The Imaging X-ray Polarimetry Explorer (IXPE) will add polarization to the properties (time, energy, and position) observed in x-ray astronomy. A NASA Astrophysics Small Explorer (SMEX) in partnership with the Italian Space Agency (ASI), IXPE will measure the 2–8-keV polarization of a few dozen sources during the first 2 years following its 2021 launch. The IXPE Observatory includes three identical x-ray telescopes, each comprising a 4-m-focal-length (grazingincidence) mirror module assembly (MMA) and a polarization-sensitive (imaging) detector unit (DU), separated by a deployable optical bench. The Observatory’s Spacecraft provides typical subsystems (mechanical, structural, thermal, power, electrical, telecommunications, etc.), an attitude determination and control subsystem for 3-axis stabilized pointing, and a command and data handling subsystem communicating with the science instrument and the Spacecraft subsystems.
The Imaging X-ray Polarimetry Explorer (IXPE) will expand the information space for study of cosmic sources, by adding polarization to the properties (time, energy, and position) observed in x-ray astronomy. Selected in 2017 January as a NASA Astrophysics Small Explorer (SMEX) mission, IXPE will be launched into an equatorial orbit in 2021. The IXPE observatory includes three identical x-ray telescopes, each comprising a 4-m-focal-length (grazing-incidence) mirror module assembly (MMA) and a polarization-sensitive (imaging) detector unit (DU). The optical bench separating the MMAs from the DUs is a deployable boom with a tip/tilt/rotation stage for DU-to-MMA (gang) alignment, similar to the configuration used for the NuSTAR observatory. The IXPE mission will provide scientifically meaningful measurements of the x-ray polarization of a few dozen sources in the 2-8 keV band, over the first two years of the mission. For several bright, extended x-ray sources (pulsar wind nebulae, supernova remnants, and an active-galaxy jet), IXPE observations will produce polarization maps indicating the magnetic structure of the synchrotron emitting regions. For many bright pulsating x-ray sources (isolated pulsars, accreting x-ray pulsars, and magnetars), IXPE observations will produce phase-resolved profiles of the polarization degree and position angle.
Hard X-ray imaging polarimeters are developed for the X-ray γ-ray polaeimtery satellite PolariS. The imaging polarimter is scattering type, in which anisotropy in the direction of Compton scattering is employed to measure the hard X-ray (10-80 keV) polarization, and is installed on the focal planes of hard X-ray telescopes. We have updated the design of the model so as to cover larger solid angles of scattering direction. We also examine the event selection algorithm to optimize the detection efficiency of recoiled electrons in plastic scintillators. We succeed in improving the efficiency by factor of about 3-4 from the previous algorithm and criteria for 18-30 keV incidence. For 23 keV X-ray incidence, the recoiled electron energy is about 1 keV. We measured the efficiency to detect recoiled electrons in this case, and found about half of the theoretical limit. The improvement in this efficiency directly leads to that in the detection efficiency. In other words, however, there is still a room for improvement. We examine various process in the detector, and estimate the major loss is primarily that of scintillation light in a plastic scintillator pillar with a very small cross section (2.68mm squared) and a long length (40mm). Nevertheless, the current model provides the MDP of 6% for 10mCrab sources, which are the targets of PolariS.
The Large Size Telescopes, LSTs, located at the center of the Cherenkov Telescope Array, CTA, will be sensitive
for low energy gamma-rays. The camera on the LST focal plane is optimized to detect low energy events based
on a high photon detection efficiency and high speed electronics. Also the trigger system is designed to detect
low energy showers as much as possible. In addition, the camera is required to work stably without maintenance
in a few tens of years. In this contribution we present the design of the camera for the first LST and the status
of its development and production.
