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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., Comment: 13 pages, 6 figures. Accepted for publication in PASJ

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 ${\rm \mu m}$), 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.

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.

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., Comment: 11 pages, 8 figures, Accepted for publication in NIM A

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., Comment: 34 pages, 29 figures, accepted for publication in Physical Review Research

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$. Gamma-rays from the Crab nebula were detected with a significance of 4.0$\sigma$ 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.

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., Comment: 22 pages, 6 figures, accepted for publication in ApJ

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., Comment: 15 pages, 4 figures

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$\pi$ 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., Comment: 22 pages, 19 figures, 1 table, accepted for publication in ApJ

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., Comment: 22 pages, 8 figures

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., Comment: 11 pages, 8 figures, SPIE Astronomical Telescopes + Instrumentation (2016)

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$\;\mu\rm{Sv/h}$ from a $^{137}$Cs gamma-ray source with a significance of 5$\sigma$ 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., Comment: 10 pages, 7 figures, proceeding of the 10th International Conference on Position Sensitive Detectors (PSD10)

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., Comment: 11 pages, 24 figures

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 $\alpha$-$\beta$ 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., Comment: 14 pages, 6 figures, 3 tables. Submitted to ApJ

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., Comment: 12 pages, 9 figures, SPIE Astronomical Telescopes + Instrumentation (2014)

Using 72 Short Gamma Ray Bursts (SGRBs) with well determined spectral data observed by BATSE, we determine their redshift and the luminosity by applying $E_p$--$L_p$ correlation for SGRBs found by \cite{tsutsui13}. For 53 SGRBs with the observed flux brighter than $4 \times 10^{-6}~{\rm erg~cm^{-2}s^{-1}}$, the cumulative redshift distribution up to $z=1$ agrees well with that of 22 {\it Swift}~SGRBs. This suggests that the redshift determination by the $E_p$--$L_p$ correlation for SGRBs works well. The minimum event rate at $z=0$ is estimated as $\rho_{SGRB}(0) = 6.3_{-3.9}^{+3.1} \times 10^{-10}~{\rm events~Mpc^{-3}yr^{-1}}$ so that the minimum beaming angle is $0.6^\circ-7.8^\circ$ assuming the merging rate of $10^{-7}-4\times 10^{-6}~{\rm events~Mpc^{-3}yr^{-1}}$ suggested from the binary pulsar data. Interestingly, this angle is consistent with that for SGRB130603B of $\sim 4^\circ-8^\circ$\citep{fong13b}. On the other hand, if we assume the beaming angle of $\sim 6^\circ$ suggested from four SGRBs with the observed value of beaming angle, the minimum event rate including off-axis SGRBs is estimated as $\rho_{SGRB,all}^{min}(0)=1.15_{-0.71}^{+0.57}\times 10^{-7}~{\rm events~Mpc^{-3}yr^{-1}}$. If SGRBs are induced by coalescence of binary neutron stars (NSs) and/or black holes (BHs), this event rate leads to the minimum gravitational-wave detection rate of $\rm 3.9_{-2.4}^{+1.9} (152_{-94}^{+75})~events~y^{-1}$ for NS-NS (NS-BH) binary, respectively, by a worldwide network with KAGRA, advanced-LIGO, advanced-Virgo, and GEO., Comment: 8 pages, 5 figures, accepted to ApJ

We have developed a prototype time-resolved neutron imaging detector employing a micro-pattern gaseous detector known as the micro-pixel chamber ({\mu}PIC) coupled with a field-programmable-gate-array-based data acquisition system. Our detector system combines 100{\mu}m-level spatial and sub-{\mu}s time resolutions with a low gamma sensitivity of less than 10^-12 and high data rates, making it well suited for applications in neutron radiography at high-intensity, pulsed neutron sources. In the present paper, we introduce the detector system and present several test measurements performed at NOBORU (BL10), J-PARC to demonstrate the capabilities of our prototype. We also discuss future improvements to the spatial resolution and rate performance., Comment: 5 pages, 7 figures, submitted to Hamon - Journal of the Japan Society for Neutron Science

We present a detailed study of the spatial resolution of our time-resolved neutron imaging detector utilizing a new neutron position reconstruction method that improves both spatial resolution and event reconstruction efficiency. Our prototype detector system, employing a micro-pattern gaseous detector known as the micro-pixel chamber ({\mu}PIC) coupled with a field-programmable-gate-array-based data acquisition system, combines 100{\mu}m-level spatial and sub-{\mu}s time resolutions with excellent gamma rejection and high data rates, making it well suited for applications in neutron radiography at high-intensity, pulsed neutron sources. From data taken at the Materials and Life Science Experimental Facility within the Japan Proton Accelerator Research Complex (J-PARC), the spatial resolution was found to be approximately Gaussian with a sigma of 103.48 +/- 0.77 {\mu}m (after correcting for beam divergence). This is a significant improvement over that achievable with our previous reconstruction method (334 +/- 13 {\mu}m), and compares well with conventional neutron imaging detectors and with other high-rate detectors currently under development. Further, a detector simulation indicates that a spatial resolution of less than 60 {\mu}m may be possible with optimization of the gas characteristics and {\mu}PIC structure. We also present an example of imaging combined with neutron resonance absorption spectroscopy., Comment: 8 pages, 8 figures, submitted to Nuclear Instruments and Methods in Physics Research A

NEWAGE is a direction-sensitive dark matter search experiment with a gaseous time-projection chamber. We improved the direction-sensitive dark matter limits by our underground measurement. After the first underground run, we replaced the detector components with radio-pure materials. We also studied the possibilities of head-tail recognition of nuclear tracks in the surface laboratory. For the future large volume detector, we are developing a pixel ASIC named QPIX. In this paper, these recent R&D activities are described., Comment: Proceedings of the 3rd International conference on Directional Detection of Dark Matter (CYGNUS 2011), Aussois, France, 8-10 June 2011

A direction-sensitive dark matter search experiment at Kamioka underground laboratory with the NEWAGE-0.3a detector was performed. The NEWAGE- 0.3a detector is a gaseous micro-time-projection chamber filled with CF4 gas at 152 Torr. The fiducial volume and target mass are 20*25*31 cm3 and 0.0115 kg, respectively. With an exposure of 0.524 kgdays, improved spin-dependent weakly interacting massive particle (WIMP)-proton cross section limits by a direction-sensitive method were achieved including a new record of 5400 pb for 150 GeV/c2 WIMPs. We studied the remaining background and found that ambient gamma-rays contributed about one-fifth of the remaining background and radioactive contaminants inside the gas chamber contributed the rest., Comment: 21 pages, 8 figures, to appear in Physics Letters B