arXiv subjects

Tatsuya Sawano

Publications and source records attributed to Tatsuya Sawano.

At least 19 recordsLinked to original sources

arXiv2026

Three-dimensional recoil-electron reconstruction using combined optical imaging and waveform readout for electron-tracking Compton cameras

Accurate reconstruction of recoil-electron directions is critical for enhancing the point-spread function of electron-tracking Compton cameras (ETCCs) in gamma-ray imaging. Although full three-dimensional (3D) readout systems achieve high-precision reconstruction, they are impractical for large-area detectors because of the enormous data volume. This study proposes and demonstrates a practical alternative for inferring the 3D recoil-electron direction in Compton scattering. This method combines a high-resolution two-dimensional optical image, a one-dimensional waveform signal, and a deep-learning-based method through simulations. The proposed method achieved an angular resolution of approximately $44^\circ$ for the recoil-electron direction in the 40-50 keV range, corresponding to an improvement of a factor of about 1.3 compared with our previous strip-readout approach using pseudo-experimental data generated by Geant4 and MAGBOLTZ simulations for an argon-based gas time projection chamber. In addition, the starting-point resolution of the electron track was improved over the previous method across the 5-50 keV electron energy range. These results demonstrate that complementary information from the transverse image and longitudinal waveform can effectively recover the 3D track topology without requiring full 3D readout. The proposed approach provides a realistic pathway for improving ETCC imaging performance.

physics.ins-det↗

arXiv2026

Development and Evaluation of a CNN-Based Charged-Particle Event Rejection Algorithm for Soft X-ray Detection in a pnCCD-Based Satellite System

All-sky surveys in the soft X-ray band are essential for detecting transient objects such as high-redshift gamma-ray bursts (GRBs), which provide key insights into the early universe. HiZ-GUNDAM is a future satellite mission designed to detect and localize high-redshift GRBs. Its wide-field X-ray monitor, EAGLE, combines Lobster Eye Optics with a pnCCD imaging detector operating in the 0.4-4 keV band. Because of limited satellite telemetry, full-frame pnCCD images cannot be downlinked, requiring onboard event selection. Charged particles in the space environment produce background events that can be misidentified as X-ray photons, degrading detection sensitivity and potentially triggering false alerts. In this study, we developed a pnCCD readout system and evaluated charged-particle rejection using conventional grade methods and a convolutional neural network (CNN). Performance was evaluated using X-ray events from an Fe-55 source and electron events from a Sr-90 beta source. The CNN reduced the misclassification rate from 10.2-11.9% for conventional grade methods to 3.1% while maintaining a high acceptance rate for X-ray events. The improvement is particularly pronounced at higher deposited energies, reflecting the CNN's ability to distinguish track-like particle events from X-ray events by capturing detailed spatial features of charge distributions. Comparison with a thin-depletion-layer CMOS sensor further indicates that the thicker depletion layer of the pnCCD enhances discrimination performance. These results demonstrate that CNN-based event classification can substantially reduce charged-particle contamination while maintaining high X-ray acceptance, making it a promising approach for onboard event selection in future pnCCD-based wide-field X-ray missions.

astro-ph.IM↗

arXiv2026

Radiation tolerance of a diamond radiation detector for space use

We present a study of the radiation tolerance of two types of diamond radiation detectors for space use. We plan to launch a 3U-size CubeSat, KSAT3-X, developed by Kanazawa University in 2027. The KSAT3-X mission is aimed to observe inflows and outflows of charged particles such as electrons and protons, particularly in the 10 - 40 keV energy range, in the Earth's magnetosphere. As the mission instrument, we have developed two diamond radiation detectors. The first is composed of a microwave plasma chemical vapor deposition (MPCVD) diamond fabricated by Element Six, and the second is based on a MPCVD diamond produced in-house at Kanazawa University. We irradiate both diamonds with 100 MeV protons and evaluate their spectroscopic performance as an indicator of radiation tolerance using characteristic X-rays from radioisotope sources. We find no significant degradation in their spectroscopic performance up to at least the 10-year equivalent irradiation under the orbital environments of KSAT3-X. We additionally irradiate the Element Six diamond with 100 MeV protons up to the 100-year equivalent. As a result, no significant degradation in the spectroscopic performance is observed. These results indicate that the two diamond radiation detectors have sufficiently high radiation tolerance. We also discuss possible physical origins of the observed difference in the spectroscopic performance between the two detectors.

