7. X-Ray Imaging Spectrometer (XIS)

**Figure 7.1:** The four XIS detectors before installation onto *Suzaku*.
$\includegraphics[totalheight=4in]{figures/xispic.ps}$

Suzaku has four X-ray Imaging Spectrometers (XISs), which are shown in Fig. 7.1. These employ X-ray sensitive silicon charge-coupled devices (CCDs), which are operated in a photon-counting mode, similar to that used in the ASCA SIS, Chandra ACIS, and XMM-Newton EPIC. In general, X-ray CCDs operate by converting an incident X-ray photon into a charge cloud, with the magnitude of charge proportional to the energy of the absorbed X-ray. This charge is then shifted out onto the gate of an output transistor via an application of time-varying electrical potential. This results in a voltage level (often referred to as ``pulse height'') proportional to the energy of the X-ray photon.

The four Suzaku XISs are named XIS0, 1, 2 and 3, each located in the focal plane of an X-ray Telescope. Those telescopes are known respectively as XRT-I0, XRT-I1, XRT-I2, and XRT-I3. After the launch of Suzaku, we had operated the four XISs successfully for more than a year. However, a large amount of leaked charge suddenly appeared in one of the sensors, XIS2, in November 2006, as explained in section 7.12. Because we could not suppress the leak of charge, we stopped using XIS2 for scientific observations. Thus, only the three XISs (XIS0, XIS1, XIS3) are currently usable.

Each CCD camera has a single CCD chip with an array of 1024 $\times$ 1024 picture elements (``pixels''), and covers an $18'\times18'$ region on the sky. Each pixel is 24 $\mu$ m square, and the size of the CCD is 25mm $\times$ 25mm. One of the XISs, XIS1, uses a back-side illuminated CCDs, while the other three use front-side illuminated CCDs. The XIS has been partially developed at MIT (CCD sensors, analog electronics, thermo-electric coolers, and temperature control electronics), while the digital electronics and a part of the sensor housing were developed in Japan, jointly by Kyoto University, Osaka University, Rikkyo University, Ehime University, and ISAS.

**Figure 7.2:** One XIS instrument. Each XIS consists of a single CCD chip with $1024 \times 1024$ X-ray sensitive cells, each 24 $\mu$ m square. *Suzaku* contains four CCD sensors (XIS0 to 3), two AE/TCUs (AE/TCE01 and AE/TCE23), two PPUs (PPU01 and PPU23), and one MPU. AE/TCU01 and PPU01 service XIS0 and XIS1, while AE/TCE23 and PPU23 service XIS2 and XIS3. Three of the XIS CCDs are front-illuminated (FI) and one (XIS1) is back-illuminated (BI).
$\includegraphics[height=6.0in,angle=0]{figures/xis_config_v2.eps}$

A CCD has a gate structure on one surface to transfer the charge packets to the readout gate. The surface of the chip with the gate structure is called the ``front side''. A front-side illuminated CCD (FI CCD) detects X-ray photons that pass through its gate structures, i.e., from the front side. Because of the additional photo-electric absorption at the gate structure, the low-energy quantum detection efficiency (QDE) of the FI CCD is rather limited. Conversely, a back-side illuminated CCD (BI CCD) receives photons from ``back,'' or the side without the gate structures. For this purpose, the undepleted layer of the CCD is completely removed in the BI CCD, and a thin layer to enhance the electron collection efficiency is added in the back surface. A BI CCD retains a high QDE even in sub-keV energy band because of the absence of gate structure on the photon-detection side. However, a BI CCD tends to have a slightly thinner depletion layer, and the QDE is therefore slightly lower in the high energy band. The decision to use only one BI CCD and three FI CCDs was made because of both the slight additional risk involved in the new technology BI CCDs and the need to balance the overall efficiency for both low and high energy photons.

Fig. 7.2 provides a schematic view of the XIS system. The Analog Electronics (AE) drives the CCD and processes its data. Charge clouds produced in the exposure area in the CCD are transferred to the Frame Store Area (FSA) after the exposure according to the clocks supplied by the AE. The AE reads out data stored in the FSA sequentially, amplifies the data, and performs the analog-to-digital conversion. The AE outputs the digital data into the memory in PPU named Pixel RAM. Subsequent data processing is done by accessing the Pixel RAM. To minimize the thermal noise, the sensors need to be kept at $\sim -90^\circ$ C during observations. This is accomplished by thermo-electric coolers (TECs), controlled by TEC Control Electronics, or TCE. The AE and TCE are located in the same housing, and together, they are called the AE/TCE. Suzaku has two AE/TCEs. AE/TCE01 is used for XIS0 and 1, and AE/TCE23 is used for XIS2 and 3. The digital electronics system for the XISs consists of two Pixel Processing Units (PPU) and one Main Processing Unit (MPU); PPU01 is associated with AE/TCE01, and PPU23 is associated with AE/TCE23. The PPUs accesses the raw CCD data in the Pixel RAM, carry out event detection, and send event data to the MPU. The MPU edits and packets the event data, and sends them to the satellite's main digital processor.

To reduce contamination of the X-ray signal by optical and UV light, each XIS has an Optical Blocking Filter (OBF) located in front of it. The OBF is made of polyimide with a thickness of 1000Å, coated with a total of 1200Å of aluminum (400Å on one side and 800Å on the other side). To facilitate the in-flight calibration of the XISs, each CCD sensor has two $^{55}$ Fe calibration sources ^7.1. These sources are located on the side wall of the housing and are collimated in order to illuminate two corners of the CCD. They can easily be seen in two corners of each CCD. A small number of these X-rays scatter onto the entire CCD.

The XIS had unexpectedly large contamination on the OBF from some material in the satellite. This reduced the low-energy efficiency of the XIS significantly. We monitor the contamination thickness by observing the calibration targets (e.g., 1E0102

72, RXJ1856.5

3754) regularly. This effect is included in the response matrices. Details of the contamination are explained in section 7.7.

It is known that the CCD performance gradually degrades in space due to the radiation damage. This is because charge traps are produced by cosmic-rays and are accumulated in the CCD. One of the unique features of the XIS is the capability to inject small amounts of charge to the pixels. Charge injection is quite useful to fill the charge traps periodically, and to make them almost harmless. This method is called the spaced-row charge injection (SCI), and the SCI has been adopted as a standard method since AO-2 to cope with the increase of the radiation damage (Uchiyama et al. 2009). The charge injection capability is also used to measure the charge transfer efficiency (CTE) of each CCD column in the case of the no-SCI mode (Nakajima et al. 2008; Ozawa et al. 2009). Details of the charge injection are explained in section 7.6.

7.2 CCD Pixels & Coordinates

A single XIS CCD chip consists of four segments (marked A, B, C and D in Fig. 7.2) and correspondingly has four separate readout nodes. Pixel data collected in each segment are read out from the corresponding readout node and sent to the Pixel RAM. In the Pixel RAM, pixels are given RAW X and RAW Y coordinates for each segment in the order of the readout, such that RAW X values are from 0 to 255 and RAW Y values are from 0 to 1023. These physical pixels are named active pixels.

