X-ray polarimetry in astronomy has not been exploited well, despite its importance. The recent innovation of instruments is changing this situation. We focus on a complementary metal–oxide–semiconductor (CMOS) pixel detector with small pixel size and employ it as an x-ray photoelectron tracking polarimeter. The CMOS detector we employ is developed by GPixel Inc. and has a pixel size of 2.5 μm × 2.5 μm. Although it is designed for visible light, we succeed in detecting x-ray photons with an energy resolution of 176 eV (FWHM) at 5.9 keV at room temperature and the atmospheric condition. We measure the x-ray detection efficiency and polarimetry sensitivity by irradiating polarized monochromatic x-rays at BL20B2 in SPring-8, the synchrotron radiation facility in Japan. We obtain modulation factors of 7.63 % ± 0.07 % and 15.5 % ± 0.4 % at 12.4 and 24.8 keV, respectively. It demonstrates that this sensor can be used as an x-ray imaging spectrometer and polarimeter with the highest spatial resolution ever tested.
X-ray Imaging Spectrometers (XIS) are the X-ray CCD cameras onboard Suzaku. They were operated in orbit from 2005 to 2015 and produced lots of findings with their good energy resolution and low non X-rat background. Precise calibration including the 10 eV accuracy in the energy scale reinforced them. Nevertheless, there has been a unresolved calibration issue in the spectral response around the Si-K edge (1.839 keV) appearing as systematic residuals up to 10%. The residual is negative peaking at 1.85 keV in the front illuminated (FI) sensors and positive peaking at 1.8 keV in the back illuminated (BI) sensor for X-ray sources dominated by continuum X-ray emission. Various attempts to eliminate these residuals by changing response parameters or quantum efficiency models have been insufficient. In this paper, we revisit this problem by focusing on the relation between incident X-ray energy and pulse height. We introduce a jump in that relation at the Si-K edge by modifying the , and optimize its value so as to minimize the residuals in the fit of the X-ray spectra for the black hole binary LMC X-3, a source dominated by continuum emission. We find the introduction of a jump significantly reduces the residuals. The optimized jump values are +4:2 channel, +4:0 channel, and -3:1 channel, corresponding to 15.3 eV, -11:3 eV, and 14.6 eV, for XIS0, XIS3 (FI), and XIS1 (BI), respectively. The direction of the jump is opposite for the FI and for the BI. We revise the response matrices generator so as to include the jump for each XIS sensor, and apply it to the X-ray spectra of the Perseus cluster of galaxies which has various emission lines in the spectra, and the blazar PKS2155-304 which was observed various epoch in the Suzakuoperation. We confirm the residuals are significantly reduced for these sources, too. We finally suggest the jump at Si-K edge in the energy and pulse height relation is qualitatively explained, if some of charges are lost in course of charger collection to the electrode of the CCD in the depletion later, and its amount is large for larger travel length in the depletion layer. If this explanation is correct, the Si-K edge problem and its solution presented in this paper is not specific only for the SuzakuXIS but also for other X-ray CCDs.
X-ray Astronomy Recovery Mission (XARM) scheduled to be launched in early 2020’s carries two soft X-ray telescopes. One is Resolve consisting of a soft X-ray mirror and a micro calorimeter array, and the other is Soft X-ray Imaging Telescope (Xtend), a combination of an X-ray mirror assembly (XMA) and an X-ray CCD camera (SXI). Xtend covers a field of view (FOV) of 38′ × 38′ , much larger than that of Resolve (3′ × 3 ′ ) with moderate energy resolution in the energy band from 0.4 keV to 13 keV, which is similar to that of Resolve (from 0.3 keV to 12 keV). Simultaneous observations of both telescopes provide complimentary data of X-ray sources in their FOV. In particular, monitoring X-ray sources outside the Resolve FOV but inside the Xtend FOV is important to enhance the reliability of super high resolution spectra obtained with Resolve. Xtend is also expected to be one of the best instruments for low surface brightness X-ray emissions with its low non X-ray background level, which is comparable to that of Suzaku XIS. The design of Xtend is almost identical to those of Soft X-ray Telescope (SXT) and Soft X-ray Imager (SXI) both on board the Hitomi satellite. However, several mandatory updates are included. Updates for the CCD chips are verified with experiment using test CCD chips before finalizing the design of the flight model CCD. Fabrication of the foils for XMA has started, and flight model production of the SXI is almost ready.
We have proposed a new type of X-ray interferometer called Multi Image X-ray Interferometer Module (MIXIM) consisting simply of a grating and an X-ray spectral imaging detector. The baseline concept of MIXIM is a slit camera to obtain the profile of X-ray sources, but aim to get a sub-arcsecond resolution. For that purpose, to avoid blurring of the image by diffraction is a key, and we select X-ray events of which energy satisfies the interferometric condition called Talbot effect. Stacking the images (X-ray interferometric fringes) with the period of the grating is another point of the method, which provides the self image of a grating slit convolved with the profile of the X-ray source. We started an experiment with a micro focus X-ray source, 4.8 μm pitch grating, and an SOI type X-ray detector XRPIX2b with a pixel size of 30 μm. The stacked self image was obtained with a magnification factor of 4.4. We, however, need finer positional resolution for the detector to obtain the self image to a parallel beam, for which the magnification factor must be 1. We thus focused on small pixel size CMOS sensors developed for visible light. We irradiated X-rays to one of such CMOS sensors GSENSE5130 with a pixel size of 4.25 μm, and found enough capability to detect X-rays, i.e., FWHM of 207 eV at 5.9 keV at room temperature. We then employed this sensor and performed an experiment at a 200 m beam line of BL20B2 in the synchrotron facility SPring8. Using a grating with a pitch of 4.8 µm and an opening fraction of f=0.5, we obtained the self image of the grating at the detector distance from the grating of 23 cm and 46 cm and the X-ray energy of 12.4 keV. We also performed an experiment using a 9.6 μm f = 0.2 grating with a detector-grating distance of 92 cm, and obtained higher contrast image of the grating. Note that the slit width of 2.4 μm at 46 cm corresponds to 1.1′′, while that of 1.9 μm at 92 cm does 0.43′′. We suggest several format of possible MIXIM missions, including MIXIM-S for very small satellite of 50cm size, MIXIM-P for parasite use of nominal X-ray observatory employing grazing X-ray telescopes with a focal length of 10 m, and MIXIM-Z in which the grating-detector distance of 100 m is acquired by formation flight or free fryers to yield 0.01” level resolution.
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.