Hard X-ray Polarimeter for Small Satellite: Design, Feasibility Study, and Ground Experiments

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1 Hard X-ray Polarimeter for Small Satellite: Design, Feasibility Study, and Ground Experiments Kiyoshi Hayashida a*, Tatehiro Mihara b, Syuichi Gunji c and Fuyuki Tokanai c a Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka , Japan; b RIKEN, 2-1 Hirosawa, Wako, Saitama , Japan; c Yamagata University, Kojirakawa, Yamagata , Japan ABSTRACT We make a plan of a hard X-ray polarimetry experiment with a small satellite. Bright point-like sources in 20-80keV are prime targets, for which we will not use focusing optics. Comparing various types of polarimeters, we adopt a scattering type in which anisotropy in scattering directions of photons is employed. After optimization of the design is considered with simplified models of scattering polarimeters, we propose to use segmented scatter targets made of plastic scintillators, with which scattering location is identified by detecting recoiled electrons. Simulations show that recoiled electrons are detectable when incident X-ray energies are above 40keV, for which higher polarimetry sensitivity is obtained. We confirmed the performance of such a polarimeter in experiments at a Synchrotron facility and performed a balloon flight in which a proto type unit of the polarimeter was onboard. We finally discuss feasibility of a small satellite experiment in which many of the polarimeter units will be employed. Twenty five units of the polarimeter enable us to detect hard X-ray polarization of 5-10% for a hundred mcrab sources. Improvement in the sensitivity to detect recoiled electrons will significantly improve the polarimetry sensitivity. We also consider a low energy extension of our system down to below 10keV in order to cover wide energy range. Keywords: X-ray polarimetry, scattering polarimeter, hard X-ray, scintillator, PMT 1. INTRODUCTION Astronomical X-ray polarimetry has a long history as X-ray astronomy itself, but is still an unexploited field. The filed has been recently activated by developments of various new technologies for stellar X-ray polarimetry, and by new understandings of importance of polarization information. It may be partly due to a kind of saturation in other fields of X- ray astronomy, spectrometry and imaging, for which huge resources will be required to make a big step. In the workshop held at SLAC in this February, many proposals and current going projects were presented 1. The authors of this paper themselves have been developing several types of X-ray polarimeters so far, some of them jointly and the other independently. Based on those experiences, we are going to make a plan for a stellar X-ray polarimetry project in near future. This is firstly motivated by discussions for the NeXT project 2, a large size X-ray mission next to the Astro-E2 in Japan, but we are planning to make a small satellite experiment specialized for X-ray polarimetry. In the next section, we explain the target of our experiment, which is polarization of bright point-like sources in hard X- ray band of 20-80keV. As shown in the same section, a polarimeter using Compton scattering is best for such a study. In section 3, we consider how to optimize the design of the scattering polarimeter. We finally propose to use a segmented target polarimeter as a unit of our experiment. In section 4, our developments of scattering polarimeters on the ground are introduced. We also briefly touch a balloon experiment recently performed by one of the authors. In the last section, we show feasibility study of a possible small satellite experiment. The experiment primarily aims at 20-80keV, but we show low energy extension below 10keV is possible by some means. * Further author information: (Send correspondence to K.H.) K.H.: hayasida@ess.sci.osaka-u.ac.jp, Telephone: +81 (6)

