XRT/XIS: Contamination thickness

Calibration status and caveats

• XIS mode: any, Processing ver: 1.x, HEASOFT ver: 6.2, CALDB ver: **
  • Contamination thickness as a function of time
    The effective area below 2 keV has been decreasing with time after the launch. Based on spectral monitoring observations of some stable X-ray sources, the extra absorber thickness is plotted as a function of time since the XIS door open in Figure 1. The evolution seems gradually slow down and is almost saturated 200-300 days after the XIS door open. Note that the absorber thickness in this plot is measured at the center of the FOV, where the absorber thickness is maximum.
    
    In order to take into account the decrease of the low-energy efficiency, users should rebuilt the ARF files with XISSIMARFGEN by themselves. Since the models shown with the dotted curves are implemented in the current version of the software, the users should be aware that the thickness tends to be underestimated as indicated from the figure. In addition, the column density was measured by fitting the X-ray spectra in the 0.4-** keV band: outside the energy range, the uncertainty would be even larger.
    
    Figure 1: Empirical model for the on-axis contamination evolution, assuming DEHP (C24H38O4, or C/O = 6 by number) as contaminant. Crosses and open circles indicate the C column density of the contaminant derived from the E0102-72 and RXJ 1856 observations, respectively. The dotted curves are the models implemented in the Suzaku analysis softwares. The solid curves indicate the best fit empirical model to the time evolution of the contamination for each sensor.
    
    
  • Contamination thickness as a function of position
    Spatial distribution of the absorber thickness for XIS1 (BI-CCD) is shown in Figure 2 as a function of the distance from the FOV center. The extra absorber is thinner near the edge of the FOV. Tracing the time history of off-axis absorption over ** months, the radial profile is found to be approximated by an analytic function of 1/[1 + {r/a(t)}^b(t)] (see Suzaku memo 2006-35 for details).
    
    In the current version of XISSIMARFGEN, the above function is implemented to describe the time-dependent, radial profile, where the distribution of the contaminant is assumed to be azimuthally uniform.
    
    
    Figure 2: Radial profile of the contamination of the BI chip (XIS1). The circles and crosses represent the data for one month and five months after the door-open, respectively.
    
  
  

Related links

• Data Processing
• Suzaku memo 2006-35 "X-ray Imaging Spectrometer (XIS) on Board Suzaku"
• Suzaku memo 2006-39 "Monte-Carlo Simulator and ARF Generator of XRT/XIS for Extended Source Analysis"
• Notes on using XISSIMARFGEN