PolariS (Polarimetry Satellite) is a Japanese small satellite mission dedicated to polarimetry of X-ray and γ-ray sources. The primary aim of the mission is to perform hard X-ray (10-80 keV) polarimetry of sources brighter than 10 mCrab. For this purpose, PolariS employs three hard X-ray telescopes and scattering type imaging polarimeters. PolariS will measure the X-ray polarization for tens of sources including extragalactic ones mostly for the first time. The second purpose of the mission is γ-ray polarimetry of transient sources, such as γ-ray bursts (GRBs). Wide field polarimeters based on similar concept as that used in the IKAROS/GAP but with higher sensitivity will be used, and polarization measurement of 10 GRBs per year is expected.
We are developing a gamma-ray burst polarimeter for a small satellite. It is Compton-scattering-type polarimeter which consists of segmented plastic scintillator and segmented GAGG scintillator. The scattering position in the plastic scintillator and the absorption position in the GAGG scintillator for incident hard X rays can be read out by multi-anode photomultipliers and avalanche photodiodes, respectively. The detection efficiency and the modulation factor amount to about 21% and 39% at 60 keV, respectively. The geometrical area for one module is about 300 cm2. If two modules will be installed, the polarimeters will measure the polarization for about thirty-five GRBs in two years. Through the observation of the polarization, the radiation mechanism of gamma-ray bursts will be clarified.
We are now investigating and studying a small satellite mission HiZ-GUNDAM for future observation of gamma-ray bursts (GRBs). The mission concept is to probe “the end of dark ages and the dawn of formation of astronomical objects”, i.e. the physical condition of early universe beyond the redshift z > 7. We will consider two kinds of mission payloads, (1) wide field X-ray imaging detectors for GRB discovery, and (2) a near infrared telescope with 30 cm in diameter to select the high-z GRB candidates effectively. In this paper, we explain some requirements to promote the GRB cosmology based on the past observations, and also introduce the mission concept of HiZ-GUNDAM and basic development of X-ray imaging detectors.
The Cherenkov Telescope Array (CTA) project aims to implement the world’s largest next generation of Very High Energy gamma-ray Imaging Atmospheric Cherenkov Telescopes devoted to the observation from a few tens of GeV to more than 100 TeV. To view the whole sky, two CTA sites are foreseen, one for each hemisphere. The sensitivity at the lowest energy range will be dominated by four Large Size Telescopes, LSTs, located at the center of each array and designed to achieve observations of high red-shift objects with the threshold energy of 20 GeV. The LST is optimized also for transient low energy sources, such as Gamma Ray Bursts (GRB), which require fast repositioning of the telescope. The overall design and the development status of the first LST telescope will be discussed.
PolariS (Polarimetry Satellite) is a Japanese small satellite mission dedicated to polarimetry of X-ray and γ-ray sources. The primary aim of the mission is to perform wide band X-ray (4-80 keV) polarimetry of sources brighter than 10 mCrab. For this purpose, Polaris employs three hard X-ray telescopes and two types of focal plane imaging polarimeters. PolariS observations will measure the X-ray polarization for tens of sources including extragalactic ones mostly for the first time. The second purpose of the mission is γ-ray polarimetry of transient sources, such as γ-ray bursts. Wide field polarimeters based on similar concept as that used in the IKAROS/GAP but with higher sensitivity, i.e., polarization measurement of 10 bursts per year, will be employed.
We have been developing a hard X-ray polarimeter to open a new window for hard X-ray astronomy. The project is
called as PHENEX (Polarimetry for High ENErgy X rays). The PHENEX detector is Compton scattering type
polarimeter and it is constructed by several unit counters. The unit counter can achieve the modulation factor and the
detection efficiency of 53% and 20% at 80 keV, respectively. Installing four unit counters, we have carried out balloon-borne
experiment in Jun.13 2006 to preliminarily observe the polarization of the Crab Nebula in hard X-ray band. The
PHENEX polarimeter successfully operated on the level flight and observed the Crab Nebula for about one hour. From
the analysis of the obtained data, it was recognized that the PHENEX polarimeter does not make much spurious
modulation and that the ratio of the signal from the Crab Nebula to the background from the blank sky is 1:3. Though we
can not precisely determine the degree and the direction of the polarization for the Crab Nebula because of the trouble of
the attitude control system, the obtained results were not inconsistent with those in the X-ray band. We will carry out
balloon-borne experiment again, fixing the trouble of the attitude control system.