2 2. HARD X-RAY, NON-IMAGING, SCATTERING TYPE POLARIMETER As mentioned above, X-ray polarimetry is still an unexploited field. It means that there are many rooms and directions to plan a polarimetry experiment. The situation is very much different from the case for X-ray imaging or spectrometry experiments. In this section, we fix the baseline of the design for X-ray polarimetry experiment with a small satellite. In short, our plan focus on hard X-ray range from 20 to 80keV, bright compact sources will be primary targets, and scattering type polarimeters will be employed for it Targets of X-ray polarimetry and its energy range One may set a specific group of targets for X-ray polarimetry for stellar sources, but it would be better to cover as many X- ray objects in different categories as possible, for the polarimetry at current circumstance. In fact, lots of theoretical prospects and models have been presented for the target of X-ray polarimetry 1, including super nova remnants, clusters of galaxies, galactic compact star binaries, pulsars, and active galactic nuclei. They can be summarized by the mechanism to produce possible polarized X-ray emission. Synchrotron radiation should produce polarized X-ray emission unless magnetic field is perfectly random. If we can map the X-ray polarization degree and direction in diffuse source, the magnetic field in the source will be known. Compton scattering process will also produce the polarized X-ray emission depending on the geometry of sources and scatters. For example, we expect polarization in scattered radiation from an accretion disk around compact objects. Most of these processes are related to non-thermal radiation usually with power law X-ray spectra. We consider such a power law component is generally emphasized in hard X-ray energies than in soft X-ray energies. This is one of the reason why we set the target energy range to be hard X-ray band, for example 20-80keV. However, it is apparent that number of photons is much larger in soft X-ray band than hard X-ray band. In fact, polarization was detected previously in the soft X- ray band, though positive detection was limited to the Crab nebulae and Sco X-1. In addition, several proposals (AXP, PLEXAS, SXP, etc) were presented for soft X-ray polarization experiments with small satellites 1, while there is only one proposal XPE, though POGO, a proposed balloon experiment, has some overlap with 20-80keV energy range 1. We think it is most important to realize a positive detection of X-ray polarization as early as possible, which has been paused nearly 30 years. Considering possible instrumentations for soft X-ray polarimetry and for hard X-ray polarimetry, the latter needs smaller amount of development work and resources for us. That is also an important reason why we adopt hard X-ray band. Of course, hard X-ray band is fascinating as it is an unexploited energy range in stellar polarimetry. There might be further discussions on hard X-ray or soft X-ray, but considering non-thermal spectra from the targets of polarization experiments, energy dependence of the polarization is important as already emphasized by authors. Therefore, hard X-ray polarization is important even after some of the soft X-ray projects succeed in detecting polarization from many sources. Finally, we note another merit of taking hard X-ray range as a target; we are able to perform balloon experiments prior to a small satellite experiment, not only for instrument verification but also for real observation Using focusing optics or not We fix the target energy range to be hard X-ray band of 20-80keV. For this energy range, usual focusing optics such as X- ray grazing incidence mirror was not available, and mechanical collimator was the only option. Recently, multi-layer coated mirror, called super mirror is available for this energy range as planed for the NeXT satellite 2. Collecting power of such a mirror is very attractive to gain the sensitivity with a small detector and to reduce background. In fact, if we place a handy size scattering polarimeter on the focal plane a super mirror on the NeXT satellite, we expect to realize hard X-ray polarimetry for ten mcrab level sources. However, for a small satellite experiment, resources and geometrical size does not allow us to use such optics. We thus have to use mechanical collimators to limit the field of view. Another merit of using focusing optics might be its imaging capability. As mentioned above, X-ray polarization mapping of diffuse objects, such as super nova remnants and clusters of galaxies, may be attractive to see magnetic field in those sources. We estimated photon fluxes in hard X-ray band for many diffuse sources from the observations in soft X-ray band. Nevertheless, it is found that with an exception of the Crab nebulae, other diffuse (if we look at arc minute resolution) sources, such that Coma cluster, or Kes75, SN1006 etc., are expected to have at brightest mcrab / arcmin 2. We therefore had better to concentrate bright point-like sources, unless hard X-ray optics with large effective area is available. That is our baseline of the design.

