arXiv subjects

Tomohiko Oka

Publications and source records attributed to Tomohiko Oka.

9 recordsLinked to original sources

arXiv2026

Cosmological $\gamma$-$\gamma$ Pair-Production Background

The origin of positrons is one of the unsolved puzzles in astrophysics as the majority of sources are still unidentified. The Cosmic Photon Background (CPB) is the isotropic radiation spanning the entire electromagnetic spectrum. Interactions of the CPB with itself may pose a promising source of positrons and secondary emission. We calculate the electron-positron pair production rate from the $\gamma$-$\gamma$ pair-production of the CPB with itself for redshifts $z \leq 10$, and determine the annihilation spectrum, Inverse Compton emission, and bremsstrahlung. The CPB is decomposed into a sum of gray body functions, of which each is being evolved according to source type luminosity functions and redshift. We compute the pair-production rate by integrating the angle- and energy-dependent cross section over the evolving CPB. The pairs produced at each redshift are then propagated towards $z=0$, taking into account a cosmological, intergalactic, energy loss function. The photon emission is calculated per redshift and then line-of-sight integrated towards a contribution of the Cosmic Gamma-Ray Background (CGB) today. The resulting pair-production emissivity increases steeply from $z=0$ of about $2 \times 10^{-36}$ to a peak of $1.8 \times 10^{-31}\,\mathrm{e^\pm\,cm^{-3}\,s^{-1}}$ at $z=2.7$, then declines again. This yields a total cosmic pair-production rate on the order of $10^{54}\,\mathrm{e^\pm\,s^{-1}}$ up to redshift $10$. The secondary emission of pairs experiencing Inverse Compton scattering off the CPB results in a sizable contribution to the CGB. The pairs from cosmological $\gamma$-$\gamma$ absorption provide a minimum level of secondary emission which needs to be taken into account for any CGB study. Especially in the range from 1 MeV to 1 GeV, this background can make up 20% of the total CGB emission and may substantially reduce the gap between MeV observations and models.

astro-ph.CO↗

arXiv2026

Stellar flares cannot explain the Galactic 511 keV emission

The origin of the 511 keV line signal in the Milky Way remains unresolved despite decades of observations. The measured flux of $\sim 3 \times 10^{-3}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$ suggests a steady-state positron injection rate of $\sim 10^{43}$-$10^{44}\,\mathrm{e^+\,s^{-1}}$. One proposed contributor to this signal is stellar flaring activity since high energy Solar flares are known to produce positrons and associated annihilation radiation. We estimate the quasi-persistent 511 keV luminosity expected from flaring stellar populations and estimate the Galactic contribution. We constrain the 511 keV fluxes for different sources in the Galaxy, and in particular globular clusters. Using Solar flare observations as a calibration baseline, we construct a hierarchical Bayesian model to link flare energy to 511 keV luminosity. We further use flare frequency-energy distributions to estimate the time-averaged positron output of stellar populations. The resulting predictions are compared with INTEGRAL/SPI observations using spatial and population-based models. We find that stellar flares fall short by several orders of magnitude in explaining the Galactic positron annihilation rate. Reproducing $\sim10\%$ of the observed luminosity in the Galactic bulge would require unphysically large maximum flare energies per star reaching up to $E_\mathrm{{max}} \gtrsim 10^{37-39}\,\mathrm{erg}$. Spatial modeling further shows that stellar flare scenarios cannot reproduce the observed 511 keV morphology. Stellar flares cannot be the dominant source of Galactic positrons. Although previous studies have shown that the measured 511 keV morphology is broadly consistent with old stellar populations, our results exclude normal stellar flaring activity as the underlying source for this emission.

astro-ph.GA↗

arXiv2026

Hadronic Scenario for Galactic PeVatron LHAASO J1912+1014u Supported by Fermi-LAT $\gamma$-ray Data and FUGIN CO Data