astro-ph.IM↗

arXiv2025

Systematic Biases from ARM-Based Sensitivity and Imaging in Compton Cameras

The angular resolution measure (ARM) is widely used in Compton camera analyses to characterize angular uncertainty and to define event selection criteria. However, the ARM distribution is fundamentally different from the point spread function (PSF), which represents the true spatial imaging response of the system on the celestial sphere. In this study, motivated by applications in gamma-ray astrophysics, we investigated the consequences of using ARM-based definitions in sensitivity estimation and image reconstruction. Through Monte Carlo simulations of 662 keV gamma-rays, we demonstrated that the background rejection power of ARM-based region-of-interest selections with the ARM resolution of 7.0$^{\circ}$ is equivalent to that of a PSF resolution of 33$^{\circ}$. As a result, treating the ARM resolution as a surrogate for the PSF resolution leads to significant overestimation of the signal-to-noise ratio, particularly in analyses assuming isotropic backgrounds. We further show that image reconstructions at pixel scales finer than the PSF resolution introduce substantial statistical uncertainties, reducing the effective number of independent spatial degrees of freedom. These findings underscore the necessity of PSF-based analysis for accurate sensitivity evaluation and statistically robust imaging in Compton cameras, particularly in applications where the background discrimination is critical.

astro-ph.IM↗

arXiv2024

Possible X-ray Cocoon Emission from GRB 050709

The detection of the short gamma-ray burst (SGRB) 050709 by the HETE-2 satellite opened a new window into understanding the nature of SGRBs, offering clues about their emission mechanism and progenitors, with the crucial aid of optical follow-up observations. Here, we revisit the prompt emission of GRB 050709. Our analysis reveals an initial hard spike ~200 ms long, followed by a subsequent soft tail emission lasting ~300 ms. These components could be common among other SGRBs originating from binary neutron merger events, such as GW/GRB 170817A. Detailed temporal and spectral analyses indicate that the soft tail emission might be attributed to the cocoon formed by the relativistic jet depositing energy into the surrounding material. We find the necessary cocoon parameters at the breakout, as consistent with numerical simulation results. We compared the physical parameters of this cocoon with those of other SGRBs. The relatively higher cocoon pressure and temperature in GRB 050709 may indicate a more on-axis jet compared to GRB 170817A and GRB 150101B.

astro-ph.HE↗

arXiv2024

High-speed Readout System of X-ray CMOS Image Sensor for Time Domain Astronomy

We developed an FPGA-based high-speed readout system for a complementary metal-oxide-semiconductor (CMOS) image sensor to observe soft X-ray transients in future satellite missions, such as HiZ-GUNDAM. Our previous research revealed that the CMOS image sensor has low-energy X-ray detection capability (0.4-4 keV) and strong radiation tolerance, which satisfies the requirements of the HiZ-GUNDAM mission. However, CMOS sensors typically have small pixel sizes (e.g., $\sim$10 ${\rm μm}$), resulting in large volumes of image data. GSENSE400BSI has 2048$\times$2048 pixels, producing 6 Mbyte per frame. These large volumes of observed raw image data cannot be stored in a satellite bus system with a limited storage size. Therefore, only X-ray photon events must be extracted from the raw image data. Furthermore, the readout time of CMOS image sensors is approximately ten times faster than that of typical X-ray CCDs, requiring faster event extraction on a timescale of $\sim$0.1 s. To address these issues, we have developed an FPGA-based image signal processing system capable of high-speed X-ray event extraction onboard without storing raw image data. The developed compact system enabled mounting on a CubeSat mission, facilitating early in-orbit operation demonstration. Here, we present the design and results of the performance evaluation tests of the proposed FPGA-based readout system. Utilizing X-ray irradiation experiments, the results of the X-ray event extraction with the onboard and offline processing methods were consistent, validating the functionality of the proposed system.