3 2.3. Polarimeter type There are basically two types of hard X-ray polarimeters (for stellar polarimetry), a photo-electron track type and a (Compton) scattering type. The former utilizes anisotropy in the emission direction of photoelectrons, while the latter use that in the scattering direction of photons. Photo-electron track type polarimeters are realized with an X-ray CCD or with gas counters. In particular, polarimeters using gas imaging detectors have been extensive developed recently. On the other hand, scattering type is somehow classical but various configurations are proposed. General arguments between these two types are beyond the scope of this paper, but we focus on the polarimetry sensitivity as a clue. In order to evaluate the performance of an X-ray polarimeter, we often use a modulation factor M, and a detection efficiency η. The modulation factor M is defined as a modulation of the signal (counting rate etc.) as a function of rotation angle, e.g. (N max -N min )/(N max +N min ), where N max and N min are the maximum and minimum of the counting rate. The detection efficiency η is defined as the ratio between the number of events detected and employed in the polarization analysis to that of the incident X-rays. For photo-electron track gas counters, it is the same as the quantum efficiency of the gas counters. For CCD, we have to consider an additional factor, the ratio of number of multi-pixel events to the total number of events. When we consider a scattering type polarimeter, as described in the next section, it is multiplications of probabilities, the probability of incident photons are scattered in a target material, the probability of the scattered photons travel to the sold angle covered by detectors, and the X-ray detection efficiency of detectors. Both M and η are the greater the better, but the sensitivity of a polarimeter is quantitatively evaluated with Minimum Detectable Polarization degree (MDP). The MDP is defined as the lower limit of the polarization degree of a target source of which polarization is positively detected. Apparently it is a function of a source intensity, exposure time, effective area, and background level, but is inversely proportional to Mη 1/2 if the background is negligible. Therefore, a polarimeter with larger M η 1/2 is better. As described below, scattering type polarimeters can provide, for example, M of 0.5 and η of 0.6 at the same time, which makes M η 1/2 of On the other hand, photo-electron track polarimeters with gas detectors are difficult to get large values of M and η at the same time. In order to get large M, scattering of electrons in gas must be suppressed, for example, by using gas with a small atomic number. According to the simulation by Pacciani et al. 3, Mη 1/2 is optimized to be 0.07 at 20keV. Although the efficiency η may be improved by stacking the detectors, to gain the factor of 5 difference in Mη 1/2, the efficiency η must be increased by factor of 25. In conclusion, we adopt the scattering type polarimeter for hard X-ray, nonimaging polarimetry. 3. OPTIMIZATION OF HARD X-RAY SCATTERING POLARIMETER DESIGN We have fixed to adopt a scattering type polarimeter. Although this type of polarimeters is classical, we reconsider its optimized design with a simplified model, a scattering target and surrounding detectors. After that, we propose to improve its simplified polarimeter by introducing segmented and active targets. Its polarimetry sensitivity is explored with simulations Design optimization with a simplified scattering porarimeter We first simplify a scattering polarimeter with a cylindrical shape target and detectors surrounding the target as shown in Fig.1 (a). The highest M ~1 is obtained when we make the height of the target and detector very small and place detectors far apart the target. Note that the anisotropy of the scattering direction is described as (1-sin 2 θcos 2 φ), the modulation is maximized at the scattering polar angle θ of 90º. That is different from the anisotropy in the photo-electron emission where the same azimuthal anisotropy for any polar angles. The scattering polarimeter of model (a) is good for ground experiment, and in fact we employed such model to calibrate the polarization degree of incident X-ray beam 4,5. However, the detection efficiency is too small to use it for stellar polarimetry. We have to make the height of the detectors larger in order to cover larger solid angle of the scattering. There is an optimized point where Mη 1/2 is maximized for a given target. If we idealize the target has a large scattering cross section and a negligible absorption cross section so that a small height target is enough to work, Mη 1/2 is maximized to be about 0.55 when we cover the scattering polar angle of about ±50º. This value should be considered to be a kind of theoretical limit of a scattering polarimeter. In reality, we have to make the target height longer,

