arXiv subjects

Shunsaku Horiuchi

Publications and source records attributed to Shunsaku Horiuchi.

At least 19 recordsLinked to original sources

arXiv2026

SN2025aico: An Interesting Case Of $^{56}$Ni Mixing, Ejecta Asymmetries, and Dust Formation in a Type IIb Supernova

Stripped-envelope supernovae provide a window into how massive stars lose their layers, mix radioactive material, produce dust, and explode asymmetrically. We present optical, near-infrared (NIR), and mid-infrared (MIR) observations of SN~2025aico, a Type~IIb supernova in LEDA~35384. Our spectroscopic sequence, spanning $+1$ to $+167$ d after explosion, follows its evolution from a photospheric phase exhibiting both hydrogen and helium features to prominent helium emission and, ultimately, nebular-phase ejecta. Using He I $1.083$ and $2.0581\,μ$m transitions, we investigated the kinematics and geometry of the helium-rich material. Both transitions exhibit a three-phase, non-monotonic velocity evolution: an initial rapid decline, a subsequent increase, and an eventual plateau. We interpret this behavior as evidence for limited outward mixing of $^{56}$Ni, such that radioactive energy deposition reaches the outer helium-rich ejecta progressively as the ejecta expand, producing the non-thermal electrons responsible for helium excitation. After $\sim100$ d, both NIR He I transitions develop double-peaked emission profiles. Similar structure in the oxygen emission indicates ejecta asymmetry. Comparison with other supernovae suggests a tentative connection between this structure and explosion energy, potentially linked to the delay between core collapse and explosion. Serendipitous JWST observations at $+124.4$d reveal an infrared excess. Modeling favors warm ($\sim800$--$1500$K) carbon dust newly formed in the ejecta, together with cooler carbon or silicate dust likely associated with pre-existing circumstellar material. SN~2025aico demonstrates how continuous optical-to-MIR observations can connect progenitor evolution, explosion physics, ejecta geometry, and dust production.

astro-ph.HE↗

arXiv2026

Survival of ultraheavy nuclei in astrophysical sources: applications to protomagnetar outflows

Outflows of rapidly rotating protomagnetars have been considered as attractive sites for the synthesis of nuclei heavier than iron, but the question remains whether these nuclei are able to survive against photodisintegration as they make their way out of their formation environments. In this work, we present new analytic fitting formulae for the photodisintegration cross sections applicable to heavy nuclei beyond iron. We confirm that the results from the TALYS simulations are consistent with the theory of the giant dipole resonance, and apply the obtained new formulae to investigate whether ultraheavy nuclei entrained in protomagnetar outflows can be disintegrated by thermal and nonthermal photons before leaving the stellar envelope. We explore two outflow models: a spherical wind model and a jetted outflow model. For nuclei accelerated to the bulk speed of these outflows, their survival depends on the model and engine properties. For spherical winds, nuclei may survive for the first $\sim100\,{\rm s}$ post-core collapse, but as the wind Lorentz factor increases, the photodisintegration optical depth sharply rises and nuclei may no longer survive. For the jetted outflows arising from progenitors surrounded with stellar envelopes, nuclei can only survive before the jet breakout time in cases where the central engine has high spin-down energy, that is, with a high magnetic field strength and shorter spin period. In progenitors with more extended envelopes, the jet break out time is much longer, allowing for nonthermal photons to readily photodisintegrate nuclei in high spin-down energy cases. These results also outline some of the necessary, but not yet sufficient, conditions to source ultrahigh-energy cosmic-ray nuclei.

