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Top > Suzaku > Analysis > XIS > CTI

CTI correction error in the pipeline product data

2007-02-26 by the XIS team

Summary:
The XIS team found that an inappropriate CTI correction was made to some of the AO1 data obtained during August - October, 2006. The inappropriate CTI correction resulted in a systematically lower energy scale, at most 20 eV at 5.9 keV, in the event files. This was caused by using old CALDB files for the CTI correction during the pipeline processing. Hence, if your data are affected by this error, please reprocess the data to get a correct energy scale using the latest CALDB files as explained below.

1. Observations and data affected by this error
Observations which satisfy the following conditions are affected by the CTI correction error:
(1) the observation did not utilize the Spaced-row Charge Injection (SCI)
(2) the observation was made between 2006-08-15 and 2006-10-31.
(3) the processing version, designated as "PROCVER", is earlier than 1.2.2.5.

Sequence numbers of all the observations affected by this error are listed below:

2006
August September October
101005050100048010101005070
101012010101005060101009010
401034010101010010400001020
401041010101011010401046030
401046010101011020401048010
401094010101011030401051010
501007010101011040401059010
501041010101011050501010010
701006010101011060501044010
801020010101011070501045010
101011080501072010
101011090501097020
101011100701093020
101011110
101011120
101011130
101011140
101011150
101011160
401043010
401046020
401047010
401052010
401056010
401058010
401092010
501042010
501043010
501063010
501064010
501065010
501066010
501067010
501068010
501069010
501070010
501071010
801049010
801049020
801049030
801049040
801091010
901001010
901002010
901003010

In the pipeline products, the unfiltered/cleaned event files are affected by the CTI correction error. The unfiltered data are labelled as aeXXXXXXXXXxi[0-3]_N_mmmmmmmm_uf.evt.gz in the "xis/event_uf" subdirectory, and, the cleaned event files are as aeXXXXXXXXXxi[0-3]_N_mmmmmmmm_cl.evt.gz in the "xis/event_cl" subdirectory, where XXXXXXXXX is a sequence number of the observation and mmmmmmmm is a file identifier. The filename convention is summarized in the following URL: http://www.darts.isas.jaxa.jp/astro/suzaku/seq_data.html.v1.2
Since the spectra in the "xis/products" are extracted from the cleaned event files, both source (aeXXXXXXXXXxi[0-3]_N_mmmmmmmm_sr.pi) and background (aeXXXXXXXXXxi[0-3]_N_mmmmmmmm_bg.pi) spectra are also affected by this error.

2. Details of the error
Since the CTI increases with time due to the radiation damage in space, the CTI-correction parameters become time dependent. The current CALDB files contain a table which provides the relevant parameters at some specific epochs. Because the processing tool to correct the CTI ("xispi") refers to the CTI parameters at the closest time of the observation, the table has many entries with enough frequency. Because the calibration of the CTI takes some time, the CTI parameters for the latest observations are not known. This means that the table should contain CTI parameters extrapolated to future. (Note that the CTI parameters in the current CALDB files are not valid to the data obtained with SCI.)

The CALDB files used incorrectly contained the extrapolated CTI values only up to 2006-08-13 00:00:00. Therefore, the parameters at this epoch were always used even for the data obtained after August 13. This caused an insufficient correction of the charge loss, and reduced the total pulse height slightly. This error occurred commonly to the 4 sensors.

Figure 1 shows a time history (2006 Jun - 2006 December) of the center energy of Mn Kalpha line from the 55Fe calibration source, which illuminate a corner of the XIS1, segment A. Each mark corresponds to a sequence of observation. Deviation of the line center energy from 5.895 keV increases with time and reaches up to 20 eV by the end of October.

png

3. Countermeasure
The error mentioned above can be corrected by applying the CTI correction tool "xispi" again with correct CTI parameters. The correction method is summarized below. Although it is much easier to apply "xispi" directly to the cleaned event files, this needs to wait for a future release of the tool (scheduled in March, 2007). Thus we describe here how to apply "xispi" to the unfiltered data and to screen them to create the cleaned event files; this is basically the same procedure used in the pipeline processing. We also explain how to apply "xispi" directly to the cleand event files, which is useful when a new version of "xispi" is released. Whole the Suzaku archive will be reprocessed with the correct CTI parameters when the processing verion 2.0 becomes available.

Correction to the unscreened event files:

1. Recalculate the pulse invariant (PI) and grade values with the latest CALDB file. In the current release, the CALDB files for the CTI correction are ae_xi[0-3]_makepi_20060823.fits. This is done by the "xispi", XIS-specific tool in HEASOFT v6.1.2.

unix% xispi infile=ae501008010xi0_1_3x3n066z_uf.evt.gz \
            trcor_caldbfile=ae_xi0_makepi_20060823.fits \
            cticor_caldbfile=ae_xi0_makepi_20060823.fits \
            grade_caldbfile=ae_xi0_makepi_20060823.fits \
            pha2pi_hkfile=ae501008010xi0_0.hk.gz \
            pha2pi_caldbfile=ae_xi0_makepi_20060823.fits \
            outfile=ae501008010xi0_1_3x3n066z_uf_cticorr.evt

The unfiltered event files and house keeping (HK) file are in the "xis/event_uf" and "xis/hk" subdirectories, respectively.