In the same segment, pixels closer to the read-out node are read-out faster and stored in the Pixel RAM faster. Hence, the order of the pixel read-out is the same for segments A and C, and for segments B and D, but different between these two segment pairs, because of the different locations of the readout nodes. In Fig. 7.2, numbers 1, 2, 3 and 4 marked on each segment and Pixel RAM indicate the order of the pixel read-out and the storage in the Pixel RAM.

In addition to the active pixels, the Pixel RAM stores the copied pixels, dummy pixels and H-over-clocked pixels (Fig. 7.2). At the borders between two segments, two columns of pixels are copied from each segment to the other. Thus these are named copied pixels. On both sides of the outer segments, two columns of empty dummy pixels are attached. In addition, 16 columns of H-over-clocked pixels are attached to each segment.

Actual pixel locations on the chip are calculated from the RAW XY coordinates and the segment ID during ground processing. The coordinates describing the actual pixel location on the chip are named ACT X and ACT Y coordinates (Fig. 7.2). It is important to note that the RAW XY to ACT XY conversion depends on the on-board data processing mode (see section 7.4).

7.3 Pulse Height Determination & Hot Pixel Rejection

When a CCD pixel absorbs an X-ray photon, the X-ray is converted to an electric charge, which in turn produces a voltage at the analog output of the CCD. This voltage (``pulse-height'') is proportional to the energy of the incident X-ray. In order to determine the true pulse-height corresponding to the input X-ray energy, it is necessary to subtract the dark levels and correct possible optical light leaks.

Dark levels are non-zero pixel pulse-heights caused by leakage currents in the CCD. In addition, optical and UV light might enter the sensor due to imperfect shielding (``light leak''), producing pulse heights that are not related to X-rays. Analysis of the ASCA SIS data, which utilized the X-ray CCD in photon-counting mode for the first time, showed that the dark levels were different for different pixels, and their average did not necessarily follow a Gaussian. On the other hand, light leaks are considered to be rather uniform over the CCD.

For the Suzaku XIS, dark levels and light leaks are calculated separately in normal mode. Dark levels are defined for each pixel and are expected to be constant for a given observation. The PPU calculates the dark levels in the Dark Initial mode (one of the special diagnostic modes of the XIS); those are stored in the Dark Level RAM. The average dark level is determined for each pixel, and if the dark level is higher than the hot-pixel threshold, this pixel is labeled as a hot pixel. Dark levels can be updated by the Dark Update mode, and sent to the telemetry by the Dark Frame mode. Analysis of the ASCA data showed that Dark levels tend to change mostly during the SAA passage of the satellite. The Dark Update mode may be employed several times a day after the SAA passage.

Hot pixels are pixels which always output over threshold pulse-heights even without input signals. Hot pixels are not usable for observation, and their output has to be disregarded during scientific analysis. XIS detects hot pixels on-board by the Dark Initial/Update mode, and their positions are registered in the Dark Level RAM. Thus, hot pixels can be recognized on-board, and they are excluded from the event detection processes. It is also possible to specify the hot pixels manually. There are, however, some pixels which output over threshold pulse-heights intermittently. Such pixels are called flickering pixels It is difficult to identify and remove the flickering pixels on board. They are inevitably output to the telemetry and need to be removed during the ground processing. Flickering pixels sometimes cluster around specific columns, which makes them relatively easy to identify.

The light leaks are calculated on board with the pulse height data after the subtraction of the dark levels. A truncated average is calculated for $256\times114$ pixels (this size was $64\times64$ before January 18, 2006) in every exposure and its running average produces the light leak. In spite of the name, light leaks do not represent in reality optical/UV light leaks to the CCD. They mostly represent fluctuation of the CCD output correlated to the variations of the satellite bus voltage. XIS has little optical/UV light leak, which is negligible unless the bright earth comes close to the XIS field of view.

The dark levels and light leaks are merged in the parallel-sum (P-Sum) mode, so Dark Update mode is not available in P-Sum mode. The dark levels, which are defined for each pixel as the case of the normal mode, are updated every exposure. It may be considered that the light leak is defined for each pixel in P-Sum mode.

7.4 On-Board Event Analysis

The main purpose of the on-board processing of the CCD data is to reduce the total amount transmitted to the ground. For this purpose, the PPU searches for a characteristic pattern of charge distribution (called an event) in the pre-processed (post- dark level and light leak subtraction) frame data. When an X-ray photon is absorbed in a pixel, the photoionized electrons can spread into at most four adjacent pixels. An event is recognized when a valid pulse-height (one between the lower and upper event thresholds) is found that exceeds the pulse-heights in the eight adjacent pixels (e.g., it is the peak value in the $3 \times 3$ pixel grid). In P-Sum mode, only the horizontally adjacent pixels are considered. The copied and dummy pixels ensure that the event search is enabled on the pixels at the edges of each segment. The RAW XY coordinates of the central pixel are considered the location of the event. Pulse-height data for the adjacent $5 \times 5$ square pixels (or in P-Sum mode 3 horizontal pixels) are sent to the Event RAM as well as the pixel location.

The MPU reads the Event RAM and edits the data to the telemetry format. The amount of information sent to telemetry depends on the editing mode of the XIS. All the editing modes (in normal mode, see section 7.5) are designed to send the pulse heights of at least 4 central pixels of an event to the telemetry, because the charge cloud produced by an X-ray photon can spread into at most 4 pixels. Information of the surrounding pixels may or may not be output to the telemetry depending on the editing mode. The $5 \times 5$ mode outputs the most detailed information to the telemetry, i.e., all 25 pulse-heights from the $5 \times 5$ pixels containing the event. The size of the telemetry data per event is reduced by a factor of 2 in $3 \times 3$ mode, and another factor of 2 in $2 \times 2$ mode. Details of the pulse height information sent to the telemetry are described in the next section.

7.5 Data Processing Modes

There are two different kinds of on-board data processing modes. The Clock modes describe how the CCD clocks are driven, and determine the exposure time, exposure region, and time resolution. The Clock modes are determined by a program (``micro-code'') loaded into the AE. The Editing modes specify how detected events are edited, and determine the format of the XIS data telemetry. Editing modes are determined by the digital electronics.

In principle, each XIS sensor can be operated with different mode combinations. However, we expect that most observations will use all three sensors^7.2 in Normal 5 $\times$ 5 or 3 $\times$ 3 mode (without Burst or Window options). Other modes are useful for bright sources (when pile-up or telemetry limitations are a concern) or if a higher time resolution (

8s) is required.

7.5.1 Clock Modes

There are two kinds of Clock modes in XIS, Normal and Parallel Sum mode. Furthermore, two options (Window and Burst options) may be used in combination with the Normal mode. However, see section 7.5.3 for the restriction in the usage of the Parallel Sum mode.