astro-ph.IM↗

arXiv2024

Peak energy--Isotropic Luminosity Correlation and Jet Opening Angle Evolution in Swift-BAT Short GRBs with Soft Tail Emission

Some short gamma-ray bursts (SGRBs) exhibit a short duration and spectral hard emission (referred to as a "hard spike") followed by a slightly longer soft emission (known as a "soft tail"). We identified nine SGRBs with known redshift in the \textit{Swift}/BAT gamma-ray burst catalog by specifically searching for the soft tail. We found that spectra of these SGRBs can be described as a cutoff power-law model for both hard spike and soft tail, and both show time variation keeping the $E_{\rm peak}$--$L_{\rm iso}$ correlation. This suggests that the emission mechanism of both phenomena is identical. Furthermore, we found a trend of luminosity evolution as a function of redshift. This phenomenon suggests that these bursts originate from sources that have intrinsically bright and/or energy density concentrated within a narrower jet at higher redshift. We demonstrate that the average jet opening angle, derived from the jet break, can be explained by considering a model based on a strongly redshift-dependent jet opening angle.

astro-ph.HE↗

arXiv2024

High-energy extension of the gamma-ray band observable with an electron-tracking Compton camera

Although the MeV gamma-ray band is a promising energy-band window in astrophysics, the current situation of MeV gamma-ray astronomy significantly lags behind those of the other energy bands in angular resolution and sensitivity. An electron-tracking Compton camera (ETCC), a next-generation MeV detector, is expected to revolutionize the situation. An ETCC tracks each Compton-recoil electron with a gaseous electron tracker and determines the incoming direction of each gamma-ray photon; thus, it has a strong background rejection power and yields a better angular resolution than classical Compton cameras. Here, we study ETCC events in which the Compton-recoil electrons do not deposit all energies to the electron tracker but escape and hit the surrounding pixel scintillator array (PSA). We developed an analysis method for this untapped class of events and applied it to laboratory and simulation data. We found that the energy spectrum obtained from the simulation agreed with that of the actual data within a factor of 1.2. We then evaluated the detector performance using the simulation data. The angular resolution for the new-class events was found to be twice as good as in the previous study at the energy range 1.0--2.0~MeV, where both analyses overlap. We also found that the total effective area is dominated by the contribution of the double-hit events above an energy of 1.5~MeV. Notably, applying this new method extends the sensitive energy range with the ETCC from 0.2--2.1 MeV in the previous studies to up to 3.5~MeV. Adjusting the PSA dynamic range should improve the sensitivity in even higher energy gamma-rays. The development of this new analysis method would pave the way for future observations by ETCC to fill the MeV-band sensitivity gap in astronomy.

astro-ph.HE↗

arXiv2022

First observation of MeV gamma-ray universe with bijective imaging spectroscopy using the Electron-Tracking Compton Telescope aboard SMILE-2+

MeV gamma-rays provide a unique window for the direct measurement of line emissions from radioisotopes, but observations have made little significant progress after COMPTEL/{\it CGRO}. To observe celestial objects in this band, we are developing an electron-tracking Compton camera (ETCC), which realizes both bijective imaging spectroscopy and efficient background reduction gleaned from the recoil electron track information. The energy spectrum of the observation target can then be obtained by a simple ON-OFF method using a correctly defined point spread function on the celestial sphere. The performance of celestial object observations was validated on the second balloon SMILE-2+ installed with an ETCC having a gaseous electron tracker with a volume of 30$\times$30$\times$30 cm$^3$. Gamma-rays from the Crab nebula were detected with a significance of 4.0$σ$ in the energy range 0.15--2.1 MeV with a live time of 5.1 h, as expected before launching. Additionally, the light curve clarified an enhancement of gamma-ray events generated in the Galactic center region, indicating that a significant proportion of the final remaining events are cosmic gamma rays. Independently, the observed intensity and time variation were consistent with the pre-launch estimates except in the Galactic center region. The estimates were based on the total background of extragalactic diffuse, atmospheric, and instrumental gamma-rays after accounting for the variations in the atmospheric depth and rigidity during the level flight. The Crab results and light curve strongly support our understanding of both the detection sensitivity and the background in real observations. This work promises significant advances in MeV gamma-ray astronomy.