LHAASO has reported 43 sub-PeV $\gamma$-ray sources, which are promising candidates for cosmic-ray (CR) accelerators above the PeV energy, commonly called as PeVatrons. Multi-wavelength observations are crucial for identifying the underlying particle species and estimating the CR energy content of these sources. In this work we investigate the region around LHAASO J1912+1014u (and HESS J1912+101) using Fermi-LAT $\gamma$-ray data and FUGIN CO data. We analyzed 15 years of Fermi-LAT data in the 0.4--409.6 GeV energy range. By improving the standard Fermi-LAT diffuse emission model, we significantly reduced the large residuals around the source in the 1.6-12.8 GeV band. We detected a statistically significant excess above the diffuse background, which likely represents $\ge$10 GeV emission associated with the LHAASO/H.E.S.S. source. The GeV excess exhibits a hard spectrum (photon index of about 2.1) and is well reproduced by interstellar gas templates with systemic velocities of about 25 $\mathrm{km~s^{-1}}$ or 60 $\mathrm{km~s^{-1}}$. We performed a comprehensive fit to the GeV--TeV spectral energy distribution. Although a leptonic scenario can reproduce the observed spectrum, a hadronic scenario is favored once electron cooling is considered. The inferred CR proton spectrum has an index of $\sim$2.2, and the total CR proton energy above 1 GeV is (1--5) $\times 10^{49}~\mathrm{erg}$, depending on the assumed velocity range of the associated interstellar gas. A stringent upper limit on diffuse X-ray emission further supports the proton PeVatron scenario.

astro-ph.HE↗

arXiv2025

Resolving the origin of the unidentified TeV source HESS J1626-490 as a relic of the ancient cosmic-ray factory SNR G335.2+0.1

While decades of observations in the TeV gamma-ray band have revealed more than 200~sources with radio or X-ray counterparts, there remain dozens of unidentified TeV sources, which may provide crucial information of cosmic ray (CR) accelerators. HESS J1626$-$490 is an unidentified TeV gamma-ray source but is expected to originate from CRs that escaped from the nearby supernova remnant (SNR) G335.2+0.1 and are interacting with dense interstellar clouds. To test this scenario, we scrutinize the properties of the SNR and search for non-thermal counterparts by analyzing observational data in the radio, X-ray, and GeV gamma-ray bands. From analysis of the H\,{\sc i} and $^{12}$CO ($J{=}1{-}0$) line data, we identify the cloud associated with the SNR and compare the morphologies of the cloud and the gamma-ray emission. The distance and age of the SNR are estimated to be $3.3 \pm 0.6$~kpc and ${\sim}5$~kyr, respectively. From X-ray and GeV gamma-ray data analyses, we find an extended GeV gamma-ray emission overlapping with the SNR and H.E.S.S. source regions but no X-ray emission. The location of the peak of the extended GeV emission changes from near the SNR at $\lesssim 1$~GeV to the H.E.S.S. source at $>10$~GeV. We find a spectral hardening at ${\sim}50$~GeV, which is consistent with the existence of two components in the gamma-ray emission. We find that a combination of emission from the escaped CRs and the SNR itself can reproduce the observed broadband spectrum, on the assumption that the SNR has accelerated protons to ${\gtrsim}100$~TeV in the past.

astro-ph.HE↗

arXiv2025

Evidence for GeV Gamma-Ray Emission from Intense GRB 240529A During the Afterglow's Shallow Decay Phase

X-ray light curves of gamma-ray burst (GRB) afterglows exhibit various features, with the shallow decay phase being particularly puzzling. While some studies report absence of the X-ray shallow decay for hyper-energetic GRBs, recently discovered GRB 240529A shows a clear shallow decay phase with an isotropic gamma-ray energy of \SI{2.2e54}{erg}, making it a highly unusual case compared to typical GRBs. In order to investigate the physical mechanism of the shallow decay, we perform the \textit{Fermi}-LAT analysis of GRB 240529A along with \textit{Swift}-XRT analysis. We find no jet break feature in the X-ray light curve and then give the lower bound of the collimation-corrected jet energy of $>10^{52}$~erg, which is close to the maximum rotational energy of a magnetar. Our LAT data analysis reveals evidence of GeV emission with a statistical significance of $4.5σ$ during the shallow decay phase, which can be interpreted as the first case for hyper-energetic GRBs with a typical shallow decay phase. The GeV to keV flux ratio is calculated to be $4.2\pm2.3$. Together with X-ray spectral index, this indicates an inverse Compton origin of the GeV emission. Multiwavelength modeling based on time-dependent simulations tested two promising models, the energy injection and wind models. Both models can explain the X-ray and gamma-ray data, while our modeling demonstrates that gamma-ray observations, along with future GeV--TeV observations by CTAO, will distinguish between them.

