arXiv subjects

Hiroki Yoneda

Publications and source records attributed to Hiroki Yoneda.

33 records · Page 2Linked to original sources

arXiv2023

The origin of MeV gamma-ray diffuse emission from the inner Galactic region

The origin of the inner Galactic emission, measured by COMPTEL with a flux of $\sim ~ 10^{-2}$ MeV cm$^{-2}$ s$^{-1}$ sr$^{-1}$ in the 1-30 MeV range, has remained unsettled since its discovery in 1994. We investigate the origin of this emission by taking into account individual sources which are not resolved by COMPTEL and the Galactic diffuse emission. The source contribution is estimated for sources crossmatched between the Swift-BAT and Fermi-LAT catalogs by interpolating the energy spectra in the hard X-ray and GeV gamma-ray ranges, as well as unmatched sources. This results in a flux of $\sim$20% of the COMPTEL excess. The Galactic diffuse emission is calculated by GALPROP to reconcile the cosmic-ray and gamma-ray spectra with observations by AMS-02, Voyager, and Fermi-LAT, resulting in a flux of $\sim$30-80% of the COMPTEL emission. Thus, we show that the COMPTEL emission could be roughly reproduced by a combination of the sources and the Galactic diffuse emission. Furthermore, combined with the extragalactic emission, we construct all-sky images in the MeV gamma-ray range to pinpoint some potential interesting targets for future missions, which would be critical for bridging the MeV gap in the spectra of gamma-ray sources.

astro-ph.HE↗

arXiv2023

Unveiling properties of the non-thermal X-ray production in the gamma-ray binary LS 5039 using the long-term pattern of its fast X-ray variability

Gamma-ray binary systems, a subclass of high-mass X-ray binaries, show non-thermal emissions from radio to TeV. While efficient electron acceleration is considered to take place in them, the nature of the acceleration mechanism and the physical environments in these systems have been a long-standing question. In this work, we report on long-term recurrent patterns in the short-term variability of the soft X-ray emission of LS 5039, one of the brightest gamma-ray binary systems. The Neutron star Interior Composition Explorer (NICER) observed LS 5039 four times from 2018 to 2021. By comparing them with the previous Suzaku and NuSTAR long-exposure observations, we studied the long-term evolution of the orbital light curve in the soft X-ray band. Although the observations by NICER and Suzaku are separated by $\sim$14 years, i.e., more than 10^3 orbits, the orbital light curves show remarkable consistency after calculating their running averages with a window width 70 ks. Furthermore, all of the light curves show short-term variability with a time scale of $\sim$10 ks. Since the column density did not vary when the flux changed abruptly, such a short-term variability seems to be an intrinsic feature of the X-ray emission. We propose that the short-term variability is caused by clumps (or inhomogeneities) of the companion star wind impacting the X-ray production site. The observed time scale matches well with the lifetime of the clumps interacting with the pulsar wind and the dynamical time scale of the relativistic intrabinary shock in the pulsar wind scenario.

astro-ph.HE↗

arXiv2023

MeV Gamma-Ray Source Contribution to the Inner Galactic Diffuse Emission

The origin of the inner Galactic emission, measured by COMPTEL with a flux of $\sim 10^{-2} ~{\rm MeV~ cm}^{-2}~ {\rm s}^{-1}~ {\rm sr}^{-1}$ in the 1-30 MeV energy range from the inner Galactic region, has remained unsettled since its discovery. In this paper, we elaborate on a model of individual MeV gamma-ray sources unresolved by COMPTEL. This is conducted for sources crossmatched between the Swift-BAT and Fermi-LAT catalogs by interpolating the energy spectra in the hard X-ray and GeV gamma-ray ranges, as well as unmatched sources between the two catalogs. We find that the source contribution to the COMPTEL emission would be at least ~20%. Combined with the Galactic diffuse emission, which is not well constrained, the COMPTEL emission can be roughly reproduced in some cases.

astro-ph.HE↗

arXiv2022

Reconstruction of multiple Compton scattering events in MeV gamma-ray Compton telescopes towards GRAMS: the physics-based probabilistic model

Aimed at progress in mega-electron volt (MeV) gamma-ray astronomy, which has not yet been well-explored, Compton telescope missions with a variety of detector concepts have been proposed so far. One of the key techniques for these future missions is an event reconstruction algorithm that is able to determine the scattering orders of multiple Compton scattering events and to identify events in which gamma rays escape from the detectors before they deposit all of their energies. We revisit previous event reconstruction methods and propose a modified algorithm based on a probabilistic method. First, we present a general formalism of the probabilistic model of Compton scattering describing physical interactions inside the detector and measurement processes. Then, we also introduce several approximations in the calculation of the probability functions for efficient computation. For validation, the developed algorithm has been applied to simulation data of a Compton telescope using a liquid argon time projection chamber, which is a new type of Compton telescope proposed for the GRAMS project. We have confirmed that it works successfully for up to 8-hit events, including correction of incoming gamma-ray energies for escape events. The proposed algorithm can be used for next-generation MeV gamma-ray missions featured by large-volume detectors, e.g., GRAMS.

