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Top > Suzaku > Analysis > XIS > window

A new recipe for generating NXB background spectra.

2015-04-24 by the XIS team

Introduction

In the standard processing of the XIS data, flickering pixels are removed. Flickering pixels are defined as those exceeding a threshold in a given period of time, thus the number of flickering pixels depends on how long the data are integrated. For the NXB database, a very long integration time is used (19 months), whereas individual observations are much shorter. Because of this, the NXB spectra generated using xisnxbgen are lower than actual background spectra. This was not a problem in the early phase of the mission when the fraction of the flickering pixels are tiny. However, the fraction keeps increasing, making the discrepancy noticeable recently.
Here, we present a workaround for this problem. A cumulative flickering pixel map ("noisy pixel map"; the map of the pixels that has experienced flickering more than once throughout the mission in the past) is generated for each setting shown below. This map is applied for both the observation files and the CALDB files to remove exactly the same number of pixels. Then, the background spectrum generator (xisnxbgen) is used. The resultant NXB spectrum matches much better than observations at a sacrifice of an effective area up to 6.5%.
In general, we recommend this method for weak sources with low surface brightness.
CI offCI on (2 keV)CI on (6 keV)
XIS0ae_xi0_npmsciof_20150220.regn/aae_xi0_npmscion_20150220.reg
XIS1ae_xi1_npmsciof_20150220.regae_xi1_npmscion_20150220.regae_xi1_npmsci6_20150220.reg
XIS2ae_xi2_npmsciof_20150220.regn/an/a
XIS3ae_xi3_npmsciof_20150220.regn/aae_xi3_npmscion_20150220.reg

Pros and Cons of this method

  • A significant improvement to reproduce the NXB spectra below 1 keV.
  • The improvement is larger for newer (more recent) data.
  • The efffective area will be decreased up to about 6.5% by applying this method.
  • Users can choose between the conventional method (by default) or the method presented here.

Recipe

Remove noisy pixels using a noisy pixel map (npm) file provided here.
Below, a recipe is described for applying this method for the XIS1 data of the sequence number 109001010 with the SCI charge 6 keV. The selection of the files are summarized in the table below.
Selection of files
NPM file Period CALDB
ae_xi[0,1,2,3]_npmsciof_20150220.reg 2005/09/11--2006/10/01
all sensor, CI-off		 ae_xi[0,1,2,3]_sciof_[DATE].fits
ae_xi[0,3]_npmscion_20150220.reg 2006/10/01--2014/12/30
XIS0,3, CI-on		 ae_xi[0,3]_scion_[DATE].fits
ae_xi1_npmscion_20150220.reg 2006/10/01--2011/06/01
XIS1, CI-on with 2keV equivalent		 ae_xi1_scion_[DATE].fits
ae_xi1_npmsci6_20150220.reg 2011/06/01--2014/12/30
XIS1, CI-on with 6keV equivalent		 ae_xi1_sci6_[DATE].fits
  1. Remove noisy pixels from an event file using xslect.
    • infile : ae109001010xi1_0_3x3n066q_cl.evt
    • npmfile: ae_xi1_npmsci6_20150220.reg
    • outfile: ae109001010xi1_0_3x3n066q_cl_npm.evt
    > xselect
xsel:SUZAKU >read events ae109001010xi1_0_3x3n066q_cl.evt
> Enter the Event file dir >[./]./
xsel:SUZAKU-XIS1-STANDARD >set xyname ACTX ACTY
xsel:SUZAKU-XIS1-STANDARD >set xybinsize 1
xsel:SUZAKU-XIS1-STANDARD >filter region ae_xi1_npmsci6_20150220.reg
xsel:SUZAKU-XIS1-STANDARD >ext ev
xsel:SUZAKU-XIS1-STANDARD >save ev
xsel:SUZAKU-XIS1-STANDARD >ae109001010xi1_0_3x3n066q_cl_npm.evt
> Use filtered events as input data file ? >[yes]
  2. Remove noisy pixels from CALDB files. This may take a long time.
    1. Use xispi for a CALDB file.
      • infile : ae_xi1_nxbsci6_20150220.fits
      • hkfile : ae_xi1_nxbvdchk_20150220.fits
      • outfile: ae_xi1_nxbsci6_20150220_mkpi.fits
      > xispi ae_xi1_nxbsci6_20150220.fits ae_xi1_nxbsci6_20150220_mkpi.fits ae_xi1_nxbvdchk_20150220.fits
    2. Use xisputpixelquality for the CALDB file.
      • infile : ae_xi1_nxbsci6_20150220_mkpi.fits
      • outfile: ae_xi1_nxbsci6_20150220_xispixq.fits
      > xisputpixelquality ae_xi1_nxbsci6_20150220_mkpi.fits ae_xi1_nxbsci6_20150220_xispixq.fits
    3. Apply the noisy pixel map for the files generated above using xselect.
      • infile : ae_xi1_nxbsci6_20150220_xispixq.fits
      • npmfile: ae_xi1_npmsci6_20150220.reg
      • outfile: ae_xi1_nxbsci6_20150220_nmp.evt
      > xselect
xsel:SUZAKU >read events ae_xi1_nxbsci6_20150220_xispixq.fits
> Enter the Event file dir >[./]./
xsel:SUZAKU-XIS1-STANDARD >set xyname ACTX ACTY
xsel:SUZAKU-XIS1-STANDARD >set xybinsize 1
xsel:SUZAKU-XIS1-STANDARD >filter region ae_xi1_npmsci6_20150220.reg
xsel:SUZAKU-XIS1-STANDARD >ext ev
xsel:SUZAKU-XIS1-STANDARD >save ev
xsel:SUZAKU-XIS1-STANDARD >ae_xi1_nxbsci6_20150220_nmp.evt
> Use filtered events as input data file ? >[yes]
  3. Set the parameter for the following xisnxbgen processes.
    4. > pset xisnxbgen nxbevent=./ae_xi1_nxbsci6_20150220_nmp.evt
    Other settings of the xisnxbgen are the same. For the source spectrum, use the one after applying the noisy pixel map.

Demonstration

Below, some comparisons are shown between the NXB spectra before and after applying the method described above. It is easily noticed that the NXB spectra generated by this method better reproduces the actualy background spectra than the conventional method.
  • Source and NXB spectra before applying the method.
    • From top to bottom, XIS0, XIS1, and XIS3 spectra.
    XIS0 XIS1 XIS3
  • Source and NXB spectra after applying the method.
    • From top to bottom, XIS0, XIS1, and XIS3 spectra.
    XIS0 XIS1 XIS3
  • The spectral fitting also improves after applying this method, especially below 1 keV.
    • From top to bottom, the background-subtracted spectra before and after applying the method.
    XIS0 XIS1
    ISAS/JAXA Department of High Energy Astrophysics