4 as long as the mean free path against Compton scattering in order to gain the efficiency to collect scattering events. (Stacking number of model (a) polarimeters is another option, but we will not consider here.) Furthermore, we have to make the target radius bigger if we have to gain an effective area of the system without employing focusing optics, as is the case of ours. Here, we introduce an additional factor, f, which is the ratio of the cross sectional area of the target to that of the polarimeter. For the polarimeter model (a), f is apparently too small. We then consider a model (b) in which the radius of the target is 1/2 of that of the circle (octagonal) of the surrounding detectors. With this model (b), M of 0.5, η of 0.6, and f of 0.25 are obtained. If we make the radius of the target larger so that it fills inside of the surrounding detectors, as model (c), f becomes 1, while M gets as worse as The figure of merit of the polarimeter, M(ηf) 1/2 is the same as the model (b). Comparing the model (b) and (c), the model (b) may be practically better, because of larger M. The factor M(ηf) 1/2 comes from purely statistics and any possible systematic errors are not taken into account. When we observe X-ray objects in the sky, it is not likely that the polarization degree of their radiation is close to 100%. If polarization degree of the source is 10%, we will observe a modulation of 0.1M. Although we are not sure about the possible systematic errors, M should be higher as possible against general kinds of systematic errors. The point here is, however, for either model (b) or model (c) there are some rooms to improve M(ηf) 1/2, by factor of 2. We describe our proposal for it in the next subsection. (a) (b) (c) Top view Detector Scattering Target X-ray incidence Side view Figure 1. Simplified models of a scattering polarimeter. (a) The target has a small radius and length. The height of the detectors are also small. The modulation factor M of nearly 1 is obtained, while η (detection efficiency) and f (geometrical covering fraction) is very small. (b) The target is longer than (a), and its radius is schematically 1/2 of the detector circle. The model gives, for example, M of 0.5 and η of 0.6. The covering fraction f is 0.25 in this case. (c) The target radius is as large as it fit the detector circle, ie, f=1. Schematically, M is 0.25 and η is 0.6. M(ηf) 1/2 is the same as the model (b) Segmented target polarimeter with recoild electron detection The reason why the model (c) polarimeter provides poorer M is that each detector collects scattered X-rays from various portion of the target, i.e., wide range of scattering angles. If we the scattering location is identified for each event, the

5 scattering azimuthal angle will be accurately determined. In order to realize such an idea, Gunji et al. 6 proposed to use segmented plastic scintillators as a scattering target. Plastic scintillators are used to detect recoiled electrons in Compton scattering to identify the scattering locations in the target cross section. Schematic design is shown in Fig.2. In the figure, plastic scintillators are placed circularly, but they may be placed in a square format. To detect the signals from recoild electrons, a Multi Anode type Photo Multiplier Tube (MAPMT) is employed. Detectors for scattered X-rays should have high quantum efficiency in hard X-ray band, e.g., heavy scintillators such as CsI(Tl) or solid state detectors such as CdTe. The whole polarimeter assembly should be surrounded by active shield, such as well type scintillators, to reduce background in real observations. Segmented target = Plastic scintillators Surrounding detectors= NaI, CsI scintillator or CdTe detetors Figure 2. Schematic view of a segmented target polarimeter. X-rays incident to plastic scintillators are scattered inside the scintillator and detected in one of the surrounding detectors. If recoiled electron energy is high enough to be detected with a plastic scintillator and MAPMT(Multi Anode Photo-Multiplier Tube) system, the scattering position is identified. It allows us to determine the scattering azimuthal angle accurately, providing large M for any incident positions. Function of this segmented target polarimeter is as follows. Detected X-ray events can be basically classified in to three cases. In the first case, an X-ray photon is scattered in one of the plastic scintillators, the scattered photon is absorbed in a surrounding detector, and a recoiled electron is detected with the MAPMT at the same time. We call this kinds of events as double hit events. The second case is similar to the first case, but the recoiled electron is not detected with the MAPMT. We call those events as single hit events. The kinetic energy of recoiled electrons is very small, e.g., 6keV for 90º scattering of incident photon energy of 60keV, but 3keV for 40keV incidence. Depending on the incident X-ray energy, some fractions of events go to single hit events. The third case is photo-absorbed events in the plastic scintillators. We performed simulation using EGS4 code, assuming uniform X-ray beam irradiation over the plastic scintillators area. The result of the simulation is summarized in Fig3. in terms of M and η. As seen in Fig.3, the double hit events are limited at higher incident X-ray energy above about 40keV. This limitation comes from the sensitivity of the plastic scintillator plus MAPMT system. Nevertheless, the modulation factor M is as high as 0.55 for the double hit events, as we expected. Note that we reconstruct the scattering azimuthal angle from the scattering position (which plastic scintillator is hit) and the absorbed position (which surrounding detector responds). On the other hand, the single hit events have M of 0.28 as is the model (c) in the previous