astro-ph.HE↗

arXiv2026

Little Red Dots as Shock-Powered High-Energy Neutrino Sources

Little Red Dots (LRDs) are compact, high-redshift sources whose physical nature remains uncertain. Their optical spectra bear many similarities to Type IIn supernovae (SNe IIn), motivating a scenario in which their emission is powered by shocks interacting with dense surrounding material. We investigate whether such interactions can power LRDs and contribute to the diffuse high-energy neutrino intensity measured by IceCube. In our simplified model, a fast central-engine outflow drives a shock through dense surrounding material before stalling near the LRD photosphere, while some material continues to flow through the shock. We explore parameter ranges motivated by SNe IIn and the observed and inferred properties of LRDs, finding solutions with shock luminosities from 2.2e43 to 3.5e44 erg/s. Using an analytical framework for cosmic-ray acceleration and hadronic interactions in dense shock environments, we calculate the resulting high-energy neutrino emission and integrate it over the cosmological LRD population. For our fiducial SNe IIn-based cosmic-ray parameters, the average predicted contribution below 2e5 GeV increases from about 0.2% for the lowest-luminosity quintile to about 2% for the highest-luminosity quintile, while an illustrative higher-efficiency case reaches about 13% of the IceCube diffuse neutrino intensity. The incompleteness of the current LRD census and uncertainties in their physical nature limit constraints on the cosmic-ray parameters and the distribution of L_s. Larger LRD samples, together with improved physical models and a full population-synthesis study, will be required to constrain the total LRD contribution to the diffuse neutrino background.

astro-ph.HE↗

arXiv2026

High-Energy Neutrinos from Cosmic-Ray Scatterings with Supernova Neutrinos

Cosmic rays scattering with neutrinos produced in supernovae induce a flux of supernova neutrinos boosted to high energies. We calculate the neutrino flux arising from this new mechanism in environments with large cosmic-ray and supernova densities, such as some Active Galactic Nuclei. Under plausible astrophysical conditions, this flux may be detectable with high-energy neutrino telescopes, just considering the proton-neutrino scattering cross section expected in the Standard Model. Furthermore, the center of mass energy of such scatterings can reach $ \sqrt{s} \sim 10-100$ TeV, where the proton-neutrino cross section may be enhanced by new physics such as extra-dimensional theories. The boosted neutrino signal benefits from such an enhancement in the cross section not only at the detection point on Earth, but also at production in astrophysical sources, which allows us to set novel constraints on the ultra-high energy proton-neutrino cross section with neutrino telescopes.

hep-ph↗

arXiv2026

Constraints on Annihilating Dark Matter from Gamma-Ray Background-Galaxy Shape Correlations: Model-independent Null Results and Moderate Template-based Signals

We revisit the cross-correlation between the unresolved $γ$-ray background and galaxy shapes to constrain the annihilation cross section of particle dark matter. Our analysis uses $γ$-ray photons from 14 years of observations with the Fermi Large Area Telescope (LAT), together with galaxy shape catalogs from the Dark Energy Survey Year 3 (DES Y3) and the Dark Energy Camera All Data Everywhere (DECADE) project, enabling us to probe cosmological large-scale signals over a common sky area of $\sim 12{,}000\,\mathrm{deg}^2$ shared by the $γ$-ray and galaxy data sets. In order to better access signals from large-scale structure, we employ a Fourier-space estimator for the cross-correlation in contrast to the previous DES Y3 analysis. We find that our measurements are consistent with a null detection in a model-independent $χ^2$ test, while template-based analyses yield signals at the $\sim 3σ$ level. Our null results exclude an enhanced annihilation cross section for wino-like dark matter with a mass of $2-3$ TeV under a modest substructure boost factor of $\sim 30$ in Milky Way-sized halos. For larger boost factors of $\sim 100$, the constraints become significantly stronger and exclude the canonical thermal annihilation cross section $\langle σv \rangle = 3 \times 10^{-26}\,\mathrm{cm}^3/\mathrm{s}$ for a $7-40$ GeV dark matter particle annihilating into $b\bar{b}$ or $τ^{+}τ^{-}$. The template-based analysis favors a power-law $γ$-ray energy dependence of the cross-correlation, but also indicates deviations from that expected based on the mean intensity of the unresolved $γ$-ray background around 100 GeV. We further consider decaying dark matter scenarios and derive $2σ$ lower limits on the particle lifetime of $\sim 10^{26}-10^{27}\,\mathrm{s}$, depending on the decay channel.

