寺田 幸功

The radioactive decay of the freshly synthesized $r$-process nuclei ejected<br /> in compact binary mergers power optical/infrared macronovae (kilonovae) that<br /> follow these events. The light curves depend critically on the energy partition<br /> among the different products of the radioactive decay and this plays an<br /> important role in estimates of the amount of ejected $r$-process elements from<br /> a given observed signal. We study the energy partition and $\gamma$-ray<br /> emission of the radioactive decay. We show that $20$-$50\%$ of the total<br /> radioactive energy is released in $\gamma$-rays on timescales from hours to a<br /> month. The number of emitted $\gamma$-rays per unit energy interval has roughly<br /> a flat spectrum between a few dozen keV and $1$ MeV so that most of this energy<br /> is carried by $\sim 1$ MeV $\gamma$-rays. However at the peak of macronova<br /> emission the optical depth of the $\gamma$-rays is $\sim 0.02$ and most of the<br /> $\gamma$-rays escape. The loss of these $\gamma$-rays reduces the heat<br /> deposition into the ejecta and hence reduces the expected macronova signals if<br /> those are lanthanides dominated. This implies that the ejected mass is larger<br /> by a factor of $2$-$3$ than what was previously estimated. Spontaneous fission<br /> heats up the ejecta and the heating rate can increase if a sufficient amount of<br /> transuranic nuclei are synthesized. Direct measurements of these escaping<br /> $\gamma$-rays may provide the ultimate proof for the macronova mechanisms and<br /> an identification of the $r$-process nucleosynthesis sites. However, the<br /> chances to detect these signals are slim with current X-ray and $\gamma$-ray<br /> missions. New detectors, more sensitive by at least a factor of ten, are needed<br /> for a realistic detection rate.

While the X-ray, GeV gamma-ray, and TeV gamma-ray skies have been extensively studied, the MeV gamma-ray sky has not yet been well investigated since the Imaging Compton Telescope (COMPTEL) scanned the sky about two decades ago. In this paper, we investigate the prospects for active galactic nuclei population studies with future MeV gamma-ray missions using recent spectral models and luminosity functions of Seyferts and flat-spectrum radio quasars (FSRQs). Both of them are plausible candidates for the origins of the cosmic MeV gamma-ray background. If the cosmic MeV gamma-ray background radiation is dominated by non-thermal emission from Seyferts, a sensitivity of 10(-12) erg cm(-2) s(-1) is required to detect several hundred Seyferts in the entire sky. If FSRQs make up the cosmic MeV gamma-ray background, a sensitivity of similar to 4x10(-12) erg cm(-2) s(-1) is required to detect several hundred FSRQs following the recent FSRQ X-ray luminosity function. However, based on the latest FSRQ gamma-ray luminosity function, with which FSRQs can explain up to similar to 30% of the MeV background, we can expect several hundred FSRQs even with a sensitivity of 10(-11) erg cm(-2) s(-1), which is almost the same as the sensitivity goal of the next-generation MeV telescopes.

High energy emissions from supernovae (SNe), originated from newly formed<br /> radioactive species, provide direct evidence of nucleosynthesis at SN<br /> explosions. However, observational difficulties in the MeV range have so far<br /> allowed the signal detected only from the extremely nearby core-collapse SN<br /> 1987A. No solid detection has been reported for thermonuclear SNe Ia, despite<br /> the importance of the direct confirmation of the formation of 56Ni, which is<br /> believed to be a key ingredient in their nature as distance indicators. In this<br /> paper, we show that the new generation hard X-ray and soft gamma-ray<br /> instruments, on board Astro-H and NuStar, are capable of detecting the signal,<br /> at least at a pace of once in a few years, opening up this new window for<br /> studying SN explosion and nucleosynthesis.

A review of the Astro-H mission is presented here on behalf of the Astro-H collaboration. The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated by the Institute of Space and Astronautical Science (ISAS). One of the main uniquenesses of the ASTRO-H satellite is the high sensitivity and imaging capability of the wide energy band from 0.3 keV to 600 keV. The coverage is achieved by combining the four instruments of the SXS, SXI, HXI, and SGD. The other main uniqueness is a spectroscopic capability not only for a point-like source but also for an extended source with high spectral resolution of ΔE~4÷7eV of SXS. Using the unique powers of these instruments, ASTRO-H will address unresolved issues in high-energy astrophysics. © Czech Technical University in Prague, 2013.

We studied a new method to monitor a beam range in heavy-ion radiation therapy by measuring low energy photons emitted from a track of the ion beam. A 290 MeV/u carbon beam was injected into a cylindrical water phantom. A CdTe semiconductor detector with a lead slit having a width of 2mm was placed at a side of the phantom. In order to measure the position dependence of the low energy photon count, the beam range was varied by changing the injection energy using a binary energy degrader placed about 100 cm upstream of the beam focal point. The measured photon count decreased when the detector got closer to the end point of the beam range and the derivative of the photon count clearly changed in front of the range position. This was explained by our theoretical study assuming the photons were secondary electron bremsstrahlung. These results indicate that this new method could estimate the range position from the observation of bremsstrahlung with an accuracy of a few mm. © 2013 IEEE.

We report the design and the performance of the engineering model of the digital signal processing system called the Pulse Shape Processor (PSP) for the Soft X-ray Spectrometer (SXS) onboard the ASTRO-H satellite. The SXS employs an X-ray microcalorimeter system, in which X-ray photons are detected as a heat pulse due to photoelectric absorption. The pixelized HgTe absorbers are cooled down to 50 mK. The required energy resolution is 7 eV (FWHM) at 6 keV. Since the data link to the satellite data recorder is limited to 200 kbit s -1, the onboard digital processor PSP plays a critical role in achieving the required resolution. The PSP is also the rate-limiting factor for other performance of the SXS, such as maximum count rate and energy range. In this paper, we show the design of the PSP, and show the performance based on a series of laboratory tests performed with the engineering models of the detector and the analog readout electronics. We found that (1) the PSP can register energy in the 0.07-18 keV band [energy range], (2) the energy resolution of the engineering model system, including the detector, analog electronics, and the PSP, is 4.8-5.7 eV at 5.9 keV [energy resolution], and (3) the PSP has sufficient processing power to handle a point-like source fainter than 0.3 Crab [maximum count rate]. These results are expected to be quite similar to those with the flight model, thus the results will be useful for the observation planning using the SXS. © 2012 IEEE.

The Pulse Shape Processor is a digital signal processing electronics for the microcalorimeter instrument onboard ASTRO-H. Receiving digitized waveform (14 bit, 12.5 kHz sample) from 2×18 channels, two identical units of PSP-A and -B trigger X-ray events, assign five kinds of event grade, and perform optimal filtering to measure energy deposit on the 6 × 6 microcalorimeter pixels. One unit of PSP is composed of one FPGA board and two CPU boards. This paper describes the event processing algorithm to fulfill requirements for the signal processing, and task sharing between FPGA and CPU. © Springer Science+Business Media, LLC 2012.