6 subsection. Below 25keV, photo-absorbed events in plastic scintillator dominate the scattering events. The photo-absorbed events are available to see the spectral and timing information from the source, but no use for polarimetry. There are two points in which this segmented target polarimeter needs to be improved. One is that the low energy limit for the double hit events is too high. The limit is almost uniquely determined by the sensitivity of the plastic scintillator and MAPMT system. This is one of the development items with highest priority. The other point is also the low energy cut off for any kinds of scattering events determined by the target material, ie, plastic. The atomic number of target material should be as small as possible in order to maximize scattering events over absorption events in the target. As a solid material, Li or Be is better than plastic or carbon. We will make a proposal for the low energy extension to the segmented target polarimeter with plastic scintillators in section 5. On the contrary, this type of polarimeter has other advantages over simplified models in the previous subsections. One important advantage is a high background rejection capability at least for the double hit events. The other point is an imaging capability, though we will not utilize it since we will not use focusing optics as mentioned above. However, such imaging capability will be very important for focal plane polarimeters Single Hit Double Hit Ex(keV) Figure 3. Detection efficiency η and the modulation factor M of a segmented target polarimeter from EGS4 simulation. X- rays are irradiated uniformly over the area of plastic scintillators. Double hit events are those which both recoiled electrons and scattered photons are detected. They are marked with closed circles, indicating M values above 0.55, but has efficiencies larger than 0.1 only above 40keV. On the other hand, single hit events are those which recoiled electrons are missed to be detected with the MAPMT. The single hit events have lower M of 0.25~ GROUND AND BALOON EXPERIMENTS We have been developing various types of X-ray polarimeters on the ground, including photo-electron track type polarimeters using CCD or gas detectors, and several models of scattering polarimeters. We introduce our development of scattering type polarimeters, especially the current status of segmented target polarimeter, in this section. We already made a proto-type of the segmented target polarimeter and performed experiments at a Synchrotron facility with it. We show the results of the ground experiment and shortly mention about possible solutions to improve the MAPMT sensitivity, which is

7 essential to gain double hit events at lower energies. Furthermore, Gunji et al. and Yamagata University group performed a balloon experiment of the segmented target polarimeter on June We briefly touch the experiment in this section Scattering type polarimeters for X-ray beam calibration Fig.4(a) shows an example of a Compton scattering polarimeter, we constructed and used for calibration of X-ray beam polarization at Synchrotron facilities 5. Similar system using a gas propotional counter was used in the other beam calibration 4. The central target is typically 5mm-10mm diameter and 7mm-10mm length and two CdZnTe detectors (only one shown in the photo) are placed 150mm apart the target. The modulation factor M of this polarimeter was calculated to be larger than The detectors are rotatable to take modulation curve. The X-ray spectra agree fairly well with those expected from the simulation with EGS4 code. Figure 4. (a) Scattering polarimeter used in polarization measurement of incident X-ray beam in Synchrotron facilities. Scattered X-rays at the central target rod is detected with a CdZnTe detector. (b) The modulation of the counting rate as a function of rotation angle is shown in the right panel 5. NaI 2 NaI 1 NaI 3 NaI 0 NaI 4 NaI 7 NaI 5 NaI 6 Figure 5. Octagonal shape configuration of a proto type GAPOM polarimeter (bottom view at the left panel and side view at the middle panel). MAPMT is used to detect scintillator signals. Counting rate modulation by 8 NaI scintillators, when 40keV polarized X-rays are irradiated at the center of polarimeter, is shown in the right panel.

