arXiv subjects

Tomonori Totani

Publications and source records attributed to Tomonori Totani.

At least 19 recordsLinked to original sources

arXiv2026

Electroweak Doublet Dark Matter for a Galactic Halo Gamma-Ray Excess

Weakly interacting massive particles provide a well-motivated framework for dark matter, naturally reproducing the observed relic abundance through thermal freeze-out. A recent claim of an indirect-detection signal from the Galactic halo, consistent with dark matter annihilation in the mass range 400--800 GeV, motivates a reexamination of minimal models that can account for such a signal while remaining consistent with existing constraints. In this paper, we analyze the simplest extensions of the Standard Model capable of explaining the signal. We show that electroweak doublet dark matter with Higgs-portal interactions provides a natural and economical explanation. The model predicts annihilation predominantly into gauge bosons with characteristic branching fractions and allows for inelastic dark matter with a mass splitting of order 100 keV, intriguingly consistent with a recent direct-detection anomaly. Possible enhancements of the present-day annihilation rate relative to the thermal value are also discussed, including a simple extension with a light scalar field that induces Sommerfeld enhancement.

hep-ph↗

arXiv2025

20 GeV halo-like excess of the Galactic diffuse emission and implications for dark matter annihilation

Fifteen years of the Fermi Large Area Telescope (LAT) data in the halo region of the Milky Way (MW) are analyzed to search for gamma rays from dark matter annihilation. Gamma-ray maps within the region of interest ($|l| \le 60^\circ$, $10^\circ \le |b| \le 60^\circ$) are modeled using point sources, the GALPROP models of cosmic-ray interactions, isotropic background, and templates of Loop I and the Fermi bubbles, and then the presence of a halo-like component is further examined. A statistically significant halo-like excess is found with a spectral peak around 20 GeV, while its flux is consistent with zero below 2 GeV and above 200 GeV. Examination of the fit residual maps indicates that a spherically symmetric halo component fits the map data well. The radial profile agrees with annihilation by the smooth NFW density profile, and may be slightly shallower than this, especially in the central region. Various systematic uncertainties are investigated, but the 20 GeV peak remains significant. In particular, the halo excess with a similar spectrum is detected even relative to the LAT standard background model, which contains non-template patches adjusted to match the observed map. The halo excess spectrum can be fitted by annihilation with a particle mass $m_χ\sim$ 0.5-0.8 TeV and cross section $\langle σ\upsilon \rangle \sim$ (5-8)$\times 10^{-25} \ \rm cm^3 \, s^{-1}$ for the $b\bar{b}$ channel. This cross section is larger than the upper limits from dwarf galaxies and the canonical thermal relic value, but considering various uncertainties, especially the density profile of the MW halo, the dark matter interpretation of the 20 GeV ``Fermi halo'' remains feasible. The prospects for verification through future observations are briefly discussed.

astro-ph.HE↗

arXiv2025

The diffuse extragalactic gamma-ray background radiation: star-forming galaxies are not the dominant component

Star-forming galaxies (SFGs) are considered to be an important component of the diffuse extragalactic gamma-ray background (EGB) radiation observed in 0.1 -- 820 GeV, but their quantitative contribution has not yet been precisely determined. In this study, we aim to provide the currently most reliable estimate of the contribution of SFGs based on careful calibration with gamma-ray luminosities of nearby galaxies and physical quantities (star formation rate, stellar mass, and size) of galaxies observed by high-redshift galaxy surveys. Our calculations are based on the latest database of particle collision cross-sections and energy spectra of secondary particles, and take into account not only hadronic but also leptonic processes with various radiation fields in a galaxy. We find that SFGs are not the dominant component of the unresolved EGB measured by Fermi; the largest contribution is around 50% -- 60% in the 1 -- 10 GeV region, and the contribution falls rapidly in lower and higher energy ranges. This result appears to contradict a previous study, which claimed that SFGs are the dominant component of the unresolved EGB, and the origin of the discrepancy is examined. In calculations of cosmic-ray production, propagation, and interaction in a galaxy, we try models developed by two independent groups and find that they have little impact on EGB.

