arXiv subjects

Antonio Capanema

Publications and source records attributed to Antonio Capanema.

9 recordsLinked to original sources

arXiv2026

Acoustic instability at shock-wave precursors

Magnetic field amplification is an integral part of the process of particle acceleration at non-relativistic shocks. It is necessary to reach the maximum energies required by observations, especially in supernova remnants, thought to be sources of the bulk of Galactic cosmic rays. Such amplification can be caused by the acoustic instability that develops when small density perturbations interact with the cosmic-ray pressure gradient in the upstream of a cosmic-ray-modified shock. The vorticity induced by the nonlinear development of the instability may lead to turbulence, which amplifies the pre-existing magnetic fields. To study this phenomenon, we use the PLUTO code to carry out 2D (and some 3D) magnetohydrodynamical simulations of the evolution of small density perturbations in the presence of an assigned cosmic-ray pressure gradient. Adopting more realistic values of Mach number and cosmic-ray acceleration efficiency than previously assumed in the literature, we show that the acoustic instability can transform small density perturbations into large nonlinear structures while the fluid crosses the precursor region of a cosmic-ray-modified shock. We study the power spectrum of turbulent magnetic fluctuations that may be important to scatter particles. We comment on the possible constructive interference between acoustic and non-resonant streaming instabilities. We discuss limitations of previous and current numerical investigations in accessing spatial scales where turbulence is expected to turn nonlinear, and outline perspectives for future investigations.

astro-ph.HE↗

arXiv2025

Expected flavor composition of supernova neutrinos

We revisit the flavor composition of neutrinos from core-collapse supernovae (SN), focusing on robust predictions that are insensitive to the poorly known dynamics of collective flavor conversion in the inner core. Assuming that the many different trajectories and microscopic histories of neutrinos lead to decoherence of the ensemble at the boundary between the region of collective effects and the Mikheyev-Smirnov-Wolfenstein (MSW) dominated layers, we show that standard matter effects alone strongly constrain the electron-flavor fraction at Earth. For normal mass ordering (NO) we obtain $f_{ν_e}^{\rm NO}\lesssim 0.5$ at all times and energies, while for inverted ordering (IO), we predict $f_{ν_e}^{\rm IO}\simeq 1/3$, i.e.\ near flavor equipartition. Shock-wave propagation through the high (H) MSW resonance drives the system toward equipartition also in NO. In this way our framework links simple assumptions about decoherence and standard matter effects to robust expectations for the flavor evolution inside core-collapse supernovae. This contribution summarizes the main results of arXiv:2403.14762.

hep-ph↗

arXiv2025

The Role of Acoustic Instability in Cosmic-Ray Self-Confinement

Over the past decades, there has been growing observational and theoretical evidence that cosmic-ray-induced instabilities play an important role in both acceleration and transport of cosmic rays (CRs). For instance, the efficient acceleration of charged particles at supernova remnant shocks requires rapidly growing instabilities, so much so that none of the proposed processes seem sufficient to warrant acceleration to PeV energies. In this work, we investigate whether an acoustic instability triggered by the presence of a CR pressure gradient can lead to significant self-confinement of charged particles in the vicinity of shocks. We validate the expected growth rates and obtain the scale and energy of magnetic field perturbations induced by such system using magnetohydrodynamical simulations. Our results suggest a strong suppression of the diffusion coefficient for particles with Larmor radius around a thousandth of the precursor scale length.

astro-ph.HE↗

arXiv2025

Flavor composition of supernova neutrinos

Predicting the flavor composition of neutrinos from supernovae is a challenging task, primarily due to the high neutrino densities at their core. In such an environment, neutrino self-interactions give rise to collective effects that have dramatic yet poorly understood consequences for their flavor evolution. In this paper, however, we show that standard matter effects in the outer layers of supernovae can significantly constrain the flavor composition of the neutrino flux. We assume that, since a large number of neutrinos undergo different evolutions within the core, their state upon entering the MSW-dominated region is affected by decoherence. This assumption simplifies the problem and suggests that the fraction of neutrinos with electron flavor reaching Earth, denoted as $f_{ν_e}$, is constrained to be less than $0.5$ for all energies throughout the emission phase in the case of normal mass ordering. In contrast, for inverted mass ordering, we anticipate neutrinos arriving in near flavor equipartition ($f_{ν_e} \approx 1/3$). These predictions, and consequently their underlying assumptions, could be tested by future observations and may provide valuable insights into the properties of neutrino fluxes emerging from supernovae.

