" as make_ascaray_images normalized the ray-traced images so, though XSPEC displays just "cm2".

ASCA GIS extended arf We may carry out spectral fitting as usual. Here we fit the CXB spectrum only in the 2 - 10 keV band with a power-law, fixing the index to the "canonical value" 1.4 and the hydrogen column density to 1.1E20cm-2(Galactic value).

      mo = wabs[1]( powerlaw[2] )
      Model Fit Model Component  Parameter  Unit     Value
      par   par comp
	1    1    1   wabs       nH       10^22     1.1000E-02 frozen
	2    2    2   powerlaw   PhoIndex            1.400     frozen
	3    3    2   powerlaw   norm                9.195     +/-  0.4650E-01
     Chi-Squared =      206.6354     using   169 PHA bins.
     Reduced chi-squared =      1.229973     for    168 degrees of freedom

    XSPEC> flux 2 10
     Model flux   8.260     photons ( 6.0486E-08 ergs)cm**-2 s**-1 (  2.000- 10.000)
As shown below, we see that the data fits well in this energy band, and another soft excess component exists below ~1 keV which is considered to have Galactic origin (Gendreau et al. 1995).

We obtain correct flux and normalization of the diffuse component, only if we are reminded that the unit of the model should be cm-2s-1str-1, not cm-2s-1.

ASCA GIS cxb fit

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This file was last modified on Tuesday, 26-Jun-2001 12:43:02 EDT

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