astro-ph.CO↗

arXiv2026

The Cosmic Neutrino Background is within Reach of Future Neutrino Telescopes

The cosmic neutrino background (C$ν$B) can be boosted to high energies due to scatterings with energetic cosmic rays (CRs) across cosmological scales. Previous calculations focused on neutral current incoherent and coherent elastic scatterings of cosmic-ray protons off relic neutrinos. However, charged current interactions and deep inelastic scatterings are also expected to occur, which enhances the boosted relic neutrino fluxes on Earth. Here, we compute the \textit{total} diffuse boosted cosmic neutrino background (DBC$ν$B) arising from CRs at all redshifts in the Universe, accounting for neutral current and charged current elastic and deep inelastic scatterings. We find that IceCube already places an upper limit on the cosmic neutrino background overdensity in cosmological scales of ~$\mathcal{O}(100-1000)$ at $E_ν=10^{10}$ GeV, for a lightest neutrino mass of $m_ν \gtrsim 0.1$ eV. We further show that IceCube-Gen2 could test $\mathcal{O}(1-10)$ C$ν$B overdensities, and the combination of $10$ future neutrino telescopes with similar sensitivity would allow us to test the $Λ$CDM expected C$ν$B density for a lightest neutrino mass compatible with the KATRIN bound.

hep-ph↗

arXiv2026

Ultra high-energy cosmic rays from relativistic outflows in accretion induced collapse of white dwarfs

When a rapidly-rotating, highly magnetized white dwarf (WD) approaches the Chandrashekhar limit through mass accretion, it can undergo an accretion-induced collapse (AIC) to form a proto-neutron star or protomagnetar. The protomagnetar can drive a magnetically-dominated relativistic outflow, whose low entropy can lead to efficient formation of heavy nuclei. In this work, we propose that such relativistic outflows from AIC of WDs can contribute as sources of ultra high-energy cosmic rays (UHECRs). We model the acceleration of heavy nuclei in these relativistic outflows, and show that AICs can dominantly power the observed UHECRs, if a majority of them host relativistic outflows. Accounting for uncertainties in the acceleration mechanisms and AIC rates, AICs can contribute $\sim$ a few $10^{43} - 10^{45}\ {\rm erg \ Mpc}^{-3} {\rm yr}^{-1}$ in UHECR energy generation rate density, assuming iron-like nuclei.

astro-ph.HE↗

arXiv2026

Ultraheavy Ultrahigh-Energy Cosmic Rays

We investigate the propagation of ultraheavy (UH) nuclei as ultrahigh-energy cosmic rays (UHECRs). We show that their energy loss lengths at $\lesssim300$ EeV are significantly longer than those of protons and intermediate-mass nuclei, and that the highest-energy cosmic rays with energies beyond $\sim100$ EeV, including the Amaterasu particle, may be UH-UHECRs. For the first time, we derive constraints on the contribution of UH-UHECR sources, and find that the current data are consistent with energy generation rate densities of UHECRs from collapsars and neutron star mergers. Our model predicts that the mean value of the depth of shower maximum is lower than that for iron nuclei beyond 100 EeV, which can be tested with future composition measurements, e.g., AugerPrime and the Global Cosmic Ray Observatory. In addition, the spectral tension between the Telescope Array (TA) and the Pierre Auger Observatory can be alleviated by considering the enhanced contribution of UHECRs -- including UH nuclei -- from a nearby transient.

astro-ph.HE↗

arXiv2026

Novel method to trace the dark matter density profile around supermassive black holes with AGN reverberation mapping