2. Screen with xselect (filter column), using GRADE and STATUS information.

xsel:SUZAKU > set datadir .
xsel:SUZAKU > read event ae501008010xi0_1_3x3n066z_uf_cticorr.evt
xsel:SUZAKU-XIS0-STANDARD > filter column "GRADE=0:0 2:4 6:6 STATUS=0:131072"

Calibration source areas are not removed in the pipeline processing. You are requested to do by yourself if needed. If you are using xselect, substitute "GRADE=0:0 2:4 6:6 STATUS=0:65535" for the above criterion.

3. Screen with xselect (select hk), using common HK, to discard the data during maneuver.

xsel:SUZAKU-XIS0-STANDARD > read hk hkfiles=ae501008010.hk.gz expand=yes
> HK file directory >[./]
xsel:SUZAKU-XIS0-STANDARD > select hk "AOCU_HK_CNT3_NML_P==1"
xsel:SUZAKU-XIS0-STANDARD > extract event
xsel:SUZAKU-XIS0-STANDARD > save event ae501008010xi0_1_3x3n066z_uf_cticorr.evt_gsmnvcut
> Use filtered events as input data file ? >[no] yes
The common HK file is in "auxil" subdirectory.

4. Screen with xselect (select hk), using XIS HK, to discard the data during data rate low. The criteria are given by "Sn_DTRATE < 3", where n=0, 1, 2, 3, depending on the sensor ID.

xsel:SUZAKU-XIS0-STANDARD > read hk hkfiles=ae501008010xi0_0.hk.gz expand=yes
> HK file directory >[./]
xsel:SUZAKU-XIS0-STANDARD > select hk "S0_DTRATE<3"
xsel:SUZAKU-XIS0-STANDARD > extract event
xsel:SUZAKU-XIS0-STANDARD > save event ae501008010xi0_1_3x3n066z_uf_cticorr.evt_dtratecut
> Use filtered events as input data file ? >[no] yes

5. Screen with xselect (select hk), using extended house keeping (EHK) file, to discard the data during SAA.

xsel:SUZAKU-XIS0-STANDARD > read hk hkfiles=ae501008010.ehk.gz expand=yes
> HK file directory >[./]
xsel:SUZAKU-XIS0-STANDARD > select hk "SAA_HXD==0 && T_SAA_HXD>436 && ANG_DIST<1.5" 
xsel:SUZAKU-XIS0-STANDARD > extract event
xsel:SUZAKU-XIS0-STANDARD > save event ae501008010xi0_1_3x3n066z_uf_cticorr.evt_saacut
> Use filtered events as input data file ? >[no] yes
The EHK file is in "auxil" subdirectory.

6. Then, cleaned events are selected with the following criteria:

xsel:SUZAKU-XIS0-STANDARD > read hk hkfiles=ae501008010.ehk.gz expand=yes
> HK file directory >[./]
xsel:SUZAKU-XIS0-STANDARD > select hk "ELV>5 && DYE_ELV>20"
xsel:SUZAKU-XIS0-STANDARD > extract event
xsel:SUZAKU-XIS0-STANDARD > save event ae501008010xi0_1_3x3n066z_cticorr_cl.evt
> Use filtered events as input data file ? >[no] no
xsel:SUZAKU-XIS0-STANDARD > exit
> Save this session? >[no]

7. Finally, "cleansis" is applied to remove hot/flickering pixels.

unix% cleansis datafile=ae501008010xi0_1_3x3n066z_cticorr_cl.evt \
               outfile=ae501008010xi0_1_3x3n066z_cticorr_cl2.evt \
               cellsize=5 logprob=-.53E+01 bthresh=3 \
               phamin=0 phamax=4095 chipcol=SEGMENT

Correction to the cleaned event files:
(Note: this is not possible with the current version of "xispi".)

1. In the near future, a new version of "xispi" applicable to the cleaned event files will be released. Then, you can recalculate the PI and the grade values by explicitly specifying the latest CALDB files as below:

unix% xispi infile=ae501008010xi0_0_3x3n066a_cl.evt.gz \
            outfile=ae501008010xi0_0_3x3n066a_cl_cticorr.evt \
            hkfile=ae501008010xi0_0.hk.gz \
            makepifile=ae_xi0_makepi_20060823.fits


ISAS/JAXA Department of High Energy Astrophysics

Last Modified: Monday, 26-Feb-2007 21:46:17 JST