7.5.2 Window & Burst Options

Table 7.1 indicates how the effective area and exposure time are modified by the Burst and Window options.

**Table 7.1:** Effective area and exposure time for different Burst and Window options.
Option	Effective area	Exposure time
	(nominal: $1024 \times 1024$ pixels)	(in 8s period)
None	$1024 \times 1024$ pixels	8s $\times$ 1 exposures
Burst	$1024 \times 1024$ pixels	s $\times$ 1 exposures
Window	$256\times1024$ pixels	2s $\times$ 4 exposures
	$128\times1024$ pixels	1s $\times$ 8 exposures
Burst & Window	$256\times1024$ pixels	s $\times$ 4 exposures
	$128\times1024$ pixels	s $\times$ 8 exposures
Note: is a dead time in the Burst option.

In the Normal Clock mode, the Window and Burst options can modify the effective area and exposure time, respectively. The two options are independent, and may be used simultaneously. These options cannot be used with the Parallel Sum Clock mode.

**Figure 7.3:** Time sequence of the exposure, frame-store transfer, CCD readout, and data transfer to the Pixel RAM in PPU is shown (1) in Normal mode without options, (2) in Normal mode with Burst option, and (3) in Normal mode with Window option. In this example, the 1/4 Window option is assumed.
$\includegraphics[height=5in]{figures/xis_option.eps}$

We show in Fig. 7.3 the time sequence of exposure, frame-store transfer, CCD readout, and storage to the Pixel RAM (in PPU) in normal mode with or without Burst/Window option. Note that, when the 1/8 Window option is applied, a significant fraction of the source photons may be lost due to the tail of the XRT point spread function. Furthermore, the fractional loss may be modulated by the attitude fluctuation of the satellite, which is synchronized with the orbital motion of the satellite.

Note on the Burst option and observing very bright sources
The XIS team discourages the use of exposure times less than 0.5s per frame with the Burst option. For such short exposures, the observation efficiency decreases significantly and the proper estimation of out-of-time events (see section 7.9.2) becomes difficult. While the XIS team plans to focus on calibrating the Timing mode for very bright sources in the AO-4 cycle, the use of this mode by Guest Observers is currently still discouraged (see following section).

7.5.3 Editing Modes

Editing modes may be divided into two categories, observation modes and diagnostics modes. We describe here mainly the observation modes. General users need not use the data in the diagnostic modes. Among the observation modes, three modes ( $5 \times 5$ , $3 \times 3$ , and $2 \times 2$ ) are usable in the Normal Clock mode, and only the Timing mode can be specified in the P-Sum mode.

We show in Fig. 7.4 the pixel pattern whose pulse height or 1-bit information is sent to the telemetry. We do not assign grades to an event on board in the Normal Clock mode. This means that a dark frame error, if present, can be corrected accurately during the ground processing even in the 2 $\times$ 2 mode. The definition of the grades in P-Sum mode is shown in Fig. 7.5.

No significant difference is found so far between the $5 \times 5$ and $3 \times 3$ modes. However, the $2 \times 2$ mode is slightly different from the $3 \times 3$ or $5 \times 5$ mode. The difference is produced during the CTE correction in the ground processing. Because the pulse height information is limited in the $2 \times 2$ mode, some of the CTE corrections cannot be applied to the $2 \times 2$ mode data. This causes a slight difference in the gain of $2 \times 2$ mode data. In the case of no-SCI observations, the gain difference has been measured, and the XIS software (xispi) takes the difference into consideration. However, the gain difference has not been clarified in the case of SCI-on observations. Thus the calibration accuracy of the $2 \times 2$ mode data with the SCI is slightly worse than the $5 \times 5$ and $3 \times 3$ mode data. The $2 \times 2$ mode is employed only when a bright source is observed (see tables 7.7 and 7.8). Those who plan to observe a bright source should be aware of these systematics in $2 \times 2$ mode. The $2 \times 2$ mode is not used in the BI CCD (XIS1), because a relatively small CTE in the BI CCD produces significant calibration difference from $5 \times 5$ and $3 \times 3$ modes.

Note on the Timing mode
In the Timing mode, the data quality may be significantly degraded compared to the Normal mode, while the calibration remains at a very preliminary stage. We do not expect that the calibration will advance quickly in the near future. Furthermore, there are some operational restrictions. For these reasons, the XIS team does not recommend to use the Timing mode. Those who nevertheless consider using the Timing mode should be aware of the considerations below, should carefully demonstrate the feasibility of their observations, and especially calculate the exposure times under the assumption that the Timing mode is not available for spectral analysis.

The Timing mode is available only for the FI CCDs. There are comparably large numbers of hot/flickering pixels in the BI CCD, which are difficult to remove completely on board. The remaining hot/flickering pixels are found to fill the telemetry easily, since a single hot pixel can produce 1024 events in 8s in the Timing mode. Due to this telemetry occupation by hot/flickering pixels, we cannot use the Timing mode for the BI CCD. Note that this does not happen in the Normal mode. The FI CCDs have only a small number of hot/flickering pixels. However, we found that we need to regularly reset the dark level for its stable update ( $\sim$ once per 96min). This means that we cannot take very long continuous data. This may not become a practical restriction, because the data acquisition becomes intermittent anyway due to the SAA passage and the earth occultation of the target.

Since only one dimensional information is available in the Timing mode, distinction between X-ray and non-X-ray events becomes inaccurate. This means that the Timing mode has a significantly higher non-X-ray background than the Normal mode. Although the actual background rate in the Timing mode is still under investigation, it can be one or two orders of magnitude larger than in the Normal mode.

The in-flight calibration of the Timing mode is very preliminary, and the performance in many areas is not calibrated yet. For example, it has not been verified whether or not its full time resolution, 7.8ms, can really be achieved on orbit. Energy calibrations, e.g., gain and energy resolution, are known to change in the Timing mode. Because the CTE correction in the Timing mode is not possible yet in the ground processing, the Timing mode has significantly different gain compared to the (CTE-corrected) Normal mode. This also degrades the energy resolution. In addition, the effective area of the CCDs may change in the Timing mode: even a small number of hot pixels produces a relatively large dead area in the CCD, which reduces the effective area. This reduction could be time dependent, because some of the hot pixels often disappear and reappear.