astro-ph.HE↗

arXiv2021

Catalog of Gamma-ray Glows during Four Winter Seasons in Japan

In 2015 the Gamma-Ray Observation of Winter Thunderstorms (GROWTH) collaboration launched a mapping observation campaign for high-energy atmospheric phenomena related to thunderstorms and lightning discharges. This campaign has developed a detection network of gamma rays with up to 10 radiation monitors installed in Kanazawa and Komatsu cities, Ishikawa Prefecture, Japan, where low-charge-center winter thunderstorms frequently occur. During four winter seasons from October 2016 to April 2020, in total 70 gamma-ray glows, minute-lasting bursts of gamma rays originating from thunderclouds, were detected. Their average duration is 58.9 sec. Among the detected events, 77% were observed in nighttime. The gamma-ray glows can be classified into temporally-symmetric, temporally-asymmetric, and lightning-terminated types based on their count-rate histories. An averaged energy spectrum of the gamma-ray glows is well fitted with a power-law function with an exponential cutoff, whose photon index, cutoff energy, and flux are $0.613\pm0.009$, $4.68\pm0.04$ MeV, and $(1.013\pm0.003)\times10^{-5}$ erg cm$^{-2}$ s$^{-1}$ (0.2-20.0MeV), respectively. The present paper provides the first catalog of gamma-ray glows and their statistical analysis detected during winter thunderstorms in the Kanazawa and Komatsu areas.

physics.ao-ph↗

arXiv2019

Exponentially Decaying Extended Emissions Following Short Gamma-Ray Bursts with Possible Luminosity -- E-folding Time Correlation

The origin of extended emissions following prompt emissions of short gamma-ray bursts (SGRBs) is in mystery. The long-term activity of the extended emission is responsible for promising electromagnetic counterparts to gravitational waves and, so that it may be a key to uncovering the progenitor of SGRBs. We investigate the early X-ray light curves of 26 SGRBs with known redshifts observed with the X-Ray Telescope aboard the {\it Neil Gehrels Swift Observatory} ({\it Swift}). We find that the exponential temporal decay model is able to describe the extended emissions comprehensively with a rest-frame e-folding time of 20 -- 200 seconds. We also estimate the isotropic equivalent energies of the extended emission with the exponential decay model and of the prompt emission, compared with those of the prompt emission. Then, it is revealed that the extended emission is 0 -- 3 orders of magnitude less powerful than the prompt emission. We find a strong correlation between the expected maximum luminosity and e-folding time which can be described by a power-law with an index of $-3.3$ and whose chance probability of $8.2\times10^{-6}$ if there is no observation bias of {\it Swift}. The exponential temporal decay may be interpreted to come from the spin-down time scale of the rotation energy of a highly magnetized neutron star, and/or fallback accretion onto a disk surrounding a black hole with an exponentially decaying magnetic flux by magnetic reconnection.

astro-ph.HE↗

arXiv2019

Systematic study for gas-to-dust ratio of short gamma-ray burst afterglows

Extra-galactic X-ray absorption and optical extinction are often found in gamma-ray burst (GRB) afterglows and they could be tracers of both circumburst and host galaxy environments. By performing spectral analyses for spectral energy distribution of 9 short GRB (SGRB) afterglows with known redshift, we investigated a ratio of the equivalent hydrogen column density to the dust extinction, N^{rest}_{H}/A^{rest}_{V}, in the rest frame of each SGRB. We found that the distribution of N^{rest}_{H}/A^{rest}_{V} is systematically smaller than the one for long GRBs, and is roughly consistent with the gas-to-dust ratio in the Milky Way. This result means that the measured gas-to-dust ratio of SGRBs would originate from the interstellar medium in each host galaxy. This scenario supports the prediction that SGRBs occur in non star-forming regions in the host galaxies.

