The amount of charge injection for the 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:
- How to know the amount of CI for XIS1.
- The cause of the NXB increase for XIS1 with CI=6 keV and how to mitigate it.
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.
How to know precisely
Check the XIS1 HK file for the measured voltage value controling the amount of the charge injection. The XIS1 HK file can be found in "${seqno}/xis/hk/ae${seqno}xi1_0.hk". You can check the value S1_DVRCHK28_CAL in the table XIS_AE_TCE_I of the file.
fstatistic "107007010/xis/hk/ae107007010xi1_0.hk.gz[XIS_AE_TCE_I]" S1_DVRCHK28_CAL maxval=0 -
The sum of the selected column is -5046.0460
The mean of the selected column is -1.2301429 (<- check this value)
The standard deviation of the selected column is 6.87220149E-04
The minimum of selected column is -1.2330000
The maximum of selected column is -1.2290000
The number of points used in calculation is 4102
- If S1_DVRCHK28_CAL is close to -1.23, then XIS1 CI=6 keV.
- If S1_DVRCHK28_CAL is close to -1.75, then XIS1 CI=2 keV.
Cause and Mitigation for 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.
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.
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 xisndbgen with the following option.
) xisnxbgen 'pixq_and = 327680' 'nxbevent = ae_xi1_nxbsci6_XXXXXXXX.fits'