**Figure 7.4:** Information sent to the telemetry is shown for $5 \times 5$ , $3 \times 3$ , and $2 \times 2$ modes. 1-bit information means whether or not the PH of the pixel exceeds the outer split threshold. In $2 \times 2$ mode, the central 4 pixels are selected to include the second and the third (or fourth) highest pixels among the 5 pixels in a cross centered at the event center.
$\includegraphics[height=6cm]{figures/xis_eventPattern.eps}$

**Figure 7.5:** Definition of the grades in the P-Sum/Timing mode. Total pulse height and the grade of the event are output to the telemetry. Note that the grades are defined referring to the direction of the serial transfer, so the central pixel of a grade 1 event has the *larger* RAW-X value, while the opposite is true for a grade 2 event.
$\includegraphics[height=5cm]{figures/xis_grade.eps}$

Diagnostic Modes
Besides the observation modes explained above, the XIS instrument has several diagnostic modes, Frame mode, Dark Initial mode, Dark Update mode, and Dark Frame mode. Frame mode simply dumps the raw CCD data to the telemetry. This mode may be useful for the health check of the CCD. Other modes are used primarily in determining/checking the dark levels. Dark Init mode is employed when the Burst/Window options are changed. Dark Update mode is used just after the SAA passage to update the hot pixel list on board. It is unlikely that those would be used by guest observers.

7.5.4 Discriminators

Three kinds of discriminators, area, grade and class discriminators, can be applied during the on-board processing. The grade discriminator is available only in the Timing mode. The class discriminator was implemented after the launch of Suzaku and was used since January, 2006. In most cases, guest observers need not to change the default setting of these discriminators.

The class discriminator classifies the events into two classes, ``X-rays'' and ``others,'' and outputs only the "X-ray" class to the telemetry when it is enabled. This class discriminator is always enabled to reduce the telemetry usage of non-X-ray events. The ``other'' class is close to, but slightly different from grade 7. When the XIS points to blank sky, more than 90% of the detected events are particle events (mostly grade 7). If we reject these particle events on board, we can make a substantial saving in telemetry usage. This is especially useful when the data rate is medium or low. The class discriminator realizes such a function in a simple manner. When all the 8 pixels surrounding the event center exceed the Inner Split Threshold, the event is classified as the ``other'' class, and the rest of the events as the ``X-ray'' class. With such a simple method, we can reject more than three quarters of the particle events. The class discriminator works only for the $5 \times 5$ and $3 \times 3$ modes. It is not available in the $2 \times 2$ mode and the timing mode.

The area discriminator is used when we want to reject some (or most) of the frame data from the event extraction. The discriminator works on the Pixel RAM. When the discriminator is enabled, a part of the Pixel RAM is not used for the event extraction. This may be useful when a bright source is present in the XIS field of view other than the target source. If we set the discrimination area to include only the bright source, we can avoid outputting unnecessary events to the telemetry. Only a single, rectangular area can be specified in a segment for discrimination. Either the inside or the outside of the area can be rejected from the event extraction. The area discriminator works on the Pixel RAM, not on the physical area of the CCD. This is important when we apply the discriminator with the Window option.

The grade discriminator is used only in the timing mode. Any combination of the 4 grades can be selected to discriminate the grade for telemetry output.

Suzaku does not have the level discriminator, which was used in ASCA SIS. The same function can be realized, however, by changing the event threshold.

7.6 Spaced-Row Charge Injection

The performance of the CCDs gradually degrades in the space environment due to radiation damage by cosmic-ray particles. Spaced-row Charge Injection (SCI) is used to restore the performance of the CCDs, especially the Charge Transfer Efficiency (CTE) and the energy resolution. The SCI has been used by default since AO-2 observations, whenever possible.

7.6.1 Principle

The performance of the XIS is gradually degrading since the launch of Suzaku due to the radiation damage. The energy resolution in Sept. 2006 measured by the on-board $^{55}$ Fe calibration source was about 200eV (FWHM) at 5.9keV, while it was $\sim$ 130-135eV just after the launch.

In principle, a precision charge injection capability can mitigate the effects of in-flight radiation damage (Bautz et al. SPIE 2004, Vol. 5501, p111). A sufficient quantity of charge injected periodically during the array readout process will tend to fill radiation-induced traps. These filled traps do not capture X-ray signal charge, and the degradation of the CTE can be mitigated. Because the traps need to be filled quite often, we inject charge every 54 rows in every exposure. The charge injected rows are not usable to detect X-ray photons and become dead areas.

The SCI has been used as a standard observation method since AO-2. Details on the calibration of the SCI-on observations can be found in Uchiyama et al. 2009.

**Figure 7.6:** Frame mode data of XIS2 taken with Spaced-row Charge Injection (SCI). The bright lines at every 54th row correspond to the charge injected rows. These rows are disconnected because the overclocked regions are also displayed at the segment boundaries.
$\includegraphics[height=4in]{figures/srci_per_xis2_frame.eps}$

7.6.2 Supported Modes

The SCI is applicable only for the Normal mode. It is not supported for the Timing mode. Some of the Burst, Window, and Window

Burst options are also supported. Those who want to know which observation modes are supported with the SCI, should consult the latest microcode file (e.g., ae_xis_ucodelst_20080112.fits) in the Suzaku CALDB. The ``UCODE_LIST_SCI'' extension of the file is devoted to the SCI and the ``COMMENT'' column is useful.

7.6.3 Data Impact

Here we summarize how the SCI affects the data compared to when no SCI is used. There are several demerits associated with the SCI. Guest observers should be aware of these characteristics of the SCI data.

7.7 Low Energy Efficiency Degradation

The good low energy response is an important unique feature of the XIS. However, we noticed the effective area below 2keV has been decreasing with time after the launch. The degradation is characterized by the additional absorption of carbon-dominated material. The thickness of this extra absorber is different for each sensor, but becomes maximal at the center of the XIS CCD and decreases toward the edge of the field of view. Although the origin of the extra absorber has not yet been identified, we consider it is most likely some contaminant accumulating on the surface of the XIS Optical Blocking Filters (OBF), which is one of the coldest parts inside the satellite.

In order to examine this efficiency degradation, we have observed soft and stable X-ray sources, such as, 1E0102

72, RXJ1856.5

3754, and the Cygnus loop, repeatedly. Atmospheric fluorescent lines of nitrogen and oxygen are also useful to evaluate the spatial dependence of the contaminant thickness over the CCD. The analysis of these data lead to our current knowledge of the extra absorber (or the contaminant on the OBF), as summarized below.

**Figure 7.7:** An empirical model for the on-axis contamination evolution, assuming DEHP (C $_{24}$ H $_{38}$ O, or C/O = 6 by number) as contaminant. Crosses indicate the C column density of the contaminant derived from the E010272 observations. Solid lines indicate the best fit empirical model to the time evolution of the contamination for each sensor.
$\includegraphics[height=0.9\textwidth,angle=270]{figures/e0102_20080904_rate.eps}$

**Figure 7.8:** Temporal evolution of the radial profile of the contamination thickness derived from N and O fluorescent lines from the day earth (i.e., Sun-lit Earth) atmosphere. Open circles and filled triangles represent the data points determined by the N and O fluorescence lines, respectively. The best fit model is also shown in the figure. Note that, for the on-axis column densities, the results in Fig. 7.7 are used.
$\includegraphics[height=0.9\textwidth,angle=-90]{figures/contami_profile_ver3.eps}$

These results, the empirical models of the contamination thickness in particular, are implemented in the Suzaku analysis software. The instrument team discourages the use of XSPEC models to describe this extra absorption (such as xisabs, xiscoabs, xiscoabh, xispcoab) but recommends the use of a modified ARF file in which the absorption is taken into account. Such an ARF file can be generated with the Suzaku FTOOL xissimarfgen. A detailed description can be found in Ishisaki et al. 2008, PASJ, 59, S113 and on the Suzaku web site. For diffuse sources, one needs to make an ARF with xissimarfgen considering the extension of the source. In this case, one should not include xisabs, xiscoabs, xiscoabh, xispcoab in the spectral fit.

