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Top > Suzaku > Analysis > XIS > SCI

Provisional method to analyze the XIS data with SCI

2007-04-10 by the XIS team

In the current pipeline products (Ver. 1.x), pulse invariant (PI) values of the data with the Spaced-row Charge Injection (SCI) are not correct. Here we introduce a provisional method to calculate the PI values with reasonable accuracy for the SCI data.

The PI values are calculated in the current processing using the Charge Transfer Inefficiency (CTI) appropriate for the no-SCI data regardless of the SCI usage. This introduces a systematic error in the PI values of the SCI data. The XIS team is now preparing tools to process the SCI data correctly, which will be used from the Ver. 2.0 processing. However, some observers may want to analyze the SCI data as soon as possible. Thus we introduce here a provisional method to analyze the SCI data in addition to the calculation of PIs with reasonable accuracy. However, this method is restricted to the FI data (XIS0,2,3) in normal mode without any options. We would like to ask the users to use this method carefully with an understanding of the systematics associated with it.

By analyzing some data taken for the SCI calibration, we have found that the CTI is almost zero when the SCI is applied. In fact, a PI calculation assuming that the CTI is zero gives an energy scale with systematic errors of < 30 eV in the iron band, except for the BI CCD (XIS1). Hence, if you want to analyze the SCI data with a reasonable energy scale, please reprocess them using the zero-CTI CALDB files, as explained below.

Note:
The principle of the SCI is described in the Suzaku Technical Description (http://www.astro.isas.jaxa.jp/suzaku/doc/suzaku_td/).
In addition, the availability and the restrictions of the SCI are summarized in http://www.astro.isas.jaxa.jp/suzaku/planning/xismode/

This document consists of three subsections below. First, we summarize systematic errors of the SCI data with the zero-CTI correction. The energy scale and resolution are described in detail. After the explanation of reprocessing the SCI data, we show how to create the XIS Redistribution Matrix File (RMF) and Auxiliary Response File (ARF) files for the spectral analysis.

1. Systematic errors of the SCI data with the zero-CTI correction
2. How to reprocess the SCI data with the zero-CTI CALDB files
3. How to create the RMF and ARF files

1. Systematic errors of the SCI data with the zero-CTI correction
Systematic errors of the SCI data with the zero-CTI correction are summarized below.

Energy scale < 30 eV at 6 keV.
Energy resolution The width of a narrow line can be reproduced relatively well, if you use an RMF created with the method in Section 3. The excess width would be much smaller than 20 eV at 6 keV.
Effective area Additional systematic errors due to the SCI are considered to be negligible, if the SCI and adjacent rows are removed (Section 2) and corresponding ARF created with "xissimarfgen" (Section 3) is used.
Quantum efficiency Because the zero-CTI is just an approximation, it may affect the quantum detection efficiency of the data through the grade classification. However, the systematic error is considered to be negligibly small unless the data statistics are extremely good.

Energy scale and resolution
Figure 1 shows a time history (2006 August - 2007 February) of the apparent center energy of Mn Kalpha line from the 55Fe calibration source, which illuminate corners of the XIS0 and XIS1, segment A. The data we used here are publicly available from DARTS/Suzaku (
http:// www.darts.isas.jaxa.jp/astro/suzaku/). However, the PIs are recalculated with the provisional method described in Section 2 below. Each mark in the plots corresponds to an observation sequence. While the XIS1 shows a systematically lower energy scale of ~30 eV, the line center energies of the XIS0 are consistent with 5.895 keV (dashed-lines in the figures) within ~10 eV.

Note that the line center energy becomes lower with time even if the SCI is applied to observations, although the energy decrease is slower than that without the SCI. The time dependence of the energy scale with the SCI and the cause of the lower energy scale of the XIS1 are now under investigation. Full treatment of the SCI data including these issues will be available in the Ver. 2.0 processing. New ftools and CALDB files for the Ver. 2.0 processing will be released in June. Once the Ver. 2.0 processing is started, whole the Suzaku archive will be reprocessed with appropriate CTI parameters.

png png
Figure 1: Time history of the apparent center energy of Mn Kalpha line. The PIs are recalculated with the zero-CTI CALDB files.