8 4.2. Scattering type polarimeter for gamma ray burst Mihara et al. have been developing a scattering polarimeter for the Gamma-ray burst Polarization Monitor (GAPOM) project which aims to detect polarization in gamma-ray bursts 7. One unit of the GAPOM is similar the simplified model (c) in Fig.1 and consists of a plastic scintillator target surrounded by six heavy scintillator detectors. Hexagonal shape allows many units to be located with little crevice in order to get large effective area in given geometrical area. In order to improve the modulation factor M, outside part of the plastic scintillator target is shielded against the X-ray incidence. They made a new version of proto type unit of the GAPOM polarimeter recently, though its cross section is not hexagonal but octagonal, and tested it at a Synchrotron facility 7. In this proto type unit, BaF2 crystal is installed at the bottom of the target plastic scintillator to detect X-rays pass through the plastic scintillator. A MAPMT (Hamamatsu R8520) with 6x6 readouts is used to detect all the signals from these scintillators, but fast-slow amplifier discrimination and position information from the MAPMT are enough to separate them. They obtained the modulation factor M of 0.57 at 30keV, 0.41 at 40keV, 0.48 at 60keV, and 0.54 at 80keV when X-ray beam is irradiated at the center of the unit. Lower M at 40keV is considered to be contamination from fluorescent from Ba. These M values and efficiencies η are almost consistent with simulations using the Geant4 code Segmented target polarimeter: proto type design and experiment at Synchrotron facility The segmented target polarimeter, introduced in the previous section, has been developed by Gunji et al 6,8 In a resent model, primarily targeting polarization of gamma ray burst, they employ Hamamatsu H8500 8x8 ch readouts MAPMT. Plastic scintillators of 5.5mmx5.5mm cross section and 40mm length are placed as a 6x6 matrix form. CsI(Tl) scintillators of the same size surround them, making 8x8 matrix of two kinds of scintillators within the cross sectional area of 48mmx48mm, which almost matches the top plate of the MAPMT(see Fig,6). This unit including MAPMT is very compact and can be placed with little dead space. Signals from each scintillator are basically separated in the MAPMT, though there is some cross talk. Countermeasures against the cross talk have been studied from many aspects, for example by using rise time discrimination and by modifying the shape of the CsI(Tl) scintillator at the contact to the MAPMT 8. Top view Side view Plastic scintillator Figure 6. Proto type model of a segmented target polarimeter. Plastic scintillators are arranged in 6x6 matrix, surrounded by 24 CsI(Tl) scintillators. This block of scintillators is placed on the Hamamatsu H8500 MAPMT. The plastic scintillators are worked as a scattering target and also as absorbers for recoiled electrons. Note that whole the unit is placed in a well type active shield made of scintillators in order to reduce backgrounds by anti coincidence rejection when it is used in real observations.

9 This proto type polarimeter unit was tested at a Synchrotron facility. X-ray beam of 80keV was irradiated to one of plastic scintillator near the center. The number of counting rate in each anode is plotted in Fig.7. Comparing this data with EGS4 simulations, the modulation factor M is evaluated to With further simulation, M against uniform incidence to the whole polarimeter is estimated to be The detection efficiency η observed is 0.17, but that for uniform incidence is estimated to be 0.21 from a simulation. As mentioned in the subsection 3.2, the sensitivity to detect recoiled electrons is essential to lower the energy threshold for the double hit events and thus improve the polarimetry sensitivity. There are several directions to search for. One is to introduce a better reflector to scintillators. Gunji et al. have tested a reflector, vikuiti ESR by 3M campany and confirmed light yield increase by about 20%. Increasing the sensitivity may also be possible, if a prism-shaped photocathode, which was found to increase the quantum efficiency of PMT by several tens percents, is implemented to the MAPMT. Figure 7. Counting rate histogram of 24 anodes signals corresponding to each of the 24 CsI scintillators surrounding 6x6 plastic scintillators. X-ray beam of 80keV was irradiated at one plastic scintillator near the center. Modulation is clearly visible. This result is reproduced with a simulation when we assume the modulation factor M of this polarimeter to be 0.65 for the X-ray irradiation near the center. M for uniform iiradiation is estimated to be Segmented target polarimeter: balloon experiments with proto-type model Gunji et al. performed a balloon experiment using another proto type model modified from the one used in the Synchrotron facility experiment. It is a flight primarily for performance verification of the instrument and for background level measurement, not necessarily for detecting gamma-ray bursts. The number of plastic scintillators was reduced from 6x6 to 3x3 with increasing their cross section, in order to save the signal processing channels. The number of CsI(Tl) scintillators was also reduced to 12. Anode signals are summed accordingly. With these modifications, the modulation factor M becomes about 0.45 according to a simulation, though detection efficiency is increased slightly. Whole this polarimeter unit was included in a CsI well, which will act as an active shield. Passive collimator was used to limit the FOV. The balloon flight was successfully done on June 3rd 2004 from Sanriku balloon center in Japan. Background measurement and Crab nebulae observation were made. Results of the observations will be reported elsewhere. 5. FEASIBILITY STUDY FOR A SMALL SATELLITE EXPERIMENT We have just started making a plan of a small satellite experiment for hard X-ray polarimetry. As mentioned in section 2, we focus on bright point-like sources by using scattering polarimeters without employing focusing optics. Although the plan has not yet been outlined, we show some feasibility studies of polarimetry observations with an assumed size of the polarimeter.