astro-ph.HE↗

arXiv2025

Time-Frequency Correlation of Repeating Fast Radio Bursts: Correlated Aftershocks Tend to Exhibit Downward Frequency Drifts

The production mechanism of fast radio bursts (FRBs)--mysterious, bright, millisecond-duration radio flashes from cosmological distances--remains unknown. Understanding potential correlations between burst occurrence times and various burst properties may offer important clues about their origins. Among these properties, the spectral peak frequency of an individual burst (the frequency at which its emission is strongest) is particularly important because it may encode direct information about the physical conditions and environment at the emission site. Analyzing over 4,000 bursts from the three most active sources--FRB 20121102A, FRB 20201124A, and FRB 20220912A--we measure the two-point correlation function $ξ(Δt, Δν_{\:\rm peak}\:)$ in the two-dimensional space of time separation $Δt$ and peak frequency shift $Δν_{\:\rm peak}\:$ between burst pairs. We find a universal trend of asymmetry about $Δν_{\:\rm peak}\:$ at high statistical significance; $ξ(Δν_{\:\rm peak}\:)$ decreases as $Δν_{\:\rm peak}\:$ increases from negative to positive values in the region of short time separation ($Δt \lesssim0.3$ s), where physically correlated aftershock events produce a strong time correlation signal. This indicates that aftershocks tend to exhibit systematically lower peak frequencies than mainshocks, with this tendency becoming stronger at shorter $Δt$. We argue that the "sad trombone effect"--the downward frequency drift observed among sub-pulses within a single event--is not confined within a single event but manifests as a statistical nature that extends continuously to independent yet physically correlated aftershocks with time separations up to $Δt \sim 0.3$ s. This discovery provides new insights into underlying physical processes of repeater FRBs.

astro-ph.HE↗

arXiv2024

Similarity to earthquakes again: periodic radio pulses of the magnetar SGR 1935+2154 are accompanied by aftershocks like fast radio bursts

It was recently discovered that the time correlations of repeating fast radio bursts (FRBs) are similar to earthquake aftershocks. Motivated by the association between FRBs and magnetars, here we report correlation function analyses in the time-energy space for the 563 periodic radio pulses and the 579 X-ray short bursts from the magnetar SGR 1935+2154, which is known to have generated FRBs. Although radio pulses are concentrated near the fixed phase of the rotational cycle, we find that when multiple pulses occur within a single cycle, their correlation properties (aftershock production probability, aftershock rate decaying in power of time, and more) are similar to those of extragalactic FRBs and earthquakes. A possible interpretation is that the radio pulses are produced by rupture of the neutron star crust, and the first pulse within one cycle is triggered by external force periodically exerted on the crust. The periodic external force may be from the interaction of the magnetosphere with material ejected in an outburst. For X-ray bursts, we found no significant correlation signal, though correlation on the same time scale as radio pulses may be hidden due to the long event duration. The aftershock similarity between the periodic radio pulsation and FRBs is surprising, given that the two are energetically very different, and therefore the energy sources would be different. This suggests that the essence of FRB-like phenomena is starquakes, regardless of the energy source, and it is important to search for FRB-like bursts from neutron stars with various properties or environments.

astro-ph.HE↗

arXiv2023

Fast radio bursts trigger aftershocks resembling earthquakes, but not solar flares

The production mechanism of repeating fast radio bursts (FRBs) is still a mystery, and correlations between burst occurrence times and energies may provide important clues to elucidate it. While time correlation studies of FRBs have been mainly performed using wait time distributions, here we report the results of a correlation function analysis of repeating FRBs in the two-dimensional space of time and energy. We analyze nearly 7,000 bursts reported in the literature for the three most active sources of FRB 20121102A, 20201124A, and 20220912A, and find the following characteristics that are universal in the three sources. A clear power-law signal of the correlation function is seen, extending to the typical burst duration ($\sim$ 10 msec) toward shorter time intervals ($Δt)$. The correlation function indicates that every single burst has about a 10-60% chance of producing an aftershock at a rate decaying by a power-law as $\propto (Δt)^{-p}$ with $p =$ 1.5-2.5, like the Omori-Utsu law of earthquakes. The correlated aftershock rate is stable regardless of source activity changes, and there is no correlation between emitted energy and $Δt$. We demonstrate that all these properties are quantitatively common to earthquakes, but different from solar flares in many aspects, by applying the same analysis method for the data on these phenomena. These results suggest that repeater FRBs are a phenomenon in which energy stored in rigid neutron star crusts is released by seismic activity. This may provide a new opportunity for future studies to explore the physical properties of the neutron star crust.