Figure 2 shows apparent line widths of Mn Kalpha obtained with the RMF that was determined by the ground calibration. Excess widths (sigma) of ~20 eV for the XIS0 and ~35 eV for the XIS1 are clear from the figure. Because the RMF has a resolution (FWHM) of ~135 eV at this band, the energy resolutions of the XIS0 and XIS1 with the SCI are estimated to be ~140 eV and ~155 eV, respectively. For the FI sensors (XIS0,2,3), the excess widths would be 0 eV at 6 keV, if you use an RMF with the resolution on 2005-08-13 (XIS door was opened). A method to create an RMF is explained in Section 3.

png png
Figure 2: Time history of the apparent line width of Mn Kalpha line. The line widths are obtained with the RMF based on the ground calibration. The excess widths would be much smaller than 20 eV at 6 keV, if the RMF created with the method in Section 3 is used.

Since charge loss due to the CTI depends on the distance from the readout nodes, i.e., the number of charge transfers, we investigated the ACTY dependence of the energy scale in the iron band. Figure 3 shows the center energies of Fe XXV Kalpha emission from the Perseus cluster observed at the end of August, 2006. We divided the Segment B of XIS0 into six regions and derived the center energy at each region. It can be seen that the ACTY dependence of the CTI with SCI is smaller than that without SCI. For Segment B and C of all sensors, we examined such ACTY dependences and fit the data by linear functions. The slopes are summarized in the Table 1. Combined with the systematic error of ~10 eV derived from the calibration sources, we now conclude that a systematic error of the energy scale due to the zero-CTI correction is probably lower than 30 eV at the center of the XIS FOV.

png
Figure 3: ACTY dependence of the center energy of Fe XXV Kalpha emission
Table 1: The ACTY dependences of the energy scale due to the CTI
Slope [eV/pix]
Segment \ DetectorXIS0 XIS1 XIS2XIS3
Segment B-(26+/-15)E-3-(2+/-18)E-3 -(11+/-11)E-3-(34+/-15)E-3
Segment C-(17+/-7)E-3-(21+/-23)E-3 -(31+/-17)E-3-(13+/-9)E-3

From the preliminary analysis of E0102.2-7219, the improvement of the energy resolution is also verified at low energies. However, further calibration at this band including the energy scale is still in progress.

2. How to reprocess the SCI data with the zero-CTI CALDB files
The PI values can be calculated with the tool "xispi" using the zero-CTI CALDB files. The recalculation method is summarized below. Recent versions (HEASOFT v6.2 or later) of "xispi" can be applied directly to the cleaned event files. However, older versions can be applied only to the unfiltered event files. Thus, we also describe how to apply it 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.

This is an example of the OBS_ID=501008010 observation. The files required in the reprocessing are as follows;

501008010/auxil/ae501008010.ehk.gz (Extended House Keeping file)
501008010/auxil/ae501008010.hk.gz  (Common House Keeping file)
501008010/xis/event_cl/ae501008010xi[0-3]_0_[35]x[35]n066a_cl.evt.gz  
                                   (Cleaned event files)
501008010/xis/event_uf/ae501008010xi[0-3]_?_[35]x[35]3n066z_uf.evt.gz
                                   (Unfiltered event files)
501008010/xis/hk/ae501008010xi[0-3]_0.hk.gz
                                   (XIS House Keeping files)

In the explanation below, these files are assumed to be linked appropriately to your working directory.

A. Correction to the cleaned event files:
(Note: this is possible with the HEASOFT v6.2 or later versions of "xispi".)

1. Make the cleaned event file only with EVENTS and GTI extensions.

unix% xselect
xsel:SUZAKU > set datadir .
xsel:SUZAKU > read event ae501008010xi0_0_3x3n066a_cl.evt.gz
xsel:SUZAKU-XIS0-STANDARD > extract event
xsel:SUZAKU-XIS0-STANDARD > save event ae501008010xi0_0_3x3n066a_cl2.evt
>Use filtered events as input data file ? >[no] no
xsel:SUZAKU-XIS0-STANDARD > exit
>Save this session? >[no] 

2. Recalculate the grade and the PI values by explicitly specifying the CALDB files in which the parallel CTI is zero. The CALDB files are labelled as ae_xi[0-3]_makepi_20060522_nocti.fits. You can download these files in the following URL: http://www.astro.isas.jaxa.jp/suzaku/analysis/xis/sci/zerocti/

unix% xispi infile=ae501008010xi0_0_3x3n066a_cl2.evt \
            outfile=ae501008010xi0_0_3x3n066a_nocti.evt \
            hkfile=ae501008010xi0_0.hk.gz \
	    makepifile=ae_xi0_makepi_20060522_nocti.fits

The house keeping (HK) files are in the "xis/hk" subdirectory.