The solar powered sail spacecraft using a huge sail is a next Japanese engineering verification satellite planned to launch in 2012. It has a hybrid propulsion system with ion engines and a huge solar sail panel of 50 m in diameter. Based on the present mission plan, it will take about 6 years to cruise to Jupiter via Earth swing-bys and 5 more years to reach the Jovian L4 Trojan asteroids. During its cruising phase, we plan to mount a gamma-ray burst (GRB) detector with polarization detection capability which also works as one of the interplanetary network (IPN) to determine the GRB positions. The emission mechanism of GRB is thought to be the synchrotron radiation from the relativistic outflows. If the emission mechanism of GRBs is really synchrotron radiation, the emitted gamma-rays should be strongly polarized. The detection principle is the anisotropy of the Compton scattering. The Compton-scattered gamma-ray photons show the strongly biased distribution toward the vertical direction against the oscillating electric field vector. The design concept of our detector is simple but carefully
avoid a fake modulation. The plastic scintillator in one Compton-length as the scattering body is placed at the center, and 12 CsI scintillators are allocated around it. To avoid a fake modulation through the satellite body scattering, these detectors work in coincidence mode. The coincidence also helps to reduce the particle background. We will use the VA-TA ASIC and FPGA as the analog readout and the digital event processing, respectively, to make the detector weight of almost 2.0 kg. In this paper, we introduce the solar sail mission and show the overview of gamma-ray polarimeter.
The Polarized Gamma-ray Observer (PoGO) is a new balloon-borne instrument designed to measure polarization from astrophysical objects in the 30-200 keV range. It is under development for the first flight anticipated in 2008. PoGO is designed to minimize the background by an improved phoswich configuration, which enables a detection of 10 % polarization in a 100 mCrab source in a 6--8 hour observation. To achieve such high sensitivity, low energy response of the detector is important because the source count rate is generally dominated by the lowest energy photons. We have developed new PMT assemblies specifically designed for PoGO to read-out weak scintillation light of one photoelectron (1 p.e.) level. A beam test of a prototype detector array was conducted at the KEK Photon Factory, Tsukuba in Japan. The experimental data confirm that PoGO can detect polarization of 80-85 % polarized beam down to 30 keV with a modulation factor 0.25 ± 0.05.
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.
We constructed an optical imaging x-ray detector with a capillary gas proportional counter (CGPC), a focusing mirror system, and an image intensified CCD camera. The CGPC consists of an absorption region for x-rays, which also operates as a drift region of electron clouds, and a gas proportional scintillation region in a capillary plate. With a gas mixture of Xe plus 2% CH4 at 1 atm and a gas gain 8000, the light output from the CGPC is several tens thousand times larger than that of a typical NaI(Tl) scintillator and it is enough to image electron clouds due to a few tens keV x rays. To investigate the imaging ability, images for tracks due to (alpha) particles (241Am) of 5.5 MeV were taken in the gas mixture of Xe plus CH4, for the several reduced fields in the drift region. We confirmed that the images were most clearly observed at around the reduced fields of 40 V/cm(DOT)atm.
The fourth Japanese x-ray astronomy satellite, ASCA, carries two imaging gas scintillation proportional counters (GIS) on its focal plane. Extensive ground calibration has established its position resolution to be 0.5 mm and FWHM energy resolution to be 8.0% both at 6 keV. When combined with the x-ray telescope, a sensitivity range becomes 0.7 - 10 keV. These properties have been confirmed through in-orbit calibrations. The in-orbit background of the GIS has been confirmed to be as low as (5 - 7) X 10-4 c s-1cm-2keV-1 over the 1 - 10 keV range. The long-term detector gain is stable within a few % for two years. Gain dependence on the position and temperature has been calibrated down to 1%. The overall energy response is calibrated very accurately. Thus the GIS is working as an all-round cosmic x-ray detector.