10 5.1. Expected sensitivity with 25 units of segmented target polarimeters We consider to employ some number of units those are similar to the one as Fig.6. It is hard to fix the number of units without fixing the specifications of the satellite platform, but we here assume 25 units as a baseline. Each polarimeter unit has a geometrical area of 5cmx5cm, and thus total area is as small as 25cmx25cm. However, since we have to put them in a well type active shield, the total size will be larger than that. The weight of the polarimeter unit is not large, but the active shield well and construction for it dominate the weight, for which we have not yet designed. Electric power is another point of concern. In the balloon experiments by Gunji et al., total (analog plus digital) electric power of 9W was used for one unit. Apparently we have to make sophisticated, lower power circuits for this polarimeter. To design the satellite body, we also have to fix what kind of attitude control system is used. Considering our main targets, severe attitude control is not required. Inexpensive spin stabilized system might work. The spin control system might be good for reducing or evaluating the systematic errors in the observed modulation. (a) 40-80keV (double hit events) (b) 20-40keV (single hit events) Considering BGD ks ks 1 Without BGD 1 1Ms 1Ms Flux (mcrab) Flux (mcrab) Figure 8. Minimum detectable polarization degree expected for 25 units of segmented target polarimeters onboard a small satellite, for two kinds of exposure time. (a) is for 40-80keV range but only for double hit events. The solid lines correspond to the values when we consider backgrounds, and dotted lines represent the values when we neglect the background. Note, however, significant fraction of the background events might be rejected by coincidence method when we pick up the double hit events. In that case we should refer the dotted line. (b) is for 20-40keV range but only for single hit events. We use the simulation result for one unit described in Fig.3 and calculate the MDP (minimum detectable polarization degree) as a function of source intensity. We consider two energy bands, 20-40keV and 40-80keV. For 20-40keV band, we take only single hit events, while only double hit events are taken for 40-80keV band. Background level is hard to be evaluated accurately, but we adopt 1x10-4 counts/s/kev/cm 2. Note that coincidence among scintillators may reject the background significantly, especially for double hit events. With 100ks observations, we expect to detect 10% and 5% polarization for a 100mCrab source, at 40-80keV and 20-40keV band respectively. For the double hit events, if the

11 background level will be significantly reduced from the value above by coincidence, 4% polarization for 100mCrab source is detected. Our targets will be a few tens of 100mCrab level sources. Therefore, we may need to increase exposure time and number of units by a factor of a few. If improvement in the sensitivity of detecting recoiled electrons is achieved, we expect to use double hit events for 20-40keV range and the polarimetry sensitivity will be significantly improved Low energy extension As shown in Fig.3, the polarimetry detection efficiency η is cut off below 20keV. This is due to the absorption in the target plastic scintillators, which is significant at lower X-ray energies. In order to avoid it, we have to prepare specialized scattering polarimeters with targets made of light elements, such as Li or Be. Employing photo-electron track polarimeters specialized for low energy X-rays might be another option. Both options should be considered, but here we propose another way of the low energy extension by modifying the segmented target polarimeter unit described above. Li target (c) 5-20keV (single hit events) ks 1 1Ms Ti filter over plastic scintillators Figure 9. Low energy