astro-ph.HE↗

arXiv2023

Solid grains ejected from terrestrial exoplanets as a probe of the abundance of life in the Milky Way

Searching for extrasolar biosignatures is important to understand life on Earth and its origin. Astronomical observations of exoplanets may find such signatures, but it is difficult and may be impossible to claim unambiguous detection of life by remote sensing of exoplanet atmospheres. Here, another approach is considered: collecting grains ejected by asteroid impacts from exoplanets in the Milky Way and then traveling to the Solar System. The optimal grain size for this purpose is around 1 $μ$m, and though uncertainty is large, about $10^5$ such grains are expected to be accreting on Earth every year, which may contain biosignatures of life that existed on their home planets. These grains may be collected by detectors placed in space, or extracted from Antarctic ice or deep-sea sediments, depending on future technological developments.

astro-ph.EP↗

arXiv2022

On the use of CHIME to Detect Long-Duration Radio Transients from Neutron Star Mergers

Short gamma-ray burst (SGRB) GRB 170817A was found to be related to a binary neutron star (BNS) merger. It is uncertain whether all SGRBs are caused by BNS mergers, and also under what conditions a BNS merger can cause a SGRB. As BNS mergers can cause SGRBs, afterglow observations will also provide an alternative measurement of the BNS merger rate independent of gravitational-wave observations. In previous work by Feng et al. (2014), the feasibility of the detection of afterglows was considered using a variety of radio observatories and a simple flux threshold detection algorithm. Here, we consider a more sophisticated detection algorithm for SGRB afterglows, and provide an estimate of the trials factors for a realistic search to obtain an updated estimate of the possibility of observing afterglows with the Canadian Hydrogen Intensity Mapping Experiment (CHIME). We estimate 893 and 312 afterglows per year can be detected using a $3 σ$ confidence level threshold with two jet models, one with half opening angle uniformly distributed in $6^\circ$ to $30^\circ$ and the other uniformly distributed in $3^\circ$ to $8^\circ$ with the median $6^\circ$. We also find 88% and 98%, respectively, of the detectable afterglows for each jet opening distribution are off-axis, which are candidates for orphan afterglows. Our result predicts fewer detectable sources per year than the earlier analysis, but confirms the essential conclusion that using CHIME to search for afterglows will be effective at constraining the astrophysical merger rate.

astro-ph.HE↗

arXiv2021

Prospects of newly detecting nearby star-forming galaxies by the Cherenkov Telescope Array

Prospects of the Cherenkov Telescope Array (CTA) for the study of very high energy gamma-ray emission from nearby star-forming galaxies are investigated. In the previous work, we constructed a model to calculate luminosity and energy spectrum of pion-decay gamma-ray emission produced by cosmic-ray interaction with the interstellar medium (ISM), from four physical quantities of galaxies [star formation rate (SFR), gas mass, stellar mass, and effective radius]. The model is in good agreement with the observed GeV--TeV emission of several nearby galaxies. Applying this model to nearby galaxies that are not yet detected in TeV (mainly from the KINGFISH catalog), their hadronic gamma-ray luminosities and spectra are predicted. We identify galaxies of the highest chance of detection by CTA, including NGC 5236, M33, NGC 6946, and IC 342. Concerning gamma-ray spectra, NGC 1482 is particularly interesting because our model predicts that this galaxy is close to the calorimetric limit and its gamma-ray spectral index in GeV--TeV is close to that of cosmic-ray protons injected into ISM. Therefore this galaxy may be detectable by CTA even though its GeV flux is below the {\it Fermi} Large Area Telescope sensitivity limit. In the TeV regime, most galaxies are not in the calorimetric limit, and the predicted TeV flux is lower than that assuming a simple relation between the TeV luminosity and SFR of M82 and NGC 253, typically by a factor of 15. This means that a more sophisticated model beyond the calorimetric limit assumption is necessary to study TeV emission from star-forming galaxies.