The calibration of the efficiency degradation due to the extra-absorption is still in progress. For example, relative contribution of oxygen to carbon has not yet been determined accurately. At present, we adopt the ratio of 1/6 in the above models, but there are indications that the ratio is different for each sensor or changing in time. We also know that for some sources an effective area decrease is evident above 2keV (typically 5%), where our current absorption models with carbon and oxygen predict little effect. Further study of this issue might be required. Furthermore, the radial profile of the extra absorber has been studied in detail only for XIS1 (BI-CCD), but not in detail for other sensors.

The future development of the efficiency degradation is difficult to predict. However, extrapolation of the current evolution curves (Fig. 7.7) will be appropriate for AO-4 proposals. For AO-4 proposers we provide an ARF in which the amount of efficiency degradation as expected for 2009 September is taken into account. These ARF files have been generated with xissimarfgen. The ARFs for diffuse sources should be generated with xissimarfgen by the proposers.

7.8 Non-X-Ray Background

All four XISs have low backgrounds, due to a combination of Suzaku's orbit and the instrumental design. The large effective area at Fe K (comparable to the XMM-Newton PN) combined with this low background makes Suzaku a powerful tool for investigating high energy sources. In the XIS, the background originates from the cosmic X-ray background (CXB) combined with charged particles (the non-X-ray background, or NXB). Currently, flickering pixels are a negligible component of the background. When observing the dark Earth (i.e., the NXB), the background rate in the 0.4-12keV band is 0.1-0.2counts/s in the FI CCDs and 0.3-0.6counts/s in the BI CCD. Note that these are the fluxes after the grade selection is applied with only grade 0, 2, 3, 4 and 6 being selected. We show in Fig. 7.9 the NXB spectra for each sensor, and their magnifications in Fig. 7.10.

**Figure 7.9:** The XIS background rate for each of the four XIS detectors, with prominent fluorescent lines marked. These spectra are based on $\sim\! 800$ ks of observations towards the dark Earth.
$\includegraphics[totalheight=4in]{figures/fig_normPix_nxb.eps}$

**Figure 7.10:** The XIS background rate for each of the four XIS detectors, showing only energies between 0.1-2.0keV. Below 0.3keV the background rate for the FI chips cannot be determined due to their low effective area.
$\includegraphics[totalheight=4in]{figures/fig_normPix_nxb_0-2keV.eps}$

There are also fluorescence features arising from the calibration sources as well as material in the XIS and XRTs. The Mn lines are due to scattered X-rays from the calibration sources. As shown in Table 7.2 they are almost negligible except for XIS0. The O lines are mostly contamination from the day Earth (see section 7.9.3). The other lines are fluorescent lines from the material used for the sensor. Table 7.2 shows the current best estimates for the strength of these emission features, along with their 90% confidence errors.

The NXB is not uniform over the chip. Its flux tends to be slightly higher at larger ACTY as shown in Fig. 7.11. This is because some fraction of NXB is produced in the frame-store region. The fraction may be different between the fluorescent lines and the continuum. This causes slight difference in the ACTY dependence of the NXB.

The total flux of the NXB depends on the cut-off rigidity (COR) as shown in Fig. 7.12. This is expected as the NXB is produced by the charged particles, the flux of which is higher at lower COR. This means that, when we use the NXB database to subtract the background, we need to select the NXB for an appropriate range of COR values, which match with the COR distribution of the on-source observation. For this purpose, the NXB database available from the Suzaku web page is sorted by COR.

The count rate of the PIN upper discriminator (PIN-UD) in the HXD also correlates with the flux of the XIS NXB. Recent studies show the PIN-UD provides a slightly better reproducibility of the XIS NXB. The reproducibility of the NXB in the 5-12keV band is evaluated to be 3-4% of the NXB, when we use the PIN-UD as the NXB sorting parameter (Tawa et al. 2008).

**Figure 7.11:** ACTY dependence of the NXB for XIS1 and XIS2. The NXB flux tends to be higher at larger ACTY, because some fraction of NXB is produced in the frame-store region.
$\includegraphics[totalheight=4in]{figures/nxb_acty2cts_v2.eps}$

**Figure 7.12:** Cut-off rigidity dependence of the NXB (average intensity in 5-10keV) for each sensor. The NXB flux varies by a factor of $\sim$ 2 depending on the cut-off rigidity.)
$\includegraphics[totalheight=4in]{figures/fig_allXIS_cor2cts_v2.eps}$

When we estimate the telemetry occupation of the background, we need to consider not only the X-ray grades (0, 2, 3, 4, 6) but also non-X-ray grades (1, 5, 7). Although the class discriminator removes a large fraction of non-X-ray background, we still have a non-negligible flux of the background. The background rate on the FI chips (including all the grades) is normally less than 10 counts/s and that of BI CCD is less than 20 counts/s. These values represent 90% upper limits of variations in the background count rates when the class discriminator is applied. These background rates are important when evaluating the possibility of the telemetry saturation at the low data rates (see section 7.9.4).

Table 7.2: Major XIS Background Emission Lines (Tawa et al. 2008).
Line Energy XIS0 XIS1 XIS2 XIS3

[keV] [ $10^{-9}$ cps/pix] [ $10^{-9}$ cps/pix] [ $10^{-9}$ cps/pix] [ $10^{-9}$ cps/pix]

Al K $\alpha$ 1.486 $1.45\pm0.11$ $1.84\pm0.14$ $1.41\pm0.10$ $1.41\pm0.10$

Si K $\alpha$ 1.740 $0.479\pm0.081$ $2.27\pm0.15$ $0.476\pm0.080$ $0.497\pm0.082$

Au M $\alpha$ 2.123 $0.63\pm0.093$ $1.10\pm0.13$ $0.776\pm0.097$ $0.619\pm0.092$

Mn K $\alpha$ 5.895 $6.92\pm0.19$ $0.43\pm0.14$ $1.19\pm0.13$ $0.76\pm0.11$

Mn K $\beta$ 6.490 $1.10\pm0.11$ $0.26\pm0.13$ $0.40\pm0.11$ $0.253\pm0.094$

Ni K $\alpha$ 7.470 $7.12\pm0.19$ $7.06\pm0.37$ $8.01\pm0.20$ $7.50\pm0.20$

Ni K $\beta$ 8.265 $0.96\pm0.10$ $0.75\pm0.22$ $1.16\pm0.11$ $1.18\pm0.11$

Au L $\alpha$ 9.671 $3.42\pm0.15$ $4.15\pm0.49$ $3.45\pm0.15$ $3.30\pm0.15$

Au L $\beta$ 11.51 $2.04\pm0.14$ $1.93\pm0.48$ $1.97\pm0.14$ $1.83\pm0.14$

The count rates are obtained from the whole CCD chip excluding regions irradiated by the calibration source. Errors are at the 90% confidence level.

