Suzaku has four X-ray Imaging Spectrometers (XISs), which are shown in Figure 6.1. 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 XIS-S0, S1, S2 and S3, 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. Each CCD camera has a single CCD chip with an array of 1024 1024 picture elements (``pixels''), and covers an region on the sky. Each pixel is 24 m square, and the size of the CCD is 25 mm 25 mm. One of the XISs, XIS-S1, 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.
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.
To minimize the thermal noise, the sensors need to be kept at C during observations. This is accomplished by thermo-electric coolers (TECs), controlled by TEC Control Electronics, or TCE. The Analog Electronics (AE) drives the CCD clocks, reads and amplifies the data from the CCDs, performs the analog-to-digital conversion, and routes the signals to the Digital Electronics (DE). 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 XIS-S0 and S1, and AE/TCE23 is used for XIS-S2 and S3. 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 receive the raw data from AE, 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 1000 Å, coated with a total of 1200 Å of aluminum (400 Å on one side and 800 Å on the other side). To facilitate the in-flight calibration of the XISs, each CCD sensor has two Fe calibration sources. One is installed on the door to illuminate the whole chip, while the other is located on the side wall of the housing and is collimated in order to illuminate two corners of the CCD. The door-mounted source will be used for initial calibration only; once the door is opened, it will not illuminate the CCD. The collimated source can easily be seen in two corners of each CCD. A small number of these X-rays scatter onto the entire CCD. In addition to the emission lines created by these sources, we can utilize a new feature of the XIS CCDs, ``charge injection capability,'' to assist with calibration. This allows an arbitrary amount of charge to be input to the pixels at the top row of the imaging region (exposure area), i.e. the far side from the frame-store region. The charge injection capability may be used to measure the CTI (charge transfer inefficiency) of each column, or even to reduce the CTI. How the charge injection capability will be used is still in progress as of this writing.
Fig. 6.2 provides a schematic view of the XIS system. Charge clouds produced in the CCD by the X-rays focused by the XRT are accumulated on the exposure area for a certain exposure period (typically 8 s in the ``normal'' mode), and the data are transferred to the Frame Store Area (FSA) after each exposure. Data stored in the Frame Store Area are read-out sequentially by the AE, and sent to the PPU after the conversion to the digital data. The data are put into the memory in PPU named Pixel RAM. Subsequent data processing is done by accessing the Pixel RAM.
A single XIS CCD chip consists of four segments (marked A, B, C and D in Fig. 6.2) 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 RAWX and RAWY coordinates for each segment in the order of the readout, such that RAWX values are from 0 to 255 and RAWY 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. 6.2, 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 (cf. Fig. 6.2). 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 (cf. Fig. 6.2). It is important to note that the RAW XY to ACT XY conversion depends on the on-board data processing mode (cf. § 6.4).
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 may enter the sensor due to imperfect shielding (``light leak''), producing pulse heights that are not related to X-rays. In the case of the ASCA SIS, these were handled via a single mechanism: Dark Levels of pixels were sampled and their (truncated) average was calculated for every exposure. Then the same average Dark Level was used to determine the pulse-height of each pixel in the sample. After the launch of ASCA, it was found that the Dark Levels of different pixels were actually different, and their distribution around the average did not necessarily follow a Gaussian. The non-Gaussian distribution evolved with time (referred to as Residual Dark-current Distribution or RDD), and resulted in a degradation of the energy resolution due to incorrect Dark Levels.
For the Suzaku XIS, Dark Levels and Light Leaks are calculated separately in normal mode. Dark Levels are defined for each pixel; those 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. Unlike the case of ASCA, Dark Levels are not determined for every exposure, but the same Dark Levels are used for many exposures unless they are initialized or updated. Analysis of the ASCA data showed that Dark Levels tend to change mostly during the SAA passage of the satellite. 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. The ASCA SIS did not identify hot pixels on-board, and all the hot pixel data were telemetered and removed during the data analysis procedure. The number of hot pixels increased with time, and eventually occupied significant parts of the telemetry. In the case of XIS, hot pixels are detected on-board by the Dark Initial/Update mode, and their positions and pulse-heights are stored in the Hot-pixel RAM and sent to the telemetry. 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 it 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 pixels (this size may be changed in the future) in every exposure and its running average produces the Light Leak. Thus, the Light Leak is basically the same as the Dark Level in ASCA SIS.