We propose a new method to determine the dark matter density profile in the vicinity of distant supermassive black holes (SMBH) using reverberation mapping (RM) measurements of active galactic nuclei (AGN). The mapping of multiple emission lines allows the measurement of the enclosed mass within different radii from the central SMBH, which can be used to infer or constrain the dark matter density profile on sub-parsec scales. We apply a toy model based on this method to a sample of fourteen AGN to test its feasibility based on current measurements. We find that for five objects, the observed enclosed mass does grow with radii, hinting towards the presence of a dark matter component at the 1-2 $σ$ level. For these sources, we find global evidence for a universal dark matter profile with a preferred radial steepness of index $γ\sim 1.6$, consistent with the scenario expected for a dark matter spike mildly relaxed by stellar heating processes. The enclosed dark matter mass, however, is found to be significantly larger than expected. We show that the current RM based mass measurements suffer from large systematic uncertainties, that limit the effectiveness of our method. Our work emphasizes the importance of applying the recent developments in mass determination techniques to target multiple emission lines with future RM and interferometry campaigns. This provides the most direct way of constraining the dark matter density in the sub-parsec regions around extragalactic SMBHs, which is crucial to our understanding of the dynamics and nature of dark matter.

astro-ph.GA↗

arXiv2026

The diffuse supernova neutrino background: an update with modern population synthesis and core-collapse simulations

We present a new, state-of-the-art computation of the Diffuse Supernova Neutrino Background (DSNB), where we use neutrino spectra from multi-dimensional, multi-second core collapse supernova simulations - including both neutron-star and black-hole forming collapses - and binary evolution effects from modern population synthesis codes. Large sets of numerical results are processed and connected in a consistent manner, using two key quantities: the mass of the star's Carbon-Oxygen (CO) core at an advanced pre-collapse stage - which depends on binary evolution effects - and the compactness parameter, which is the main descriptor of the post-collapse neutrino emission. The method enables us to model the neutrino emission of a very diverse, binary-affected population of stars, which cannot unambiguously be mapped in detail by existing core collapse simulations. We find that including black hole-forming collapses enhances the DSNB by up to 50% at energies greater than 30-40 MeV. Binary evolution effects can change the total rate of collapses and generate a sub-population of high core mass stars that are stronger neutrino emitters. However, the net effect on the DSNB is moderate - up to a 15% increase in flux - due to the rarity of these super-massive cores and to the relatively modest dependence of the neutrino emission on the CO core mass. The methodology presented here is suitable for extensions and generalizations, and therefore it lays the foundation for modern treatments of the DSNB.

astro-ph.HE↗

arXiv2025

Mineral Detection of Neutrinos and Dark Matter 2025 Proceedings

The third ``Mineral Detection of Neutrinos and Dark Matter'' (MD$ν$DM'25) meeting was held May 20-23, 2025 in Yokohama, Japan, hosted by the Yokohama Institute for Earth Sciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC). These proceedings compile contributions from the workshop and update the progress of mineral detector research. MD$ν$DM'25 was the third such meeting, following the first in October of 2022 held at the IFPU in Trieste, Italy and the second in January of 2024 hosted by the Center for Neutrino Physics at Virginia Tech in Arlington, USA. Mineral detectors record and retain damage induced by nuclear recoils in synthetic or natural mineral samples. The damage features can then be read out by a variety of nano- and micro-scale imaging techniques. Applications of mineral detectors on timescales relevant for laboratory experiments include reactor neutrino monitoring and dark matter detection, with the potential to measure the directions as well as the energies of the induced nuclear recoils. For natural mineral detectors which record nuclear recoils over geological timescales, reading out even small mineral samples could be sensitive to rare interactions induced by astrophysical neutrinos, cosmic rays, dark matter and heavy exotic particles. A series of mineral detectors of different ages could measure the time evolution of these fluxes, offering a unique window into the history of our solar system and the Milky Way. Mineral detector research is highly multidisciplinary, incorporating aspects of high energy physics, condensed matter physics, materials science, geoscience, and AI/ML for data analysis. Although realizing the scientific potential of mineral detectors poses many challenges, the MD$ν$DM community looks forward to the continued development of mineral detector experiments and the possible discoveries that mineral detectors could reveal.