**Table 7.2:** Major XIS Background Emission Lines (Tawa et al. 2008).
Line	Energy	XIS0	XIS1	XIS2	XIS3
	[keV]	[ $10^{-9}$ cps/pix]	[ $10^{-9}$ cps/pix]	[ $10^{-9}$ cps/pix]	[ $10^{-9}$ cps/pix]
Al K $\alpha$	1.486	$1.45\pm0.11$	$1.84\pm0.14$	$1.41\pm0.10$	$1.41\pm0.10$
Si K $\alpha$	1.740	$0.479\pm0.081$	$2.27\pm0.15$	$0.476\pm0.080$	$0.497\pm0.082$
Au M $\alpha$	2.123	$0.63\pm0.093$	$1.10\pm0.13$	$0.776\pm0.097$	$0.619\pm0.092$
Mn K $\alpha$	5.895	$6.92\pm0.19$	$0.43\pm0.14$	$1.19\pm0.13$	$0.76\pm0.11$
Mn K $\beta$	6.490	$1.10\pm0.11$	$0.26\pm0.13$	$0.40\pm0.11$	$0.253\pm0.094$
Ni K $\alpha$	7.470	$7.12\pm0.19$	$7.06\pm0.37$	$8.01\pm0.20$	$7.50\pm0.20$
Ni K $\beta$	8.265	$0.96\pm0.10$	$0.75\pm0.22$	$1.16\pm0.11$	$1.18\pm0.11$
Au L $\alpha$	9.671	$3.42\pm0.15$	$4.15\pm0.49$	$3.45\pm0.15$	$3.30\pm0.15$
Au L $\beta$	11.51	$2.04\pm0.14$	$1.93\pm0.48$	$1.97\pm0.14$	$1.83\pm0.14$

7.9 XIS Observations

7.9.1 Photon Pile-Up

The XIS is essentially a position-sensitive integrating instrument, with a nominal interval of 8s between readouts. If during the integration time more than one photon strikes the same CCD pixel, or one of its immediate neighbors, these cannot be correctly detected as independent photons: this is the phenomenon of photon pile-up. Here, the modest angular resolution of the Suzaku XRT is an advantage. However, photon pile-up could be a problem when observing bright sources.

If pile-up occurs, both the image and spectral data are distorted. The energy spectrum tends to become harder for the following reason: In most cases, the soft photon flux is much larger than the hard photon flux. This means that, when pile-up occurs, soft photons tend to pile up rather than the hard photons. When soft photons pile up, they may be either discarded as a non-X-ray event or regarded as a single hard photon. This results in a flux decrease in the soft band and a flux increase in the hard band. The pile-up tends to harden the energy spectrum. Because the decrease of the soft photons overwhelms the increase of the hard photons, the pile-up tends to decrease the total photon flux. In an extreme case, all the events may be discarded as non-X-ray events at the image center, and the local photon flux becomes effectively zero. This means that a point source image would have a hole at the center. This is the simplest method to detect photon pile-up.

Since the impact of the pile-up on the energy spectrum is different from source to source, it is difficult to give a general criterion of photon pile-up applicable to all sources. For example, if one tries to look for a weak hard tail in a soft source, photon pile-up has to be strictly removed as it can easily mimic the hard tail. On the other hand, a heavily absorbed source with a hard spectrum may be rather insensitive to the photon pile-up. However, some kind of guideline might be useful for the guest observers to roughly estimate the significance of pile-up. We give a detailed estimation below, but the results may be summarized as follows: In practice, point sources with counts/exposure can be observed in the Normal mode (Full Window). For somewhat brighter sources, Window options can be used to reduce the exposure time per frame (the count rate limit is inversely proportional to the exposure time -- the 1/4 Window option reduces the exposure time from 8s to 2s, and raises the limit from $\sim 12.5$ counts/s to $\sim$ 50counts/s).

A detailed calculation of photon pile-up is given in Table 7.3. In this table, we summarize the estimated count rates at which the pile-up fraction in an annulus reaches

, and

%. For example, for a source with the expected count rate of $\sim 50$ counts/s,

% of the events within $\sim 5$ pixels of the image center are piled up. On the other hand, the pile-up fraction is restrained to below

% outside the circle of

pixels in radius. Note that the attitude fluctuation of the satellite does not mitigate the photon pile-up, because the attitude drift is negligible within the exposure of

**Table 7.3:** Total count rate which causes the indicated fraction of pile-up at the annulus of the PSF.
Radius [pix]	Pile-up fraction
	3%	20%	50%
$\sim$ 5	13.4	30.0	45.9
5 $\sim$ 10	17.6	43.3	67.7
10 $\sim$ 15	25.6	64.3	101
15 $\sim$ 20	33.8	85.4	134
20 $\sim$ 25	42.6	108	171
25 $\sim$ 30	55.8	142	224
30 $\sim$ 35	70.6	180	285
35 $\sim$ 40	88.2	226	356
40 $\sim$ 45	100	256	404
45 $\sim$ 50	119	304	480
50 $\sim$ 55	141	362	571
Note: in units of counts/s.
1pixel = 1.04arcsec.

When pile-up occurs, the effects need to be removed in the course of the data analysis. The most simple method is to remove the image center from the event extraction region. This means that the events are extracted from an annular region to calculate the energy spectra. In this case, an appropriate ARF file needs to be calculated depending on the inner and outer radius of the annulus.

7.9.2 Out-Of-Time Events

X-ray photons detected during the frame-store transfer do not correspond to the true image, but instead appear as a streak or blur in the readout direction. These events are called out-of-time events, and they are an intrinsic feature of CCD detectors. Similar streaks are seen from bright sources observed with Chandra and XMM-Newton. Out-of-time events produce a tail in the image, which can be an obstacle to detecting a low surface brightness feature in an image around a bright source. Thus the out-of-time events reduce the dynamic range of the detector. Since XIS spends 25ms (without SCI) or 156ms (with SCI) in the frame-store transfer, about 0.3% ( $=0.025/8\times100$ , without SCI) and 2.0% (with SCI) of all events will be out-of-time events. However, because the orientation of the CCD chip is different among the sensors, one can in principle distinguish a true feature of low surface brightness and the artifact due to the out-of-time events by comparing the images from two or more XISs.