astro-ph.HE↗

arXiv2017

Imaging Polarimeter for a Sub-MeV Gamma-Ray All-Sky Survey using an Electron-Tracking Compton Camera

X-ray and gamma-ray polarimetry is a promising tool to study the geometry and the magnetic configuration of various celestial objects, such as binary black holes or gamma-ray bursts (GRBs). However, statistically significant polarizations have been detected in few of the brightest objects. Even though future polarimeters using X-ray telescopes are expected to observe weak persistent sources, there are no effective approaches to survey transient and serendipitous sources with a wide field of view (FoV). Here we present an electron-tracking Compton camera (ETCC) as a highly-sensitive gamma-ray imaging polarimeter. The ETCC provides powerful background rejection and a high modulation factor over a FoV of up to 2$π$ sr thanks to its excellent imaging based on a well-defined point spread function. Importantly, we demonstrated for the first time the stability of the modulation factor under realistic conditions of off-axis incidence and huge backgrounds using the SPring-8 polarized X-ray beam. The measured modulation factor of the ETCC was 0.65 $\pm$ 0.01 at 150 keV for an off-axis incidence with an oblique angle of 30$^\circ$ and was not degraded compared to the 0.58 $\pm$ 0.02 at 130 keV for on-axis incidence. These measured results are consistent with the simulation results. Consequently, we found that the satellite-ETCC proposed in Tanimori et al. (2015) would provide all-sky surveys of weak persistent sources of 13 mCrab with 10% polarization for a 10$^{7}$ s exposure and over 20 GRBs down to a $6\times10^{-6}$ erg cm$^{-2}$ fluence and 10% polarization during a one-year observation.

astro-ph.IM↗

arXiv2017

Establishment of Imaging Spectroscopy of Nuclear Gamma-Rays based on Geometrical Optics

Since the discovery of nuclear gamma-rays, its imaging has been limited to pseudo imaging, such as Compton Camera (CC) and coded mask. Pseudo imaging does not keep physical information (intensity, or brightness in Optics) along a ray, and thus is capable of no more than qualitative imaging of bright objects. To attain quantitative imaging, cameras that realize geometrical optics is essential, which would be, for nuclear MeV gammas, only possible via complete reconstruction of the Compton process. Recently we have revealed that "Electron Tracking Compton Camera" (ETCC) provides a well-defined Point Spread Function (PSF). The information of an incoming gamma is kept along a ray with the PSF and that is equivalent to geometrical optics. Here we present an imaging-spectroscopic measurement with the ETCC. Our results highlight the intrinsic difficulty with CCs in performing accurate imaging, and show that the ETCC surmounts this problem. The imaging capability also helps the ETCC suppress the noise level dramatically by ~3 orders of magnitude without a shielding structure. Furthermore, full reconstruction of Compton process with the ETCC provides spectra free of Compton edges. These results mark the first proper imaging of nuclear gammas based on the genuine geometrical optics.

physics.ins-det↗

arXiv2016

Development of wide-field low-energy X-ray imaging detectors for HiZ-GUNDAM

We are planning a future gamma-ray burst (GRB) mission HiZ-GUNDAM to probe the early universe beyond the redshift of z > 7. Now we are developing a small prototype model of wide-field low-energy X-ray imaging detectors to observe high-z GRBs, which cover the energy range of 1 - 20 keV. In this paper, we report overview of its prototype system and performance, especially focusing on the characteristics and radiation tolerance of high gain analog ASIC specifically designed to read out small charge signals.