physics.ins-det↗

arXiv2025

Gamma-ray emission from decays of boosted nuclei in protomagnetar jets

We examine the detectability of $γ$-ray emission originating from the radioactive decays of unstable nuclei that are synthesized in relativistic outflows launched in magnetorotational core-collapse supernovae. The observed lines have enhanced energies due to the Lorentz boosted nuclei and can also be seen until later times due to time dilation of the rest-frame half-lives. We find that instruments like \textit{e-ASTROGAM} and \textit{INTEGRAL/SPI} are sensitive to these boosted line emissions from hundreds of keV to tens of MeV at a distance of 10 kpc over timescales of tens of days. For favorable viewing angles, these decays can be detected to extragalactic distances for rapidly spinning protomagnetar models. On the other hand, detection for off-axis jets is challenging, even for a supernova at the Galactic Center. Measuring multiple decay lines in addition to the integrated luminosity over $\sim10$ days postbounce would allow for the ability to distinguish between models and shed light on central engine properties like magnetic field and spin.

astro-ph.HE↗

arXiv2025

Low-Energy Supernovae Bounds on Sterile Neutrinos

Sterile neutrinos can be produced through mixing with active neutrinos in the hot, dense core of a core-collapse supernova (SN). The standard bounds on the active-sterile mixing ($\sin^2 θ$) from SN arise from SN1987A energy-loss, requiring $E_{\text{loss}}<10^{52}~{\rm erg}$. In this work, we discuss a novel bound on sterile neutrino parameter space arising from the energy deposition through its decays inside the SN envelope. Using the observed underluminous SN IIP population, this energy deposition is constrained to be below $\sim 10^{50}~{\rm erg}$. Focusing on sterile neutrino mixing only with tau neutrino, for heavy sterile masses $m_s$ in the range $100$-$500$ MeV, we find stringent constraints on $\sin^2 θ_τ$ reaching two orders of magnitude lower than those from the SN1987A energy loss argument, {thereby probing the mixing angles required for Type-I seesaw mechanism}. Similar bounds will also be applicable to sterile mixing only with muons ($\sin^2 θ_μ$).

hep-ph↗

arXiv2025

Diffuse Boosted Cosmic Neutrino Background

Energetic cosmic rays scatter off the cosmic neutrino background throughout the history of the Universe, yielding a diffuse flux of cosmic relic neutrinos boosted to high energies. We calculate this flux under different assumptions of the cosmic-ray flux spectral slope and redshift evolution. The non-observation of the diffuse flux of boosted relic neutrinos with current high-energy neutrino experiments already excludes an average cosmic neutrino background overdensity larger than $\sim 10^{4}$ over cosmological distances. We discuss the future detectability of the diffuse flux of boosted relic neutrinos in light of neutrino overdensity estimates and cosmogenic neutrino backgrounds.

hep-ph↗

arXiv2025

Three-dimensional core-collapse supernova models with phenomenological treatment of neutrino flavor conversions

We perform three-dimensional supernova simulations with a phenomenological treatment of neutrino flavor conversions. We show that the explosion energy can increase to as high as ~10^51 erg depending on the critical density for the onset of flavor conversions, due to a significant enhancement of the mean energy of electron antineutrinos. Our results confirm previous studies showing such energetic explosions, but for the first time in three-dimensional configurations. In addition, we predict neutrino and gravitational wave (GW) signals from a nearby supernova explosion aided by flavor conversions. We find that the neutrino event number decreases because of the reduced flux of heavy-lepton neutrinos. In order to detect GWs, next-generation GW telescopes such as Cosmic Explorer and Einstein Telescope are needed even if the supernova event is located at the Galactic center. These findings show that the neutrino flavor conversions can significantly change supernova dynamics and highlight the importance of further studies on the quantum kinetic equations to determine the conditions of the conversions and their asymptotic states.