astro-ph.IM↗

arXiv2022

Event reconstruction of Compton telescopes using a multi-task neural network

We have developed a neural network model to perform event reconstruction of Compton telescopes. This model reconstructs events that consist of three or more interactions in a detector. It is essential for Compton telescopes to determine the time order of the gamma-ray interactions and whether the incident photon deposits all energy in a detector or it escapes from the detector. Our model simultaneously predicts these two essential factors using a multi-task neural network with three hidden layers of fully connected nodes. For verification, we have conducted numerical experiments using Monte Carlo simulation, assuming a large-area Compton telescope using liquid argon to measure gamma rays with energies up to $3.0\,\mathrm{MeV}$. The reconstruction model shows excellent performance of event reconstruction for multiple scattering events that consist of up to eight hits. The accuracies of hit order prediction are around $60\%$ while those of escape flags are higher than $70\%$ for up to eight-hit events of $4π$ isotropic photons. Compared with two other algorithms, a classical model and a physics-based probabilistic one, the present neural network method shows high performance in estimation accuracy particularly when the number of scattering is small, 3 or 4. Since simulation data easily optimize the network model, the model can be flexibly applied to a wide variety of Compton telescopes.

astro-ph.IM↗

arXiv2021

Broad-Band High-Energy Emission of the Gamma-ray Binary System LS 5039: Spectral and Temporal Features using NuSTAR and Fermi Observations

We report on detailed analysis of the hard X-ray and GeV gamma-ray spectra of LS 5039, one of the brightest gamma-ray binary system in the Galaxy. The NuSTAR observation covering its entire orbit in 2016 allowed us for the first time to study the orbital variability of the spectrum above 10 keV. The hard X-ray spectrum is well described with a single power-law component up to 78 keV. The X-ray flux showed a slight deviation from those observed previously with Suzaku in 2007. The fast X-ray brightening observed with Suzaku, around the inferior conjunction, was not observed in this observation. We also analyzed 11 years of Fermi Large Area Telescope data of LS 5039. The GeV spectrum around the inferior conjunction was well described with two non-thermal components; a power law with a photon index of $\sim 3$ and a cut-off power law with a cutoff energy of $\sim 2$ GeV. The orbital flux variability also changed gradually around a few GeV. These results indicate that there are two emission components in the GeV band, and the dominant component above $\sim 1$ GeV does not depend on the orbital phase. By combining these results, we update the spectral energy distribution of LS 5039 with the highest available statistics. Theoretical models proposed so far cannot explain the obtained multi-wavelength spectrum, especially the emission from $\sim$ 1 MeV to $\sim$ 400 MeV, and we discuss a possibility that particle acceleration in LS 5039 is different from the shock acceleration.

astro-ph.HE↗

arXiv2021

Tomographic Imaging by a Si/CdTe Compton Camera for In-111 and I-131 Radionuclides

Tomographic imaging with radionuclides commonly used in nuclear medicine, such as $^{111}$In (171 and 245 keV) and $^{131}$I (364 keV), is in high demand for medical applications and small animal imaging. The Si/CdTe Compton camera with its high angular and high energy resolutions is an especially promising detector to extend the energy coverage for imaging to the range that covers gamma-ray emitted from these radionuclides. Here, we take the first steps towards short-distance imaging by conducting experiments using three-dimensional phantoms composed of multiple sphere-like solutions of $^{111}$In and $^{131}$I with a diameter of 2.7 mm, placed at a distance of 41 mm. Using simple back-projection methods, the positions of the sources are reproduced with a spatial resolution of 11.5 mm and 9.0 mm (FWHM) for $^{111}$In and $^{131}$I, respectively. We found that a LM-MLEM method gives a better resolution of 4.0 mm and 2.7 mm (FWHM). We resolve source positions of a tetrahedron structure with a source-to-source separation of 28 mm. These findings demonstrate that Compton Cameras have the potential of close-distance imaging of radioisotopes distributions in the energy range below 400 keV.

physics.ins-det↗

arXiv2021

An application of a Si/CdTe Compton camera for the polarization measurement of hard x-rays from highly charged heavy ions