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

Calibration and performance of the readout system based on switched capacitor arrays for the Large-Sized Telescope of the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is the next-generation ground-based very-high-energy gamma-ray observatory. The Large-Sized Telescope (LST) of CTA is designed to detect gamma rays between 20 GeV and a few TeV with a 23-meter diameter mirror. We have developed the focal plane camera of the first LST, which has 1855 photomultiplier tubes (PMTs) and the readout system which samples a PMT waveform at GHz with switched capacitor arrays, Domino Ring Sampler ver4 (DRS4). To measure the precise pulse charge and arrival time of Cherenkov signals, we developed a method to calibrate the output voltage of DRS4 and the sampling time interval, as well as an analysis method to correct the spike noise of DRS4. Since the first LST was inaugurated in 2018, we have performed the commissioning tests and calibrated the camera. We characterised the camera in terms of the charge pedestal under various conditions of the night sky background, the charge resolution of each pixel, the charge uniformity of the whole camera, and the time resolutions with a test pulse and calibration laser.

astro-ph.IM↗

arXiv2022

Detection of gamma rays around SNR HB9 and its implication to the diffusive shock-acceleration history

We analyze the GeV gamma-ray emission data from the vicinity of the supernova remnant (SNR) HB9 (G160.9+2.6) from the \textit{Fermi}-LAT 12-year observations to quantify the evolution of diffusive shock acceleration (DSA) in the SNR. In the vicinity of HB9, there are molecular clouds whose locations do not coincide with the SNR shell in the line of sight. We detect significant gamma-ray emissions above 1~GeV spatially coinciding with the two prominent cloud regions, as well as a emission from the SNR shell, the latter of which is consistent with the results of previous studies. The energy spectrum above 1~GeV in each region is fitted with a simple power-law function of $dN/dE \propto E^{-Γ}$. The fitting result indicates harder spectra with power-law indices of $Γ$ = $1.84 \pm 0.18$ and $1.84 \pm 0.14$ than that at the SNR shell with $Γ$= $2.55 \pm 0.10$. The observed spectra from the cloud regions are found to be consistent with the theoretically expected gamma-ray emissions originating in the protons that escaped from SNR HB9, where particles can be accelerated up to higher energies than those at the shell at present. The resultant diffusion coefficient in the vicinity of the SNR is comparable to that of the Galactic mean.

astro-ph.HE↗

arXiv2021

Latest results of ultra-high-energy cosmic ray measurements with prototypes of the Fluorescence detector Array of Single-pixel Telescopes (FAST)

The origin and nature of ultra-high-energy cosmic rays (UHECRs) remain an open question in astroparticle physics. Motivated by the need for an unprecedented aperture for further advancements, the Fluorescence detector Array of Single-pixel Telescopes (FAST) is a prospective next-generation, ground-based UHECR observatory that aims to cover a huge area by deploying a large array of low-cost fluorescence detectors. The full-scale FAST prototype consists of four 20 cm photomultiplier tubes at the focus of a segmented mirror 1.6 m in diameter. Over the last five years, three prototypes have been installed at the Telescope Array Experiment in Utah, USA, and one prototype at the Pierre Auger Observatory in Mendoza, Argentina, commencing remote observation of UHECRs in both hemispheres. We report on the latest results of these FAST prototypes, including telescope calibrations, atmospheric monitoring, ongoing electronics upgrades, development of sophisticated reconstruction methods, and UHECR detections.

astro-ph.IM↗