astro-ph.IM↗

arXiv2015

X-raying extended emission and rapid decay of short gamma-ray bursts

Extended emission is a mystery in short gamma-ray bursts (SGRBs). By making time resolved spectral analyses of brightest nine events observed by ${\it Swift}$ XRT, we obviously classify the early X-ray emission of SGRBs into two types. One is the extended emission with exponentially rapid decay, which shows significant spectral softening during hundreds seconds since the SGRB trigger and is also detected by ${\it Swift}$-BAT. The other is a dim afterglow only showing power-law decay over $10^4$ s. The correlations between the temporal decay and spectral indices of the extended emissions are inconsistent with the $α$-$β$ correlation expected for the high-latitude curvature emission from a uniform jet. The observed too-rapid decay suggests the emission from a photosphere or a patchy surface, and manifests the stopping central engine via such as magnetic reconnection at the black hole.

astro-ph.HE↗

arXiv2015

New readout and data-acquisition system in an electron-tracking Compton camera for MeV gamma-ray astronomy (SMILE-II)

For MeV gamma-ray astronomy, we have developed an electron-tracking Compton camera (ETCC) as a MeV gamma-ray telescope capable of rejecting the radiation background and attaining the high sensitivity of near 1 mCrab in space. Our ETCC comprises a gaseous time-projection chamber (TPC) with a micro pattern gas detector for tracking recoil electrons and a position-sensitive scintillation camera for detecting scattered gamma rays. After the success of a first balloon experiment in 2006 with a small ETCC (using a 10$\times$10$\times$15 cm$^3$ TPC) for measuring diffuse cosmic and atmospheric sub-MeV gamma rays (Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment I; SMILE-I), a (30 cm)$^{3}$ medium-sized ETCC was developed to measure MeV gamma-ray spectra from celestial sources, such as the Crab Nebula, with single-day balloon flights (SMILE-II). To achieve this goal, a 100-times-larger detection area compared with that of SMILE-I is required without changing the weight or power consumption of the detector system. In addition, the event rate is also expected to dramatically increase during observation. Here, we describe both the concept and the performance of the new data-acquisition system with this (30 cm)$^{3}$ ETCC to manage 100 times more data while satisfying the severe restrictions regarding the weight and power consumption imposed by a balloon-borne observation. In particular, to improve the detection efficiency of the fine tracks in the TPC from $\sim$10\% to $\sim$100\%, we introduce a new data-handling algorithm in the TPC. Therefore, for efficient management of such large amounts of data, we developed a data-acquisition system with parallel data flow.

astro-ph.IM↗

arXiv2015

A performance study of an electron-tracking Compton camera with a compact system for environmental gamma-ray observation

An electron-tracking Compton camera (ETCC) is a detector that can determine the arrival direction and energy of incident sub-MeV/MeV gamma-ray events on an event-by-event basis. It is a hybrid detector consisting of a gaseous time projection chamber (TPC), that is the Compton-scattering target and the tracker of recoil electrons, and a position-sensitive scintillation camera that absorbs of the scattered gamma rays, to measure gamma rays in the environment from contaminated soil. To measure of environmental gamma rays from soil contaminated with radioactive cesium (Cs), we developed a portable battery-powered ETCC system with a compact readout circuit and data-acquisition system for the SMILE-II experiment. We checked the gamma-ray imaging ability and ETCC performance in the laboratory by using several gamma-ray point sources. The performance test indicates that the field of view (FoV) of the detector is about 1$\;$sr and that the detection efficiency and angular resolution for 662$\;$keV gamma rays from the center of the FoV is $(9.31 \pm 0.95) \times 10^{^-5}$ and $5.9^{\circ} \pm 0.6^{\circ}$, respectively. Furthermore, the ETCC can detect 0.15$\;μ\rm{Sv/h}$ from a $^{137}$Cs gamma-ray source with a significance of 5$σ$ in 13 min in the laboratory. In this paper, we report the specifications of the ETCC and the results of the performance tests. Furthermore, we discuss its potential use for environmental gamma-ray measurements.

physics.ins-det↗