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 |
101005050 | 100048010 | 101005070 |
101012010 | 101005060 | 101009010 |
401034010 | 101010010 | 400001020 |
401041010 | 101011010 | 401046030 |
401046010 | 101011020 | 401048010 |
401094010 | 101011030 | 401051010 |
501007010 | 101011040 | 401059010 |
501041010 | 101011050 | 501010010 |
701006010 | 101011060 | 501044010 |
801020010 | 101011070 | 501045010 |
101011080 | 501072010 | |
101011090 | 501097020 | |
101011100 | 701093020 | |
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.
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] yesThe 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] yesThe 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