The Dark Levels and the 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.
The main purpose of the on-board processing of the CCD data is to reduce the total amount transmitted to ground. For this purpose, the PPU searches for a characteristic pattern of charge distribution (called an event) in the pre-processed (post- Dark Levels and Light Leaks 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 Event Lower and Upper Thresholds) is found that exceeds the pulse-heights in the eight adjacent pixels (e.g. it is the peak value in the 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. Again, in the case of P-Sum mode only the inner one of the two columns of Copied or Dummy pixels on each side of the Segment is necessary and used. The RAW XY coordinates of the central pixel are considered the location of the event. Pulse-height data for the adjacent 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 §6.5) 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 output to the telemetry depending on the editing mode. The mode outputs the most detailed information to the telemetry, i.e. all 25 pulse-heights from the pixels containing the event. The size of the telemetry data per event is reduced by a factor of 2 in mode, and another factor of 2 in mode. Details of the pulse height information sent to the telemetry are described in the next section.
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 kind of program loaded to the AE. The Editing modes specify how detected events are edited, and determine the formats of the XIS data telemetry. Editing modes are determined by the digital electronics.
It is possible to select different mode combinations for the four XISs independently. However, we expect that most observations will use all four in Normal 5 5 or 3 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 (8 s) is required.
The following two kinds of Clock Modes are available. Furthermore, two options (Window and Burst options) may be used in combination with the Normal Mode.
Table 6.1 indicates how the effective area and exposure time are modified by the Burst and Window options.
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.
We show in Fig. 6.3 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 a dead time is introduced when the Burst option is used.
We explain only the observation modes here. Three modes (, , and ) are usable in normal modes, and only the timing mode in the P-Sum mode.
We show in Fig. 6.4 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 2 2 mode. The definition of the grades in P-Sum mode is shown in Fig. 6.5.
There are slight differences between the , and modes. Currently, there is effectively no difference between the and modes. However, mode is slightly different from or mode. This difference is smaller in the FI CCDs and is larger in BI CCD. For example, although the CTE effect on gain (see §6.8) can be corrected in both and modes, the accuracy of the gain correction is slightly worse in mode. CTE also affect the quantum detection efficiency, and therefore the correction of the CTE effect on QDE is also worse in mode. Although the Suzaku XIS team may eventually need to prepare different calibration data for the mode, the differences are not very large in the FI CCD. Therefore at present, you may be able to use the calibration data of (55) mode for 22 FI data, unless you require high accuracy. However, the BI chip has a relatively large CTI and the difference between the 22 and 33 modes is also large (compared to the FI chips). For this reason, we discourage using the 22 mode with the BI chip, although it is usable for the FI CCDs, and has been used with bright sources already. We note that the XIS team cannot guarantee accurate calibration of the 2x2 mode for the BI chip. Suzaku operation team will try not to use the 22 mode for the BI chip unless otherwise specified.
Besides the observation modes given above, the XIS instrument has several diagnostic modes, used primarily in determining the dark current levels. It is unlikely that those would be used by guest observers.
Note on the timing mode
In timing mode, data quality may be significantly degraded compared to the normal mode. Degradation is possible in terms of the background rate, the energy resolution, the effective area, the energy range, among others. Users should be aware of this when choosing the timing mode.
Because only one dimensional information is available in timing mode, distinction between X-ray and non-X-ray events becomes inaccurate. This means that timing mode has significantly higher non-X-ray background than the normal mode. Actual background rate in timing mode is under investigation using the flight data. We therefore discourage the use of the timing mode for a faint source.