astro-ph.HE↗

arXiv2024

The Red Supergiant Problem: As Seen from the Local Group's Red Supergiant Populations

The red supergiant (RSG) problem, which describes the apparent lack of high-luminosity progenitors detected in Type II supernova (SN) pre-images, has been a contentious topic for two decades. We re-assess this problem using a new RSG population of the Milky Way supplemented with RSGs from other galaxies in the Local Group. In particular, we quantify the uncertainties inherent to assumptions made regarding the star's temperature or spectral type and the corresponding bolometric correction. We find that only M3 or later RSGs reproduce the steepness seen from the SN II pre-imaged sample. To assess the significance of the RSG problem, we build a metallicity-weighted cumulative luminosity distribution of M3 or later RSGs and directly compare it to the luminosity distribution of SN II pre-imaged progenitors. We find no evidence of missing high-luminosity pre-imaged progenitors since the uncertainties on the pre-imaged SN progenitors and single-band derived luminosity are too large to meaningfully infer population differences.

astro-ph.SR↗

arXiv2024

Neutrino Diffusion within Dark Matter Spikes

Multi-messenger observations of astrophysical transients provide powerful probes of the underlying physics of the source as well as beyond the Standard Model effects. We explore transients that can occur in the vicinity of supermassive black holes at the center of galaxies, including tidal disruption events (TDEs), certain types of blazars, or even supernovae. In such environments, the dark matter (DM) density can be extremely high, resembling a dense spike or core. We study a novel effect of neutrino diffusion sustained via frequent scatterings off DM particles in these regions. We show that for transients occurring within DM spikes or cores, the DM-neutrino scattering can delay the arrival of neutrinos with respect to photons, but this also comes with a suppression of the neutrino flux and energy loss. We apply these effects to the specific example of TDEs, and demonstrate that currently unconstrained parameter space of DM-neutrino interactions can account for the sizable $O$(days) delay of the tentative high-energy neutrinos observed from some TDEs.

hep-ph↗

arXiv2024

Diffuse supernova neutrino background with up-to-date star formation rate measurements and long-term multidimensional supernova simulations

The sensitivity of current and future neutrino detectors like Super-Kamiokande (SK), JUNO, Hyper-Kamiokande (HK), and DUNE is expected to allow for the detection of the diffuse supernova neutrino background (DSNB). However, the DSNB model ingredients like the core-collapse supernova (CCSN) rate, neutrino emission spectra, and the fraction of failed supernovae are not precisely known. We quantify the uncertainty on each of these ingredients by (i) compiling a large database of recent star formation rate density measurements, (ii) combining neutrino emission from long-term axisymmetric CCSNe simulations and strategies for estimating the emission from the protoneutron star cooling phase, and (iii) assuming different models of failed supernovae. Finally, we calculate the fluxes and event rates at multiple experiments and perform a simplified statistical estimate of the time required to significantly detect the DSNB at SK with the gadolinium upgrade and JUNO. Our fiducial model predicts a flux of $5.1\pm0.4^{+0.0+0.5}_{-2.0-2.7}\,{\rm cm^2~s^{-1}}$ at SK employing Gd-tagging, or $3.6\pm0.3^{+0.0+0.8}_{-1.6-1.9}$ events per year, where the errors represent our uncertainty from star formation rate density measurements, uncertainty in neutrino emission, and uncertainty in the failed-supernova scenario. In this fiducial calculation, we could see a $3σ$ detection by $\sim2030$ with SK-Gd and a $5σ$ detection by $\sim2035$ with a joint SK-Gd/JUNO analysis, but background reduction remains crucial.

astro-ph.HE↗