7.9.3 Day Earth Contamination

When the XIS field of view is close to the day Earth (i.e., Sun-lit Earth), fluorescent lines from the atmosphere contaminate the low-energy part of the XIS data, especially in the BI chip. Most prominent is the oxygen line, but the nitrogen line may also be noticed (Fig. 7.10). These lines are mostly removed when we apply the standard data screening criteria (XIS FOV is at least 20degree away from the day Earth) during the ground processing. However, the flux of the atmospheric fluorescent lines is more or less variable and could become rather large depending on solar activity.

A small amount of contamination could remain after the standard data screening. Even in this case, the lines may be removed if we subtract background extracted in the same XIS field of view. If background data from other observations or the background database are used, the background subtraction may not completely eliminate the atmospheric lines. In this case, the data screening criterion, i.e., 20degree from the day earth, may need to be changed.

7.9.4 Telemetry Limit

If a large number of events are detected in a frame, only a part of the events is output to the telemetry. The data which cannot be output to the telemetry are discarded. This phenomenon is called telemetry saturation.

The current telemetry allocation among the XRS/XIS/HXD detectors is summarized in Table 7.4. Although the XRS is not functioning, a small amount of telemetry is allocated to monitor its HK data. The ratio depends on the data rate and may be changed in the future. Due to staffing constraints, the available telemetry is slightly lower over the weekend. The ``High'' telemetry rate is always 144kbits/s, but the ``Medium'' rate is 60kbits/s during the week and 25kbits/s over the weekend. Therefore, the mission operation team tries not to allocate observation time of bright sources on the weekend.

**Table 7.4:** Telemetry allocation (kbit/s).
Data rate	Weekday	Weekend
	(XRS/XIS/HXD)	(XRS/XIS/HXD)
Superhigh	10/144/66	--
High	10/144/66	--
Medium	1/60/40	1/25/40
Low	1/15/10	1/1/10

Since the on-board XIS data are processed in units of 8 seconds in accordance with the standard exposure time, the available telemetry per sensor is 44236bytes for the ``High'' date rate, 18432bytes for the ``Medium (weekday)'' data rate, and 7680bytes for the ``Medium (weekend)'' data rate. Here we assume a distribution of telemetry allocation among the 4 sensors as XIS0:XIS1:XIS2:XIS3 = 3:3:1:3. We restricted the telemetry output of the XIS2 after the onset of the XIS2 trouble. The HK data and the frame header occupy a constant fraction of the telemetry, the rest is allocated to the X-ray events.

In Table 7.5 the typical size of an event is shown for each Editing mode. Because the event is slightly compressed on board, the event sizes listed in the table should be regarded as representative values. The event compression efficiency depends on the grades of the events, i.e., on the spectral shape of the source. Table 7.6 shows the estimated telemetry limits of XIS in various Editing modes and the telemetry data rates. We take into account $+3 \sigma$ fluctuation of the count rate. The upper limit of source count rate can be estimated by subtracting the background count rate (10/20 counts/s for FI/BI at 90% upper limits) from the values in Table 7.6.

**Table 7.5:** Typical event size after compression [byte].
Edit mode	Minimum	Maximum	Average
5 $\times$ 5	28	52	40
3 $\times$ 3	15	23	19
2 $\times$ 2	8	11	9
Timing	4	4	4

**Table 7.6:** Telemetry limits [counts/s/XIS].
Data rate	5 $\times$ 5	3 $\times$ 3	2 $\times$ 2	timing
Superhigh	134 (123)	283 (266)	599 (574)	1364 (1326)
High	134 (123)	283 (266)	599 (574)	1364 (1326)
Medium (weekday)	54 (47)	115 (104)	243 (227)	563 (538)
Medium (weekend)	21 (16)	44 (38)	94 (84)	227 (212)
Low (weekday)	11 (8)	25 (20)	53 (45)	135 (123)

7.10 Clock/Editing Mode Selection

When we consider an appropriate XIS Clock/Editing mode for a target, we generally need to consider both the telemetry limit and the photon pile-up. However, only the telemetry limit will be a concern for diffuse sources, which are more extended than the angular resolution of the XRT.

We give recommended Editing modes for a diffuse object depending on count rate in Table 7.7. In the table, we consider $+3 \sigma$ fluctuation of the source count rate and the contribution of the background count rate. Here we assume that the source count rates for the FI CCDs are nearly equal to that for the BI CCD. However, the ratio of source count rates depends on the energy spectrum. In case of a ratio of

, the boundaries of the source count rate is changed by a factor of $1.3 \sim 1.5$ and some of telemetry modes cannot be used.

**Table 7.7:** Recommended XIS modes for a diffuse source.
Rate	Telemetry Mode
counts/s	High	Medium
0-84	5 $\times$ 5	3 $\times$ 3
84-103	5 $\times$ 5	FI: 2 $\times$ 2, BI: 3 $\times$ 3
103-122	3 $\times$ 3	FI: 2 $\times$ 2, BI: 3 $\times$ 3
122-246	3 $\times$ 3	(not usable in standard settings)
246-340	FI 2 $\times$ 2, BI: 3 $\times$ 3	(not usable in standard settings)
340	Burst option

We summarize the recommended XIS modes for a point source in Table 7.8. Here we adopt 20cts/s as pile-up limit, which corresponds to a pile-up fraction of

50% at

15 pixels from the image center. However, as explained previously, the appropriate pile-up limit depends on the spectral shape of the source and the science goal of the observations. If pile-up needs to be avoided completely, one should consider using the Window option even if the source count rate is $\sim 20$ counts/s. For a source brighter than $\sim 320$ counts/s, it may be difficult to avoid photon pile-up and/or telemetry saturation. Guest observers who plan to observe very bright sources should consult with the Suzaku personnel at ISAS/JAXA or the NASA Suzaku GOF for the optimal observation mode.

**Table 7.8:** Recommended XIS modes for a point source
Rate	Telemetry Mode		Clock	Efficiency
counts/s	High	Medium		%
0-20	5 $\times$ 5	3 $\times$ 3	Normal	100
20-80	5 $\times$ 5	3 $\times$ 3	1/4 Window	100
80-160	5 $\times$ 5	3 $\times$ 3	1/4 Window	50
			1s Burst
160-320	5 $\times$ 5	3 $\times$ 3	1/4 Window	25
			0.5s Burst
84-122	3 $\times$ 3	FI: 2 $\times$ 2, BI: 3 $\times$ 3	1/8 Window	100
122-168	5 $\times$ 5	3 $\times$ 3	1/8 Window	50
			0.5s Burst
168-244	3 $\times$ 3	FI: 2 $\times$ 2, BI: 3 $\times$ 3	1/8 Window	50
			0.5s Burst
244-320	3 $\times$ 3	(not usable in standard settings)	1/8 Window	50
			0.5s Burst

7.11 In-Flight XIS Calibration

In this section, we summarize the current status of the XIS flight calibration. Because the calibration information is updated frequently, it is expected that the description in this section may become obsolete quite soon. The latest calibration information may be found in the Suzaku web page. The XIS team has mainly been calibrating the $5 \times 5$ and $3 \times 3$ data in the Normal mode, because those are the most frequently used modes. As of Oct. 1, 2008, the calibrations for the Window option without the SCI and for the $2 \times 2$ mode without the SCI have also been conducted. However, the calibration of the Timing mode remains very preliminary. Since the introduction of the SCI changes the XIS performance considerably, the XIS team is also working on the recalibration of the SCI data.