As described in 6.3 the Dark Levels are defined in each pixel in timing mode. From the analysis of the flight data, it is apparent that the fluctuation of the Dark Levels due to the particle events is rather large in timing mode. The fluctuation may introduce an excess noise in the calculation of the pulse height. This means that the energy resolution in the timing mode may be slightly worse compared to the normal mode. Furthermore, very low energy part of the data (say 0.4 keV) might not be available in the timing mode. A more complete determination of the usable energy range in the timing mode is under investigation now.
Effective area of the CCDs may change in the timing mode. There are some numbers of hot pixels in the CCDs. The hot pixels introduce only a very small dead area in normal mode. However, a hot pixel might kill a column in timing mode. This means that hot pixels could bring relatively large reduction in the effective area of CCDs. Furthermore, the reduction of effective area may not be stable because some hot pixels appear and disappear in time and the satellite attitude may fluctuate. Thus we do not recommend timing mode to measure the source flux accurately.
Two kinds of discriminators, area and grade discriminators, can be applied during the on-board processing. The grade discriminator is available only in 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 set, 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 inside or outside of the area can be rejected from the event extraction. The area discriminator works on the Pixel RAM, not for 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.
As of this writing, the XIS team plans to add one more discriminator, a class discriminator, to XIS DE. The class discriminator will become available before the start of AO1. The class discriminator classify the events into two classes, ``X-rays'' and ``others,'' and output only the "X-ray" class to the telemetry when it is enabled. The ``other'' class is close to, but slightly different from grade 7. When XIS points to blank sky, more than 90% of the detected events is 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 exceeds 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 quarter of the particle events. The class discriminator works only for 5x5 and 3x3 modes. It is not available in 2x2 and timing mode.
The XIS is essentially a position-sensitive integrating instrument, with the nominal interval between readouts of 8 s. If during the integration time one or more photons strike 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: the central pixel area receives 2% of the total counts of a point source, and 10% of the counts fall within 0.15 arcmin of the image center. We calculated the count rate at which 50% of the events within the central pixels are piled-up (the pile-up fraction goes down as we move out of the image center; this fraction is 5% for the 0.15 arcmin radius) -- although we offer no formal justification for this particular limit, this is compatible with our ASCA SIS experience (i.e., at this level, the pile-up effects do not dominate the systematic uncertainties). In practice, point sources with cts/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 -- 1/8 window option reduces the exposure time from 8 s to 1 s, and raises the limit from cts/s to 100 cts/s). For even brighter sources, timing mode may be used: because of the extremely short effective exposure time (8 s/1024 7.8 ms), the pile-up limit is several thousand cps (despite the on-board summing of rows and the one dimensional nature of the event detection algorithm).
In case of questions, Suzaku personnel at ISAS/JAXA or the NASA Suzaku GOF will work with the observers to assure the optimum yield of every observation via selection of the best XIS mode for a given target.
All four XISs have low backgrounds, due to a combination of the Suzaku orbit and the instrumental design. Below 1 keV, the high sensitivity and energy resolution of the XIS-S1 combined with this low background means that Suzaku is the superior instrument for observing soft sources with low surface brightness. At the same time, the large effective area at Fe K (comparable to the XMM pn) combined with this low background make Suzaku a powerful tool for investigating hot and/or high energy sources as well.
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 backgrond. When observing the dark earth (i.e. the NXB), the background rate between 1-12 keV in is 0.11 cts/s
in the FI CCDs and 0.40 cts/s in the BI CCD; see
Figure 6.6. Note that these are the fluxes after the grade selection
is applied with only grade 0, 2, 3, 4 and 6 selected.
There are also fluorescence features arising
from the calibration source as well as material in the XIS and XRTs.
The Mn lines are due to the scattered X-rays from the calibration sources. As shown in
Table 6.2 the Mn lines are almost negligible except for XIS-S0.