astro-ph.HE↗

arXiv2021

Origin of Galactic Spurs: New Insight from Radio/X-ray All-sky Maps

In this study, we analyze giant Galactic spurs seen in both radio and X-ray all-sky maps to reveal their origins. We discuss two types of giant spurs: one is the brightest diffuse emission near the map's center, which is likely to be related to Fermi bubbles (NPSs/SPSs, north/south polar spurs, respectively), and the other is weaker spurs that coincide positionally with local spiral arms in our Galaxy (LAS, local arm spur). Our analysis finds that the X-ray emissions, not only from the NPS but from the SPS are closer to the Galactic center by ~5 deg compared with the corresponding radio emission. Furthermore, larger offsets of 10-20 deg are observed in the LASs; however, they are attributed to different physical origins. Moreover, the temperature of the X-ray emission is kT ~ 0.2 keV for the LAS, which is systematically lower than those of the NPS and SPS (kT ~ 0.3 keV) but consistent with the typical temperature of Galactic halo gas. We argue that the radio/X-ray offset and the slightly higher temperature of the NPS/SPS X-ray gas are due to the shock compression/heating of halo gas during a significant Galactic explosion in the past, whereas the enhanced X-ray emission from the LAS may be due to the weak condensation of halo gas in the arm potential or star formation activity without shock heating.

astro-ph.HE↗

arXiv2020

FRB 181112 as a Rapidly-Rotating Massive Neutron Star just after a Binary Neutron Star Merger?: Implications for Future Constraints on Neutron Star Equations of State

The light curve of the fast radio burst (FRB) 181112 is resolved into four successive pulses, and the time interval ($\sim0.8$ ms) between the first and third pulses coincides with that between the second and fourth pulses, which can be interpreted as a neutron star (NS) spinning at a period of about $0.8$ ms. Although this period is shorter than the most rapidly rotating pulsar currently known ($1.4$ ms), it is typical for a simulated massive NS formed immediately after the coalescence of binary neutron stars (BNS). Therefore, a BNS merger is a good candidate for the origin of this FRB if the periodicity is real. We discuss the future implications that can be obtained if such a periodicity is detected from FRBs simultaneously with gravitational waves (GW). The remnant spin period $P_{\rm rem}$ inferred from the FRB observation is unique information which is not readily obtained by current GW observations at the post-merger phase. If combined with the mass of the merger remnant $M_{\rm rem}$ inferred from GW data, it would set a new constraint on the equation of state of nuclear matter. Furthermore, the post-merger quantity $P_{\rm rem}/M_{\rm rem}$, or the tidal deformability of the merger remnant, is closely related to the binary tidal deformability parameter $Λ$ of NSs before they merge, and a joint FRB-GW observation will establish a new limit on $Λ$. Thus, if $Λ$ is also well measured by GW data, a comparison between these two will provide further insights into the nature of nuclear matter and BNS mergers.

astro-ph.HE↗

arXiv2020

Detectability of radio afterglows from binary neutron star mergers and implications for fast radio bursts

Binary neutron star (BNS) mergers are one of the proposed origins for both repeating and non-repeating fast radio bursts (FRBs), which associates FRBs with gravitational waves and short gamma-ray bursts (GRBs). In this work, we explore detectability of radio afterglows from BNS mergers and compare it to the observed radio limits on FRB afterglows. We calculate the afterglow flux powered by the two components: a relativistic jet and a slower isotropic ejecta, and quantify the detection probability as a function of the source redshift, observing time, and flux sensitivity. The model parameter distributions inferred from short GRB afterglows are adopted, and viewing angle distributions (uniform spherical, gravitational-wave, on-axis biased) are assumed to reflect different searching scenario. Assuming that FRBs are not strongly beamed, we make comparison to FRBs detected with reported radio limits and find the detection probabilities are 1--10% in general, and hence not a strong constraint on the BNS progenitor model considering the small sample number (<10). In particular for some nearby FRBs (e.g. 180916.J0158+65, 190608), we find a high chance of detection (>20% at 10$μ$Jy sensitivity) for the isotropic component that would peak around $\sim$1--10 years after the merger. Therefore a long term radio monitoring of persistent radio emission for these objects is important.

