1. Energy Response of the X-ray Imaging Spectrometer (XIS) on Suzaku
H. Matsumoto, H. Nakajima, H. Yamaguchi, T. G. Tsuru, K. Koyama (kyoto Univ.),
K. Hayashida, K. Torii, M. Namiki, H. Tsunemi (Osaka Univ.),
H. Murakami, M. Ozaki, T. Dotani (ISAS/JAXA), B. LaMarr, S. E. Kissel, and M. W. Bautz (MIT)
Abstract
The X-ray Imaging Spectrometer on the Suzaku Satellite consists three front-illuminated (FI) and one back-illuminated (BI) CCD cameras. The energy response of the XIS consists of at least 6 components: (1) a main peak, (2) a sub peak, (3) a triangle component, (4) a Si escape, (5) a Si line, and (6) a constant component. The gain was determined based on the center pulse height of the main peak. The gain is divided into two parts at the energy of the Si edge (1.839 keV) and each part is described with a linear function. Thus there is a discontinuity at keV in the XIS gain. We have monitored the variation of the gain and energy resolution in orbit by observing calibration isotopes of 55Fe illuminating corners of each CCD.
1. Introduction
The XIS is four sets of an X-ray CCD camera. Characteristics of the XIS are summarized below.
Figure 4: Schematic description of the XIS response. Physical interpretations are following.
4 sensors:
XIS0, 2, 3 … FI CCD
XIS … BI CCD
(4) Si escape
After an X-ray photon is absorbed, the fluorescent Si line is generated and it escapes from the place where the X-ray photon is absorbed.
(5) Si line
The fluorescent Si line is absorbed again far from the place where the X-ray photon is absorbed.
(6) Constant Component
An X-ray photon is absorbed at a place close to the boundary between the depletion layer and the insensitive layer.
(1) Main peak
An X-ray photon is absorbed in the depletion layer.
(2) Sub peak
An event splits over several pixels. Some of them are not counted since their PHs are below the split threshold.
(3) Triangle Component
An X-ray photon is absorbed at a channel stop. This component is required only in the FI CCD.
FOV
18min. x 18min.
Energy Range
0.2—12keV
Imaging Area
1024 x 1024 pixels
Energy Resolution (FWHM)
～130eV at 6keV
Effective Area at 1.5keV
FI: 340cm2 BI: 390cm2
at 8keV
FI: 150cm2 BI: 100cm2
Readout noise (RMS)
～3 electrons
Time resolution
8s (normal mode), 7.8ms (P-sum mode)
Figure 1: Schematic view of an XIS sensor.
3. Gain and Energy Resolution
XIS2(FI)
XIS1(BI)
Figure 5: Gain of the XIS determined with the main peak.
The gain cannot be described with a single linear function. This is probably because there is a sudden (but small) change in the so-called W value at the Si edge energy (1.839keV). The gain is divided into two parts: E1.839keV and E<1.839keV.
Then each part can be described by a linear function of PHA=A*E+B.
Figure 2: X-ray spectra obtained with the XIS:
(left) the Galactic center region and (right) SNR
The energy resolution of the BI CCD is almost comparable to the FI CCD.
Best fit parameters:
XIS2(FI): E<1.839keV A= ±0.029, B=2.447±0.031 E>1.839keV A= ±0.010, B=9.353±0.052
XIS1(BI): E<1.839keV A= ±0.050, B=-1.983±0.035 E>1.839keV A= ±0.017, B=-2.494±0.093
2. Energy Response
XIS2(FI)
XIS1(BI)
Figure 6: Energy resolution of the XIS determined with the main peak.
The resolution is fitted with a function
Best fit parameters:
XIS2(FI): A=81.63±5.09, B=2374.7±18.9, C=236.2±13.3
XIS1(BI): A=-48.66±7.42, B=3086.7±44.5, C=825.0±24.5
4. History of gain and resolution
There are two 55Fe calibration sources for each sensor (Fig. 1).
We have monitored the history of the gain and resolution by using them.
We estimate the CTI parameters from these data and do the CTI correction to all data distributed to observers.
Figure 3: XIS response to monochromatic X-rays of Mn Kα(5.89keV),
Mn Kβ(6.49keV), and O Kα(0.52keV)
Figure 7: Change of the gain and resolution.