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Imaging Area. Frame-store Area. Hiroya Yamaguchi , Hiroshi Nakajima, Hideyuki Mori, Hironori Matsumoto, Takeshi G. Tsuru (Kyoto Univ.), Kiyoshi Hayashida, Ken ’ ichi Torii, Satoru Katsuda, Masahiro Uchino (Osaka Univ.), Dai Takei (Rikkyo Univ.), Eric Miller (MIT), and XIS team

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the onboard calibration of xis

Imaging Area

Frame-store Area

Hiroya Yamaguchi, Hiroshi Nakajima, Hideyuki Mori, Hironori Matsumoto,

Takeshi G. Tsuru (Kyoto Univ.), Kiyoshi Hayashida, Ken’ichi Torii, Satoru Katsuda,

Masahiro Uchino (Osaka Univ.), Dai Takei (Rikkyo Univ.), Eric Miller (MIT), and XIS team


The Onboard Calibration of XIS

Abstract: X-ray Imaging Spectrometers (XISs) are main detectors of Suzaku. They have high quantum efficiency and moderate energy resolution. Although we constructed response function for the XIS using the results of ground calibration, the performance of the XIS is gradually changing on orbit. The increase of charge transfer inefficiency (CTI) is especially remarkable, and the energy resolution and gain had been degraded as the result. Then we have been monitoring and compensated the CTI with 55Fe calibration sources which irradiate the edge of the CCD chip, and reconstructed the response function for the CTI corrected spectra. Furthermore, the XIS equips charge injection (CI) capability, which is functioning for the first time on orbit, then we can compensate the CTI column by column. We will explain our correction method using the CI capability and report the results of the correction. The change of the quantum efficiency is also confirmed on orbit, and we calibrated it using the result of the observations of Crab nebula.

3. CTI correction

  • Degradation of performance due to Radiation Damage

CTI consists of 3 components.

CTI N(pixels)

Frame-store tarnsfer Cf 1024

Parallel transfer Cp Np

Serial tarnsfer Cs Ns


Energy Resolution

Performance of XIS is gradually degrading

since the launch of Suzaku due to

the radiation damage.

Q’ = Q (1-Cf)1024(1-Cp)Np(1-Cs)Ns

Q : Real PH (ADU)

Q’ : Readout PH (ADU)

Gain ~ 2%/year

ΔE ~ 50eV/year (@5.9keV)

・ CTI cannot be measured only from calibration source data.

・ Cp/Cf can be decided from diffuse source data

with strong emission line (e.g. Cygnus Loop, A426, Galactic center).

2. Charge trailing

ACTY dependence of 55Fe PH.

Before the correction.

After the correction.

(rev0.7, 1.2)

Ex (keV)

trailing pixel

・ A part of charge of each event trails to a pixel

opposite to the direction of the vertical transfer.

・ The amount of trailing charge increases

in the charge transfer process.


Frame-store component





Parallel component

preceding pixel

Time evolution of charge trailing




6.4keV line from Sgr C region.



Time evolution of CTI successfully corrected in rev0.7 and 1.2 data.

However systematic gain discrepancy (due to AE temperature?) still remains.

trailing pixel

・ Time dependence of charge

trailing has not been considered

in XISPI process yet.

・This effect will be corrected

properly in future version of

XISPI (probably after v2.0).

1.19±0.05 ADU/yr



Average PH (ADU)

Average PH (ADU)

1.11±0.07 ADU/yr

Possible gain shift

in rev 0.7 and 1.2 data.

trailing pixel

〜±0.2 %

preceding pixel

preceding pixel

< 0.2% @ > 1keV

< 5 eV @ < 1keV

XIS0 Seg0

Day since launch

Day since launch



4. Energy resolution

6. Checker-flag Charge Injection

Time depending Ex-ΔE relation


Schematic view of checker-flag CI method.

・ Large column-to-column variations in

the CTI degrades the energy resolution.

・ These variations can be measured with

checker-flag CI and corrected with

remarkable accuracy.

・ XIS team has conducted on-orbit CI

experiment every months from 2006-05.

・ After CTI correction, energy resolution is calibrated

with 55Fe (high energy), and E0102 (low energy) data.

・ The calibration results are reflected to xisrmfgen

2006-10-17 version (utilized for rev 0.7 and 1.2 data).

・ Spaced-raw CI succeed to improve the resolution

(~200eV → ~140eV). <See Torii et al. (poster)>.

The calibration result of S-CI will be reflected to

future version of xisrmfgen.

Green: 2005-08-15

Blue: 2005-09-15

Red: Ground calibration

(rmf provided before 20060213).

Other colors: onboard calibration.

PHA distribution of the test and reference events.

Frame image of XIS during CI experiment.

E0102 spectra fitted with old and new rmf

old rmf (20060213ver.)

new rmf (use xisrmfgen)

Observation date

Black: 20051216

Red: 20060521

55Fe spectra

・ Column-to-column CTI correction significantly

reduces the “tail” component of spectra.

・ FWHM: 190 →170eV @ 5.9keV

・ These results are not reflected in rev 0.7 and

1.2 data, and will be applied in future version

of XISPI process (after rev 2.0).

・ Since Spaced-row CI method is used at present,

Checker-flag CI experiments are interrupted.

Average CTI


(rev 0.7 and 1.2)

5. Quantum efficiency

QE calibration with Crab data (XIS0).

・ QE at high energy is calibrated with Crab data.

・ CALDB for QE parameter was updated in xisrmfgen

2006-10-17 version (only XIS0).

・ The thickness of depletion layer of XIS0, 1, 2, and 3

are assumed as 68, 45, 70, and 66μm, respectively.


CTI correction


QE degradation

We thank to all the Suzaku SWG members and associated graduate students,

especially Y. Ishisaki, Y. Maeda, N. Ota, Y. Takei, S. Okada, and K. Suzuki.

・ QE at low energy is calibrated with E0102 data.

・ Low energy QE had been degraded by OBF

contamination <see Hayashida et al. (poster)>.

・ @ >5keV, the influence of the contamination is

almost ignorable.


Bautz M.W. et al. Proc. SPIE, 2004, 5501, 111

Koyama K. et al. PASJ, 2006, accepted

LaMarr B. et al. Proc SPIE, 2004, 5501, 385

Nakajima H. et al. 2006, in preparing