The methods to measure the polarization of the x-rays from highly charged heavy ions with a significantly higher accuracy than the existing technology is needed to explore relativistic and quantum electrodynamics (QED) effects including the Breit interaction. We developed the Electron Beam Ion Trap Compton Camera (EBIT-CC), a new Compton polarimeter with pixelated multi-layer silicon and cadmium telluride counters. The EBIT-CC detects the three-dimensional position of Compton scattering and photoelectric absorption, and thus the degree of polarization of incoming x-rays can be evaluated. We attached the EBIT-CC on the Tokyo Electron Beam Ion Trap (Tokyo-EBIT) in the University of Electro-Communications. An experiment was performed to evaluate its polarimetric capability through an observation of radiative recombination x-rays emitted from highly charged krypton ions, which were generated by the Tokyo-EBIT. The Compton camera of the EBIT-CC was calibrated for the 75 keV x-rays. We developed event reconstruction and selection procedures and applied them to every registered event. As a result, we successfully obtained the polarization degree with an absolute uncertainty of 0.02. This uncertainty is small enough to probe the difference between the zero-frequency approximation and full-frequency-dependent calculation for the Breit interaction, which is expected for dielectronic recombination x-rays of highly charged heavy ions.

physics.atom-ph↗

arXiv2021

Cross-match between the latest Swift-BAT and Fermi-LAT catalogs

We report the results of a cross-match study between the hard X-ray and GeV gamma-ray catalogs, by making use of the latest 105-month Swift-BAT and 10-yr Fermi-LAT catalogs, respectively. The spatial cross-matching between the two catalogs results in the matching of 132 point-like sources, including ~5% of false-match sources. Additionally, 24 sources that have been identified as the same identifications are matched. Among the 75 extended sources in the Fermi-LAT catalog, 31 sources have spatial coincidences with at least one Swift-BAT source inside their extent. All the matched sources consist of blazars (>60%), pulsars and pulsar wind nebulae (~13%), radio galaxies (~7%), binaries (~5%), and others. Compared to the original catalogs, the matched sources are characterized by a double-peaked photon index distribution, higher flux, and larger gamma-ray variability index. This difference arises from the different populations of sources, particularly the large proportion of blazars (i.e., FSRQ and BL Lac). We also report 13 cross-matched and unidentified sources. The matched sources in this study would be promising in the intermediate energy band between the hard X-ray and GeV gamma-ray observations, that is the unexplored MeV gamma-ray domain.

astro-ph.HE↗

arXiv2021

A NuSTAR confirmation of the 36 ks hard X-ray pulse-phase modulation in the magnetar 1E 1547.0$-$5408

This paper describes an analysis of the NuSTAR data of the fastest-rotating magnetar 1E 1547$-$5408, acquired in 2016 April for a time lapse of 151 ks. The source was detected with a 1-60 keV flux of $1.7 \times 10^{-11}$ ergs s$^{-1}$ cm$^{-2}$, and its pulsation at a period of $2.086710(5)$ sec. In 8-25 keV, the pulses were phase-modulated with a period of $T=36.0 \pm 2.3$ ks, and an amplitude of $\sim 0.2$ sec. This reconfirms the Suzaku discovery of the same effect at $T=36.0 ^{+4.5}_{-2.5} $ ks, made in the 2009 outburst. These results strengthen the view derived from the Suzaku data, that this magnetar performs free precession as a result of its axial deformation by $\sim 0.6 \times 10^{-4}$, possibly caused by internal toroidal magnetic fields reaching $\sim 10^{16}$ G. Like in the Suzaku case, the modulation was not detected in energies below $\sim 8$ keV. Above 10 keV, the pulse-phase behaviour, including the 36 ks modulation parameters, exhibited complex energy dependences: at $\sim 22$ keV, the modulation amplitude increased to $\sim 0.5$ sec, and the modulation phase changed by $\sim 65^\circ$ over 10--27 keV, followed by a phase reversal. Although the pulse significance and pulsed fraction were originally very low in $>10$ keV, they both increased noticeably, when the arrival times of individual photons were corrected for these systematic pulse-phase variations. Possible origins of these complex phenomena are discussed, in terms of several physical processes that are specific to ultra-strong magnetic fields.

astro-ph.HE↗

arXiv2020

Snowmass 2021 Letter of Interest: The GRAMS Project: MeV Gamma-Ray Observations and Antimatter-Based Dark Matter Searches

The Gamma-Ray and AntiMatter Survey (GRAMS) project is a proposed next-generation balloon/satellite mission targeting both MeV gamma-ray observations and antimatter-based dark matter searches. A cost-effective, large-scale Liquid Argon Time Projection Chamber (LArTPC) detector technology will allow GRAMS to have a significantly improved sensitivity to MeV gamma rays while extensively probing dark matter parameter space via antimatter measurements.

