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Amont of Charge Injection for XIS1 and NXB Increase with CI=6 keV

2012-03-30 by the XIS team
Updated 2012-11-29 by the GOF

The amount of charge injection for XIS1 was increased from 2 to 6 keV equivalent for better high energy response in 2010. The changes were made gradually for different clocking modes and retrospectively for some calibration observations. Also, the increase in the amount of CI lead to an increase in the NXB level in the XIS1.

In this article, we explain the following:

  1. How to know the amount of CI for XIS1.
  2. The cause of the NXB increase for XIS1 with CI=6 keV and how to account for it.
  3. Method for mitigating the NXB increase for XIS1 with CI=6 keV.


How to know the amount of CI for XIS1.

General rule

A general rule is that most (including all GO) observations were made with CI=2 keV before the change and with CI=6 keV after the change made at the dates shown below. Some exception can be found in XIS calibration observations made in 2010-2011; some calibration observations were made with CI=6 keV before the changes and CI=2 keV after the changes to track the long-term gain change in both CIs.

DateUnitMode
2011-06-01XIS1Injection charge increased to 6 keV for Normal (no option).
2011-08-22XIS1Injection charge increased to 6 keV for Normal (1/4 win).
2011-09-01XIS1Injection charge increased to 6 keV for Normal (0.1s burst).
2011-10-06XIS1Injection charge increased to 6 keV for Normal (1/4 win+1.0s burst, 1/8 win).
2011-10-11XIS1Injection charge increased to 6 keV for Normal (1/4 win+0.1s, 0.3s, & 0.5s burst).
2011-10-25XIS1Injection charge increased to 6 keV for Normal (0.5s, & 0.62s burst).

How to know precisely

Before 2011-04-01

Check the submode ID in your XIS1 event file with the file name. If the file name includes "u" (e.g., ae105027010xi1_0_3x3n069*u*_cl.evt.gz), it was taken with CI=6 keV. If it is "b" (e.g., ae105027010xi1_0_3x3n069*b*_cl.evt.gz), it was taken with CI=2 keV.

After 2011-04-01

Check the microcode ID in your XIS1 event file.
) fkeyprint ae103001020xi0_0_3x3n066c_cl.evt.gz  CODE_ID
# KEYNAME: CODE_ID
 
# EXTENSION:    0
CODE_ID =                   66 / micro-code ID
    
If your ID is one of the followings, your observation was made with CI=6 keV.
Code_ID Comment
129 BI Normal Mode Periodic CI 1CI/54rows speed4x IA TrailMask (Obso  
130 BI Normal Mode Periodic CI 1CI/54rows speed4x IA TrailMask        
131 BI Normal 1/4 Win at the XIS pos. with SCI and TrailMask          
132 BI Burst 0.100sec SCI trail mask for CI=6keV                      
133 BI Burst 1/4Win 0.994sec SCItrailmask CI=6keV XIS1 XIS-nominal    
134 BI Normal 1/8Win SCItrailmask CI=6keV XIS1 XIS-nominal            
135 BI Burst 1/4Win 0.130sec SCItrailmask CI=6keV XIS1 XIS-nominal    
136 BI Burst 1/4Win 0.299sec SCItrailmask CI=6keV XIS1 XIS-nominal    
137 BI Burst 1/4Win 0.495sec SCItrailmask CI=6keV XIS1 XIS-nominal    
138 BI Burst 0.500sec SCItrailmask CI=6keV                            
139 BI Burst 0.620sec SCItrailmask CI=6keV                            
    

Cause and estimation of NXB Increase for XIS1 with CI=6 keV.

In the SCI operation, artificial charges are injected in every 54 rows. Some fraction of the charges are carried over to the adjacent rows. We remove events in the charge injected rows (SCI_ROW) and those next to them (SCI_TRAILING_ROW) onboard, but a small fraction of the charges are carried over to the next next rows (SCI_2ND_TRAILING_ROW). Due to the increase in the total amount of injected charges, the remaining charges in the second trailing rows increased, resulting in an increase in the NXB level.

Schematic of CTI

The figure below shows the NXB count rate in the 0.4-7.0 keV for the XIS1 as a function of cut-off rigidity. The increase of the level is obvious from the CI=2 keV to CI=6 keV (including the second trailing rows) data. However, when the events in the second trailing rows are removed for CI=6 keV, the level is consistent with that for CI=2 keV.

Plot of background level with SCI

The tool, xisnxbgen, when used with the appropriate nxbevent file in the CALDB, will estimate the NXB level appropriate for the amount of charges injected. Due to the current structure of the CALDB, xisnxbgen is unable to pick the correct file automatically for XIS1 data taken with CI = 6 keV. For such data, please explicitly specify the nxbevent calibration file.
xisnxbgen nxbevent=ae_xi1_nxbsci6_20120601.fits
If you have a copy of the Suzaku CALDB locally installed, this file can be found in the CALDB data/suzaku/xis/bcf directory. If you are using the HEASARC version of the CALDB remotly, it is probably best to download this specific file from:
ftp://legacy.gsfc.nasa.gov/caldb/data/suzaku/xis/bcf/


Method to Mitigate NXB Increase for XIS1 with CI=6 keV.

In the pipe-line processing, events in the second trailing rows are NOT removed. Users should make their own choice of whether they remove second trailing rows for a lower NXB level with a smaller effective area. The recipe for doing so can be found below.

  • Spectrum: Remove events in the second trailing rows using the STATUS bit mask pattern. Construct the spectrum using the output.evt file.
    ) fselect infile.evt outfile.evt '(STATUS%(2**19))<2**18'
        
  • RMF: Use xisrmfgen. No option is required.
  • ARF: Use xissimarfgen with the following option.
    ) xissimarfgen 'pixq_and = 327680'
        
  • NXB; Use xisnxbgen with the following option.
    ) xisnxbgen 'pixq_and = 327680' 'nxbevent = ae_xi1_nxbsci6_XXXXXXXX.fits'
        
    Please note that the nxbevent calibration must be specified explicitly in this case.


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This file was last modified on Thursday, 29-Nov-2012 14:46:31 EST

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