3. Next, filter out the SCI and its previous/next rows in the cleaned event file to remove fake events created by the SCI. We prepare a convenient shell script, "cut3rows.csh", instead of a direct input of selection criteria in xselect. The script can be downloaded at the following URL: http://www.astro.isas.jaxa.jp/suzaku/analysis/xis/sci/zerocti/

unix% ./cut3rows.csh \
      ae501008010xi0_0_3x3n066a_nocti.evt \
      ae501008010xi0_0_3x3n066a_cut3row_nocti.evt

The SCI rows are currently defined as follows; ACTY = 52, 106, 160, 214, 268, 322, 376, 430, 484, 538, 592, 646, 700, 754, 808, 862, 916, 970, 1022. Then you can filter out these rows by directly specifying the ACTY columns.

unix% xselect
xsel:SUZAKU > set datadir .
xsel:SUZAKU > read event ae501008010xi0_0_3x3n066a_nocti.evt
xsel:SUZAKU-XIS0-STANDARD > filter column "ACTY=0:50 54:104 108:158 162:212 216:266"
xsel:SUZAKU-XIS0-STANDARD > extract event
xsel:SUZAKU-XIS0-STANDARD > save event ae501008010xi0_0_3x3n066a_nocti.evt_cuttmp1
>Use filtered events as input data file ? >[no] yes
xsel:SUZAKU-XIS0-STANDARD > filter column "ACTY=270:320 324:374 378:428 432:482 486:536"
xsel:SUZAKU-XIS0-STANDARD > extract event
xsel:SUZAKU-XIS0-STANDARD > save event ae501008010xi0_0_3x3n066a_nocti.evt_cuttmp2
>Use filtered events as input data file ? >[no] yes
xsel:SUZAKU-XIS0-STANDARD > filter column "ACTY=540:590 594:644 648:698 702:752 756:806"
xsel:SUZAKU-XIS0-STANDARD > extract event
xsel:SUZAKU-XIS0-STANDARD > save event ae501008010xi0_0_3x3n066a_nocti.evt_cuttmp3
>Use filtered events as input data file ? >[no] yes
xsel:SUZAKU-XIS0-STANDARD > filter column "ACTY=810:860 864:914 918:968 972:1020"
xsel:SUZAKU-XIS0-STANDARD > extract event
xsel:SUZAKU-XIS0-STANDARD > save event ae501008010xi0_0_3x3n066a_cut3row_nocti.evt
>Use filtered events as input data file ? >[no] no
xsel:SUZAKU-XIS0-STANDARD > exit
>Save this session? >[no]

B. Correction to the unscreened event files:
(Note: this is a bit cumbersome. We recommend the update to the latest version of "xispi")

1. Recalculate the grade and PI values with the zero-CTI CALDB files.

unix% xispi infile=ae501008010xi0_1_3x3n066z_uf.evt.gz \
            trcor_caldbfile=ae_xi0_makepi_20060522_nocti.fits \
            cticor_caldbfile=ae_xi0_makepi_20060522_nocti.fits \
            grade_caldbfile=ae_xi0_makepi_20060522_nocti.fits \
            pha2pi_hkfile=ae501008010xi0_0.hk.gz \
            pha2pi_caldbfile=ae_xi0_makepi_20060522_nocti.fits \
            outfile=ae501008010xi0_1_3x3n066z_uf_nocti.evt

The unfiltered event files are in the "xis/event_uf" subdirectory.

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

unix% xselect
xsel:SUZAKU > set datadir .
xsel:SUZAKU > read event ae501008010xi0_1_3x3n066z_uf_nocti.evt
xsel:SUZAKU-XIS0-STANDARD > filter column "GRADE=0:0 2:4 6:6 STATUS=0:65535"

Calibration source areas are not removed in the pipeline processing. You are requested to do this 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_nocti.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_nocti.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_nocti.evt_saacut
>Use filtered events as input data file ? >[no] yes

The EHK file is in "auxil" subdirectory.

6. Cleaned events are then 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_uf_nocti.evt_elvcut
>Use filtered events as input data file ? >[no] no
xsel:SUZAKU-XIS0-STANDARD > exit
>Save this session? >[no]

8. Remove hot/flickering pixels by applying "cleansis".

unix% cleansis datafile=ae501008010xi0_1_3x3n066z_uf_nocti.evt_elvcut \
               outfile=ae501008010xi0_1_3x3n066z_nocti.evt \
               cellsize=5 logprob=-.53E+01 bthresh=3 \
               phamin=0 phamax=4095 chipcol=SEGMENT