extension by replacing some plastic scintillators with a Li target. In order to protect against 5-20keV X-rays incident to plastic scintillators, they are need to be covered with, e.g., Ti filter of 50µm. The right panel shows expected minimum detectable polarization degree with 25 units of this model of polarimeters. Solid lines are background considered and dashed lines not. The idea is very simple; remove the central four plastic scintillators from a unit and put one Li target at the center. X- rays incident to the Li target will be scattered and will be detected by some of the plastic scintillators (see Fig.9). In order to avoid scattered events of low energy X-ray incidence at surrounding plastic scintillators, we employ a filter, for example Ti 50µm. Expected polarimetry sensitivity of this system is displayed in Fig9. in terms of MDP. Although the Li target is very small, it is compensated by large number of incoming photons at lower X-ray energies. The polarization sensitivity MDP is comparable to those for 20-40keV and 40-80keV bands. Although we have not yet done a full simulation of the system, this Flux (mcrab)

12 low energy extended version of the segmented target polarimeter will provide wide energy range of polarimetry from 5keV to 80keV. It will be important to see energy dependence of the X-ray polarization, as mentioned in section Action items Apparently we have many things to do before starting our project. To design scattering polarimeters, we know simulations by EGS4 or GEANT4 are very effective and accurate enough. Detailed design and optimization of the polarimeter unit will be done with such simulations. However, the most important point to improve the polarization sensitivity is in increasing the sensitivity to detect recoiled electrons. For example, if we can make the lower boundary for the double hit events to down to 20keV, we expect about factor 2 increase in M or decrease in MDP. Prism-shaped photocathode for MAPMT will be its principle target. To make sophisticated circuits is also important to realize experiments with many units of the polarimeter. On the other hand, it is not clear how we should do for the satellite platform, since there is not a fixed roadmap for a small satellite experiment in JAXA/Japan, not like SMEX or MEDX in NASA. However, we have to fix the platform in near future to start the project. In parallel, we should continue balloon experiments with proto type polarimeters. ACKNOWLEDGMENTS KH, TM, SG, and TF acknowledge JSPS Grant-in-aid No , No , No , and No , respectively. REFERENCES 1. Presented materials in X-ray Polarimetry Workshop at SLAC, Stanford, Calfornia on February (see 2. Kunieda, H., Hard X-ray Telescope Mission (NeXT), in Proc. SPIE 5488, in press, Pacciani, L. et al., The sensitivity of a photoelectric X-ray polarimeter for Astronomy: The impact of gas mixture and pressure, in Proc. SPIE 4843, pp , K. Hayashida, N. Miura, H. Tsunemi, K. Torii, K., H. Murakami, Y. Ohno, K. Tamura, X-ray Polarimetory with a conventional gas proportional counter through rise-time analysis, in Nuclear Instruments and Methods A, Vol.421, pp , F. Tokanai, H. Sakurai, S. Gunji, S. Motegi, H. Toyokawa, M. Suzuki, K. Hirota, S. Kishimoto and K. Hayashida, Hard X-ray polarization measured with a Compton polarimeter at synchrotron radiation facility, in Nuclear Instruments and Methods A, in press, S. Gunji, E. Kudo, and H. Sakurai, "Improvement of the Modulation Factor for a Compton Scattering Type Polarimeter Using Subdivided Scintillators," IEEE Nuclear Science Symposium. Record, pp , Mihara, T., and H. Miyamoto, Octagonal scintillator for hard X-ray polarimetry, in X-ay polarimetry workshop at SLAC, 8. S. Gunji, T. Suzuki, F. Sato, H. Sakurai, F. Tokanai, Y. Saito, A. Kubota, "Development of hard X-ray polarimeter for γ-ray bursts," in Advances in Space Research, vol.33, p , 2004.