In the following, we describe the current status of the on-board calibration. The calibration items are as follows; (1) charge transfer efficiency (CTE), (2) energy scale, (3) energy resolution, (4) pulse height distribution function, (5) quantum efficiency in the low X-ray energy band (contamination) and (6) quantum efficiency in the high X-ray energy band.

7.11.1 Charge Transfer Efficiency & Energy Scale

The CTE is decreasing since the launch of Suzaku due to irradiation by charged particles on-orbit. In other words, the charge transfer inefficiency (CTI), defined as $1-{\rm CTE}$ , is increasing. We continuously measure the central energy and FWHM of the main peak from the two $^{55}$ Fe calibration sources. These calibration sources illuminate CCD segments A and D at the far corners from the read-out nodes. Fig. 7.13 shows the on-orbit time history of the apparent central energy of the main peak. The linear trend is due to charge loss caused by the gradual decrease of the CTE.

We measured the CTE with observations of the Galactic Center region, the Perseus galaxy cluster, the Cygnus loop, and 1E0102

72 as well as the calibration source $^{55}$ Fe. Since the X-ray sources are extending over the FOV of the XIS and have strong emission lines, they are suitable for the calibration of the CTE as a function of distance from the read-out node. Fig. 7.14 shows the time history of the CTE-corrected central energy of Mn K $\alpha$ .

**Figure 7.13:** Time history of the central energy of Mn K $\alpha$ from the $^{55}$ Fe calibration sources for the XIS0 (FI) and the XIS1 (BI) without CTE correction.
$\includegraphics[height=3in]{figures/CTIGainXIS01.eps}$

The SCI has been applied to observations by default since we verified its performance on August 2006, when the central energy of the $^{55}$ Fe calibration sources was improved almost to its theoretical value without any corrections.

The absolute energy uncertainty measured with the $^{55}$ Fe calibration sources is within $\pm 0.2$ %, and that for the energy band below 1keV is within $\pm5$ eV, as measured with 1E0102

72. As of Oct. 1, 2008, the calibration around $\sim 2$ keV, including the silicon edge, still shows some uncertainties. These are the main uncertainties in the energy scale calibration. They are expected to be improved soon.

**Figure 7.14:** Time history of the CTE-corrected central energy of Mn K $\alpha$ from the $^{55}$ Fe calibration sources for the XIS0 (FI) and the XIS1 (BI), for observations without (left) and with (right) SCI.
$\includegraphics[height=2in]{figures/MnKapeak_xis01_scioff_v2.eps}$ $\includegraphics[height=2in]{figures/MnKapeak_xis01_scion_v2.eps}$

7.11.2 Energy Resolution & Pulse Height Distribution Function

The calibration of the degradation of the energy resolution with time is done with the $^{55}$ Fe calibration source and observations of E0102

72, and is verified with data of the Galactic Center region and the Cygnus loop. Fig. 7.15 shows the time history of the energy resolution of the main peak of the calibration line at 5.9keV X-ray. Fig. 7.16 shows the time history of the width of the O VIII K $\alpha$ line ( $\sim$ 0.65keV) obtained from the 1E0102

72 data.

**Figure 7.15:** Time history of the energy resolution of Mn K $\alpha$ from the $^{55}$ Fe calibration sources for the XIS0 (FI) and the XIS1 (BI) after CTE correction, for observations without (left) and with (right) SCI.
$\includegraphics[height=2.1in]{figures/MnKfwhm_xis01_scioff.eps}$ $\includegraphics[height=2.1in]{figures/MnKfwhm_xis01_scion.eps}$

**Figure 7.16:** Time history of the width of the O VIII K $\alpha$ line ( $\sim$ 0.65keV) after CTE correction, for observations without (left) and with (right) SCI.
$\includegraphics[height=2in]{figures/Ofwhm_scioff.eps}$ $\includegraphics[height=2in]{figures/Ofwhm_scion.eps}$

The evolution of the XIS energy resolution ( $\Delta E(E_{\rm X}, t)$ ) can be expressed by the following formula,

Column-to-column variations of the CTE contribute much to the degradation of the energy resolution (Ozawa et al. 2009). In the case of the no-SCI mode, the CTE of each column can be measured using the checker flag charge injection (Nakajima et al. 2008; Ozawa et al. 2009). Thus, the CTE can be corrected for each column. However, the checker flag charge injection has not been conducted in the SCI mode, because the operation is too complicated. Segment-averaged CTE values are used in the SCI mode. The degradation of the energy resolution is taken into account by the RMF. Guest observers can create the appropriate RMF for their observations using xisrmfgen. The XIS team does not expect to change the pulse height distribution function (line profile response to monochromatic X-rays) on-orbit, except for the energy resolution. Results from pre-flight ground experiments are used in xisrmfgen.

7.11.3 Quantum Efficiency

Contamination is the main issue for the calibration of the quantum efficiency in the low X-ray energy band. The status of the calibration is described in section 7.7. The calibration of the quantum efficiency above $\sim\!1-2$ keV is mainly done with Crab nebula observations. The current status is given in chapter 6.

7.12 XIS2 Anomaly

XIS2 suddenly showed an anomaly on Nov. 9, 2006, 1:03 UT. About 2/3 of the image was flooded with a large amount of charge, which had leaked somewhere in the imaging region. When the anomaly occurred, the satellite was out of the SAA and the XISs were conducting observations in the Normal mode (SCI on, without options).

We carried out various tests to check the condition of XIS2, and found that (1) the 4 readout nodes of the CCD and the corresponding analog chain were all working fine, (2) charge injection was not directly related to the anomaly, but may have helped to spread the leaked charge. When we changed the clock voltages in the imaging region, the amount of leaked charge changed. This indicates a short between the electrodes and the buried channel. Possible mechanisms to cause the short include a micro-meteoroid impact on the CCD, as seen, e.g., on XMM-Newton (5 times) and Swift (once). Although there is no direct evidence to indicate the micro-meteoroid impact, the phenomenon observed in XIS2 is not very different from that expected from a micro-meteoroid impact. The low-earth orbit and the low-grazing-angle mirrors of Suzaku may have enhanced the probability of a micro-meteoroid impact.

We tried to reduce the leaked charge by changing the clock pattern and voltages in the imaging region. However, the attempt was not successful. Therefore it was decided to stop operating the sensor. It is unlikely that operation of the XIS2 will be resumed in the future.