astro-ph.HE↗

arXiv2020

Sign of hard X-ray pulsation from the gamma-ray binary system LS 5039

To understand the nature of the brightest gamma-ray binary system LS 5039, hard X-ray data of the object, taken with the Suzaku and NuSTAR observatories in 2007 and 2016, respectively, were analyzed. The two data sets jointly gave tentative evidence for a hard X-ray periodicity, with a period of $\sim 9$ s and a period increase rate by $\sim 3 \times 10^{-10}$ s s$^{-1}$. Therefore, the compact object in LS 5039 is inferred to be a rotating neutron star, rather than a black hole. Furthermore, several lines of arguments suggest that this object has a magnetic field of several times $\sim 10^{10}$ T, two orders of magnitude higher than those of typical neutron stars. The object is hence suggested to be a magnetar, which would be the first to be found in a binary. The results also suggest that the highly efficient particle acceleration process, known to be operating in LS 5039, emerges through interactions between dense stellar winds from the massive primary star, and ultra-strong magnetic fields of the magnetar.

astro-ph.HE↗

arXiv2017

Search For Gravitational Redshifted Absorption Lines In LMXB Serpens X-1

The equation of state for ultra-dense matter can be tested from observations of the ratio of mass to radius of neutron stars. This could be measured precisely from the redshift of a narrow line produced on the surface. X-rays bursts have been intensively searched for such features, but so far without detection. Here instead we search for redshifted lines in the persistent emission, where the accretion flow dominates over the surface emission. We discuss the requirements for narrow lines to be produced, and show that narrow absorption lines from highly ionized iron can potentially be observable in accreting low mass X-ray binaries (low B field) which have either low spin or low inclination so that Doppler broadening is small. This selects Serpens X-1 as the only potential candidate persistent LMXB due to its low inclination. Including surface models in the broad band accretion flow model predicts that the absorption line from He-like iron at 6.7 keV should be redshifted to $\sim$ 5.1 - 5.7 keV (10 - 15 km for $1.4\mathrm{M_\odot}$) and have an equivalent width of 0.8 - 8.0 eV for surface temperatures of 7 - 10 $\times$ 10$^6$ K. We use the high resolution Chandra grating data to give a firm upper limit of 2 - 3 eV for an absorption line at $\sim 5$ keV. We discuss possible reasons for this lack of detection (the surface temperature and the geometry of the boundary layer etc.). Future instruments with better sensitivity are required in order to explore the existence of such features.

astro-ph.HE↗

arXiv2017

Development of Si-CMOS hybrid detectors towards electron tracking based Compton imaging in semiconductor detectors

Electron tracking based Compton imaging is a key technique to improve the sensitivity of Compton cameras by measuring the initial direction of recoiled electrons. To realize this technique in semiconductor Compton cameras, we propose a new detector concept, Si-CMOS hybrid detector. It is a Si detector bump-bonded to a CMOS readout integrated circuit to obtain electron trajectory images. To acquire the energy and the event timing, signals from N-side are also read out in this concept. By using an ASIC for the N-side readout, the timing resolution of few us is achieved. In this paper, we present the results of two prototypes with 20 um pitch pixels. The images of the recoiled electron trajectories are obtained with them successfully. The energy resolutions (FWHM) are 4.1 keV (CMOS) and 1.4 keV (N-side) at 59.5 keV. In addition, we confirmed that the initial direction of the electron is determined using the reconstruction algorithm based on the graph theory approach. These results show that Si-CMOS hybrid detectors can be used for electron tracking based Compton imaging.

astro-ph.IM↗

arXiv2016

Sensitivity of the Fe K-alpha Compton shoulder to the geometry and variability of the X-ray illumination of cosmic objects

In an X-ray reflection spectrum, a tail-like spectral feature generated via Compton down-scattering, known as a Compton shoulder (CS), appears at the low-energy side of the iron K$α$ line. Despite its great diagnostic potential, its use as a spectral probe of the reflector has been seriously limited due to observational difficulties and modelling complexities. We revisit the basic nature of the CS by systematic investigation into its dependence on spatial and temporal parameters. The calculations are performed by Monte-Carlo simulations for sphere and slab geometries. The dependence is obtained in a two-dimensional space of column density and metal abundance, demonstrating that the CS solves parameter degeneration between them which was seen in conventional spectral analysis using photoelectric absorption and fluorescence lines. Unlike the iron line, the CS does not suffer from any observational dependence on the spectral hardness. The CS profile is highly dependent on the inclination angle of the slab geometry unless the slab is Compton-thick, and the time evolution of the CS is shown to be useful to constrain temporal information on the source if the intrinsic radiation is variable. We also discuss how atomic binding of the scattering electrons in cold matter blurs the CS profile, finding that the effect is practically similar to thermal broadening in a plasma with a moderate temperature of $\sim$5 eV. Spectral diagnostics using the CS is demonstrated with grating data of X-ray binary GX 301$-$2, and will be available in future with high-resolution spectra of active galactic nuclei obtained by microcalorimeters.

astro-ph.HE↗