astro-ph.HE↗

arXiv2020

Emergence of life in an inflationary universe

Abiotic emergence of ordered information stored in the form of RNA is an important unresolved problem concerning the origin of life. A polymer longer than 40--100 nucleotides is necessary to expect a self-replicating activity, but the formation of such a long polymer having a correct nucleotide sequence by random reactions seems statistically unlikely. However, our universe, created by a single inflation event, likely includes more than $10^{100}$ Sun-like stars. If life can emerge at least once in such a large volume, it is not in contradiction with our observations of life on Earth, even if the expected number of abiogenesis events is negligibly small within the observable universe that contains only $10^{22}$ stars. Here, a quantitative relation is derived between the minimum RNA length $l_{\min}$ required to be the first biological polymer, and the universe size necessary to expect the formation of such a long and active RNA by randomly adding monomers. It is then shown that an active RNA can indeed be produced somewhere in an inflationary universe, giving a solution to the abiotic polymerization problem. On the other hand, $l_{\min}$ must be shorter than $\sim$20 nucleotides for the abiogenesis probability close to unity on a terrestrial planet, but a self-replicating activity is not expected for such a short RNA. Therefore, if extraterrestrial organisms of a different origin from those on Earth are discovered in the future, it would imply an unknown mechanism at work to polymerize nucleotides much faster than random statistical processes.

q-bio.PE↗

arXiv2019

The Galactic Halo Contribution to the Dispersion Measure of Extragalactic Fast Radio Bursts

A new model of the Milky Way (MW) halo component of the dispersion measure (DM) for extragalactic sources, such as fast radio bursts (FRBs), is presented in light of recent diffuse X-ray observations. In addition to the spherical component of isothermal gas ($kT\sim0.3$ keV) in hydrostatic equilibrium with the Galactic gravitational potential, our model includes a disk-like non-spherical hot gas component to reproduce the directional dependence of the observed X-ray emission measure (EM). The total gas mass ($1.2\times10^{11}\,M_{\odot}$) is dominated by the spherical component, and is consistent with the total baryon mass of the MW expected from the dark matter mass and the cosmic baryon-to-dark-matter ratio. Our model predicts a mean halo DM of $43\:\,{\rm pc\:cm^{-3}}$, with a full range of $30$-$245\:\,{\rm pc\:cm^{-3}}$ over the whole sky. The large scatter seen in the X-ray EM data implies a $\sim0.2$ dex (rms) fluctuation of the MW halo DM. We provide an analytic formula to estimate the MW halo DM of our model along any line of sight, which can be easily used to compute the total MW component of DM toward extragalactic sources, in combination with existing DM models of the warm ionized medium associated with the Galactic disk.

astro-ph.HE↗

arXiv2019

A Blind Search for Prompt Gamma-ray Counterparts of Fast Radio Bursts with Fermi-LAT Data

Fast Radio Bursts (FRBs) are a mysterious flash phenomenon detected in radio wavelengths with a duration of only a few milliseconds, and they may also have prompt gamma-ray flashes. Here we carry out a blind search for msec-duration gamma-ray flashes using the 7-year Fermi Large Area Telescope (Fermi-LAT) all-sky gamma-ray data. About 100 flash candidates are detected, but after removing those associated with bright steady point sources, we find no flash events at high Galactic latitude region (|b|>20 deg). Events at lower latitude regions are consistent with statistical flukes originating from the diffuse gamma-ray background. From these results, we place an upper limit on the GeV gamma-ray to radio flux ratio of FRBs as xi \equiv (nu L_nu)_gamma / (nu L_nu)_radio < 10^8, depending on the assumed FRB rate evolution. This limit is comparable with the largest value found for pulsars, though xi of pulsars is distributed in a wide range. We also compare this limit with the spectral energy distribution of the 2004 giant flare of the magnetar SGR 1806-20.