The ASTRO-E satellite is scheduled for launch in 2000 by the Institute of Space and Astronautical Science (ISAS). In this paper the design and performance of the hard x ray detector (HXD) developed for ASTRO-E are described. The HXD is a combination of YAP/BGO phoswich scintillators and silicon PIN diodes covering a wide energy band of 10 - 700 keV. The detector background is reduced down to several times 10-6c/s/cm2/keV, and the sensitivity of the HXD is more than one order of magnitude higher than any other past missions in the range of a few 10 keV to several 100 keV. Thus ASTRO-E HXD is expected to achieve an extreme high performance for detecting cosmic hard x rays and low-energy gamma rays. Astrophysics to be explored with the HXT are expected to be extremely widespread and rich.
We designed a new type of polarimeter for polarized hard x rays from stellar objects, utilizing the principle that the direction of the Compton scattered x rays depends on the electric vector of the polarized incident x rays. The characteristics of the new designed polarimeter were investigated by measurements of basic data and computer simulations. From them, it is shown that the polarimeter is capable of observing the interesting astrophysical objects by using a balloon or a satellite. In this paper, we report the characteristics and the expected performance of the new designed hard x-ray polarimeter.
We have developed a new kind of phoswich counters that are capable of detecting low flux hard X-ray/γ-ray from localized sources. The counter consists of a small inorganic scintillator with a fast decay time (the detection part) glued to the interior bottom surface of a well-shaped block of another inorganic scintillator with a slow decay time (the shielding part). The well-shaped shielding part acts as an active collimator as well as an active shield. The whole assembly is viewed by a phototube from the exterior bottom surface of the shielding part. By using an appropriate pulse-shape discriminator (PSD), hard X-rays/gamma-rays that have deposited energy only in the detection part can be selected.
The first counter was built by using a new scintillator (GSO) for the detection part and CsI(Tt) for the shielding part. A detector system consisting of 64 such phoswich counters (total area ~740cm2) was flown three times on board a balloon,
setting a limit to the 57Co line flux from SN1987A at around 10-4cm-2s, determining the pulsating hard X/γ-ray flux of PSR15O9-58, determining the hard X/γ-ray spectra of CenA and GX339-04.
Analyses have revealed the fact that background counts due to the Compton scattering, nuclear reactions, and β-γ radioactivities in the detector are largely suppressed because they are likely to register at least one extra count in the shielding part. The ultimate sensitivity of the detector will then be determined by the level of radioactive contamination.
Other scintillator combinations such as GSO/BGO, NaI(Tl)/CsI(Tl), and YAIO3/BGO have also been studied. Efforts
to reduce the radioactive contamination in scintillators have also been actively pursued. In near future we expect to reach a sensitivity (3σ) around a few times 10-7cm-2s-1keV-1 for continuum in a 104s balloon observation.
We have developed a low background hard X-ray/gamma-ray telescope for balloon-borne experiments. The telescope called Welcome-1 (Well type Compound Eye) utilizes newly developed well-type phoswich counters. In the well-type phoswich counter, the background from external and internal sources are reduced significantly, Welcome-1 is designed for observation in the energy range from 60 keV to 800-1000 keV. The effective area of Welcome-1 is 740 sq cm at 122 keV and 222 sq cm at 511 keV line. We flew Welcome-1 in 1990 and 1991 to detect hard X-rays from SN1987A, PSR1509-58, Cen-A and others. The background levels at an altitude of 4g/sq cm are 1 x 10 exp -4/sq cm photons/s/keV at 122 keV and 3 x 10 exp -5 photons/sq cm/s/keV at 511 keV. The data obtained during the flight shows that the detector in fact has the 3 sigma sensitivity of about a few x 10 exp -6 photons/sq cm/s/keV and about 10 exp -4 photons/sq cm/s in a 10 exp 4 s observation for the continuum spectrum and line spectrum, respectively.