9. Finally, as explained in the step (A-3), filter out the SCI row and its previous/next rows in the cleaned event file.

3. How to create the RMF and ARF files
The energy resolution is improved from ~200-220 eV to ~140-150 eV at the iron band with the application of the SCI. The improved resolution is almost the same as that just after the XIS door opened. Therefore, in order to create the RMF for the SCI data by "xisrmfgen", the RMF generator in HEASOFT v6.2, set the observation date on 2005-08-13T00:00:00 as follows;

unix% xisrmfgen \
      phafile=NONE \
      instrume=XIS0 \
      clk_mode=normal \
      date_obs=2005-08-13T00:00:00 \
      ebinfile=none \
      quantefffile=ae_xi0_quanteff_20060825.fits \
      rmfparamfile=ae_xi0_rmfparam_20061024.fits \
      makepifile=ae_xi0_makepi_20060522_nocti.fits \
      outfile=xis0_with_SCI.rmf 

As explained in the step (A-2), the SCI and its adjacent rows are removed as dead lines. The dead lines correspond to 5.6% of the image. This ratio is changed by the source-extraction region. Therefore, if you want to estimate an X-ray flux of a source more precisely, you need to exclude these lines in the ARF calculation. It is easy to prepare an image FITS file of the source-extraction region, in which the dead lines are already removed, and to give it to an input parameter "regfileN". Note that the image must be created in DETX/Y coordinates since the SCI rows are parallel to RAWX. The ARF for the SCI data is created as below. Users should refer to the paper by Ishisaki et al. (PASJ, 59, SP1, S113 (2007), astro-ph/0610118) to understand the proper usage of this tool.

1. Create image FITS files of a region from which a source spectrum is extracted.

unix% xselect
xsel:SUZAKU > set datadir .
xsel:SUZAKU > read event ae501008010xi0_0_3x3n066a_cut3row_nocti.evt
xsel:SUZAKU-XIS0-STANDARD > set xyname DETX DETY
xsel:SUZAKU-XIS0-STANDARD > set xybinsize 1
xsel:SUZAKU-XIS0-STANDARD > filter region xis0_source_detxy.reg
xsel:SUZAKU-XIS0-STANDARD > extract image
xsel:SUZAKU-XIS0-STANDARD > save image xis0_source_detxy.img
xsel:SUZAKU-XIS0-STANDARD > set xyname X Y
xsel:SUZAKU-XIS0-STANDARD > set xybinsize 1
xsel:SUZAKU-XIS0-STANDARD > filter region xis0_source_skyxy.reg
xsel:SUZAKU-XIS0-STANDARD > extract image
xsel:SUZAKU-XIS0-STANDARD > save image xis0_source_skyxy.img
xsel:SUZAKU-XIS0-STANDARD > exit
>Save this session? >[no]

2. Create mask images by "ftimgcalc", an image manipulator in HEASOFT v6.2. When creating a mask image of the detector, you need to prepare a region file excluding the SCI and its adjacent rows. Attach a corresponding region file of the dead lines to your source region file. The region files can be downloaded at the following URL: http://www.astro.isas.jaxa.jp/suzaku/analysis/xis/sci/zerocti/

Detector

unix% cat xis0_source_detxy.reg xis03_scirows_detxy.reg \
>xis0_source_detxy_cut3row.reg
unix% ftimgcalc \
      outfile=xis0_source_cut3row.img \
      expr='regfilter("xis0_source_detxy_cut3row.reg", A.P1, A.P2) ? (1) : (0)' \
      a=xis0_source_detxy.img

Sky (Uniform sky is assumed here)

unix% ftimgcalc \
      outfile=xis0_source_uniform_sky.img \
      expr='regfilter("xis0_source_skyxy.reg", A.P1, A.P2) ? (1) : (0)' \
      a=xis0_source_skyxy.img

3. Run "xissimarfgen" to create the ARF.

unix% xissimarfgen \
      instrume=XIS0 \
      source_mode=SKYFITS \
      source_image=xis0_source_uniform_sky.img \
      num_region=1 \
      region_mode=DETFITS \
      regfile1=xis0_source_detxy_cut3row.img \
      arffile1=xis0_with_SCI.arf \
      limit_mode=NUM_PHOTON num_photon=400000 \
      accuracy=0.005 \
      phafile=none \
      detmask=none \
      gtifile=none \
      date_obs=2006-09-26T14:18:16 \
      attitude=none \
      ea1=266.4995 ea2=119.1678 ea3=184.9994 \
      rmffile=xis0_with_SCI.rmf \
      estepfile=default

ISAS/JAXA Department of High Energy Astrophysics

Last Modified: Wednesday, 04-Jul-2007 12:46:32 JST