astro-ph.HE↗

arXiv2019

Nonthermal afterglow of the binary neutron star merger GW170817: a more natural modeling of electron energy distribution leads to a qualitatively different new solution

The observed nonthermal afterglow spectrum of the binary neutron star (BNS) merger GW170817 from radio to X-ray are consistent with synchrotron radiation by shock-accelerated electrons. However, previous afterglow modeling studies were based on a simplified assumption that the acceleration efficiency is extremely high, i.e. all electrons in the shock are accelerated as a nonthermal population. This affects the estimate of the minimum electron energy and hence $ν_m$, the peak frequency of the afterglow spectrum. Here we present Bayesian fitting to the observed data with a more natural electron energy distribution, in which the acceleration efficiency is a free parameter. Interestingly, the maximum likelihood solutions are found with radio flux below $ν_m$ in the early phase, in contrast to previous studies that found the radio frequency always above $ν_m$. Therefore the $ν_m$ passage through the radio band could have been clearly detected for GW170817, if sufficient low-frequency radio data had been taken in early time. In the new solutions, the lowest energy of electrons is found close to equipartition with the post shock protons, but only a small fraction ($<$10\%) of electrons are accelerated as nonthermal particles. The jet energy and interstellar medium density are increased by 1--2 orders of magnitude from the conventional modeling, though these are still consistent with other constraints. We encourage to take densely sampled low-frequency radio data in the early phase for future BNS merger events, which would potentially detect $ν_m$ passage and give a strong constraint on electron energy distribution and particle acceleration efficiency.

astro-ph.HE↗

arXiv2018

Optical follow-up observation of Fast Radio Burst 151230

The origin of fast radio bursts (FRBs), bright millisecond radio transients, is still somewhat of a mystery. Several theoretical models expect that the FRB accompanies an optical afterglow (e.g., Totani 2013; Kashiyama et al. 2013). In order to investigate the origin of FRBs, we perform $gri$-band follow-up observations of FRB~151230 (estimated $z \lesssim 0.8$) with Subaru/Hyper Suprime-Cam at $8$, $11$, and $14$~days after discovery. The follow-up observation reaches a $50\%$ completeness magnitude of $26.5$~mag for point sources, which is the deepest optical follow-up of FRBs to date. We find $13$ counterpart candidates with variabilities during the observation. We investigate their properties with multicolor and multi-wavelength observations and archival catalogs. Two candidates are excluded by the non-detection of FRB~151230 in the other radio feed horns that operated simultaneously to the detection, as well as the inconsistency between the photometric redshift and that derived from the dispersion measure of FRB~151230. Eight further candidates are consistent with optical variability seen in AGNs. Two more candidates are well fitted with transient templates (Type IIn supernovae), and the final candidate is poorly fitted with all of our transient templates and is located off-center of an extended source. It can only be reproduced with rapid transients with a faint peak and rapid decline and the probability of chance coincidence is $\sim3.6\%$. We also find that none of our candidates are consistent with Type Ia supernovae, which rules out the association of Type Ia supernovae to FRB~151230 at $z\leq0.6$ and limits the dispersion measure of the host galaxy to $\lesssim300$~pc~cm$^{-3}$ in a Type Ia supernova scenario.

astro-ph.HE↗

arXiv2018

A search for optical transients associated with Fast Radio Burst 150418

We have searched for optical variability in the host galaxy of the radio variable source which is possibly associated with fast radio burst (FRB) 150418. We compare images of the galaxy taken 1 day after the burst using Subaru/Suprime-Cam with images taken $\sim$ 1 year after the burst using Gemini-South/GMOS. No optical variability is found between the two epochs with a limiting absolute magnitude $\gtrsim -19$ (AB). This limit applies to optical variability of the putative active galactic nucleus in the galaxy on a timescale of $\sim$ 1 year, and also to the luminosity of an optical counterpart of FRB~150418 one day after the burst should it have occurred in this galaxy.

astro-ph.HE↗