The O lines are mostly contamination from the day earth (6.7.2).
The other lines are fluorescent lines from the material used for the sensor.
Table 6.2 shows the current best estimates for the
strength of these emission features, along with their 90% upper and
The background rate on the FI chips (including all the grades) is normally less than 400 counts/frame (50 cts/s) when no class discriminator is applied. On the BI chip, the rate is normally less than 150 counts/frame (18.75 cts/s). The background rate on the FI chips is expected to reduce significantly when the class discriminator is applied. But little change is anticipated for the BI chip. Since , , and modes require on average 40, 20, and 10 bytes per event, the minimum telemetry required for any source is kbits/s for mode, kbits/s for , and kbits/s for mode, if no class discriminator is used. Due to staffing constraints, the available telemetry is slightly lower over the weekend. ``High'' rate telemetry is always 144 kbits/s, but ``Medium'' rate is 70 kbits/s during the week and 30 kbits/s over the weekend. Therefore, mission operation team tries not to allocate bright sources in the weekend. In sum, the recommended XIS mode for any combination of count rate and detector is given in Table 6.3.
|55||33||Window (avoid pileup)|
|33||FI: 22||Window (avoid pileup)|
|FI: 22||FI: 22||Window (+ area discriminator)|
|BI: 33||BI: 33||Burst (+ window)|
|FI: 22||FI: 22||Burst (+ window)|
|BI: 33||BI: 33|
|Timing mode may be used depending on the objectives.|
|Count rate for a single XIS sensor.|
|Data rate||5 5||3 3||2 2||timing|
|Note: Nominal telemetry allocation for XIS and its equal distribution among the 4 sensors are assumed.|
Table 6.4 shows the estimated telemetry limits of XIS in various editing modes and the telemetry data rates. As the NXB occupies some constant fraction of the telemetry, the rest is available for the X-ray events. The NXB rate of the FI CCD when the class discriminator is applied is not available as of the writing of this document; see the Suzaku websites for updated information (see App C). On the other hand, the NXB rate of the BI chip is normally less than 20 c/s regardless of the class discriminator. When we calculate the telemetry limits, we assumed a nominal telemetry allocation ratio among XIS, and HXD. The ratio depends on the data rate and may be changed in future. The telemetry limits also depend on the data compression efficiency. We apply a simple data compression algorithm to the event data, whose efficiency may depend on the energy spectrum of the source. Thus the telemetry limits listed in the table should be regarded as only approximate values.
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 25 ms in the frame-store transfer, about 0.3% ( ) 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.
When the XIS field of view is close to the day earth (i.e. Sun lit Earth), fluorescent lines from the atmosphere contaminate low-energy part of the XIS data, especially in the BI chip. Most prominent is the oxygen line, but the nitrogen line may be also noticed (see Fig. 6.7). These lines are mostly removed when we apply the standard data screening criteria (XIS FOV is at least 20 degree away from the day earth) during the ground processing. However, small amount of contamination can remain. This contamination may be further reduced if we subtract appropriate background. This subtraction, however, may be imperfect. Thus, when neutral oxygen or nitrogen lines are detected in the XIS data, contamination from day earth should be suspected.
The performance of X-ray CCDs gradually degrades in the space environment due to the radiation damage. This generally causes an increase in the dark current and a decrease of the charge transfer efficiency (CTE). In the case of XIS, the increase of the dark current is expected to be small due to the low (C) operating temperature of the CCD. However, a decrease in CTE is unavoidable. Thus, continuous calibration of CCD on orbit is essential to the good performance of the XIS. For this purpose, we use a radio isotope source and charge injection as explained below:
(i) Each XIS carries Fe calibration sources near the two corners of the chip, which will be used to monitor the instrument gain.
(ii) Each XIS CCD is equipped with charge injection capability, which may be useful to measure and even suppress CTI.
Nonetheless, it is difficult to predict based on existing data how well we can calibrate the long-term performance change of XIS on orbit.