hep-ph↗

arXiv2025

$γ$-Cascade V4: A Semi-Analytical Code for Modeling Cosmological Gamma-Ray Propagation

Since the universe is not transparent to gamma rays with energies above around one hundred GeV, it is necessary to account for the interaction of high-energy photons with intergalactic radiation fields in order to model gamma-ray propagation. Here, we present a public numerical software for the modeling of gamma-ray observables. This code computes the effects on gamma-ray spectra from the development of electromagnetic cascades and cosmological redshifting. The code introduced here is based on the original $γ$-Cascade, and builds on it by improving its performance at high redshifts, introducing new propagation modules, and adding many more extragalactic radiation field models, which enables the ability to estimate the uncertainties inherent to EBL modeling. We compare the results of this new code to existing Monte Carlo electromagnetic transport models, finding good agreement within EBL uncertainties.

astro-ph.HE↗

arXiv2023

Ultra-high energy neutrinos from high-redshift electromagnetic cascades

We study the impact of the muon pair production and double pair production processes induced by ultra-high energy photons on the cosmic microwave background. Although the muon pair production cross section is smaller than the electron pair production one, the associated energy loss length is comparable or shorter than the latter (followed by inverse Compton in the deep Klein-Nishina regime) at high-redshift, where the effect of the astrophysical radio background is expected to be negligible. By performing a simulation taking into account the details of $e/γ$ interactions at high energies, we show that a significant fraction of the electromagnetic energy injected at $E\gtrsim 10^{19}\,$eV at redshift $z\gtrsim 5$ is channeled into neutrinos. The double pair production plays a crucial role in enhancing the multiplicity of muon production in these electromagnetic cascades. The ultra-high energy neutrino spectrum, yet to be detected, can in principle harbour information on ultra-high energy sources in the young universe, either conventional or exotic ones, with weaker constraints from the diffuse gamma ray flux compared to their low redshift counterparts.

hep-ph↗

arXiv2021

Evaporating Primordial Black Holes in Gamma Ray and Neutrino Telescopes

A primordial black hole in the last stages of evaporation and located in the local neighborhood can produce a detectable signal in gamma ray and neutrino telescopes. We re-evaluate the expected gamma ray and neutrino fluxes from these transient point events and discuss the consequences for existing constraints. For gamma rays we improve the current bounds by a factor of few, while for neutrinos we obtain significantly different results than the existing literature. The capability and advantages of neutrino telescopes in the search for primordial black holes is discussed thoroughly. The correlations of gamma ray and neutrino energy and time profiles will be promoted as a powerful tool in identifying the primordial black holes, in case of detection.

hep-ph↗

arXiv2021

Where do IceCube neutrinos come from? Hints from the diffuse gamma-ray flux

Despite the spectacular discovery of an astrophysical neutrino flux by IceCube in 2013, its origin remains a mystery. Whatever its sources, we expect the neutrino flux to be accompanied by a comparable gamma-ray flux. These photons should be degraded in energy by electromagnetic cascades and contribute to the diffuse GeV-TeV flux precisely measured by the Fermi-LAT. Population studies have also permitted to identify the main classes of contributors to this flux, which at the same time have not been associated with major neutrino sources in cross-correlation studies. These considerations allow one to set constraints on the origin and spectrum of the IceCube flux, in particular its low-energy part. We find that, even accounting for known systematic errors, the Fermi-LAT data exclude to at least 95% C.L. any extragalactic transparent source class, irrespective of its redshift evolution, if the neutrino spectrum extends to the TeV scale or below. If the neutrino spectrum has an abrupt cutoff at $\sim10$ TeV, barely compatible with current observations, the tension can be reduced, but this way out requires a significant modification to the current understanding of the origin of the diffuse extragalactic gamma-ray flux at GeV energies. In contrast, these considerations do not apply if a sizable fraction of IceCube data originates within the Galactic halo (a scenario however typically in tension with other constraints) or from a yet unidentified class of "opaque" extragalactic emitters, which do not let the high-energy gamma rays get out.

hep-ph↗

arXiv2020

New constraints on the origin of medium-energy neutrinos observed by IceCube

The recent IceCube publication claims the observation of cosmic neutrinos with energies down to $\sim 10$ TeV, reinforcing the growing evidence that the neutrino flux in the 10-100 TeV range is unexpectedly large. Any conceivable source of these neutrinos must also produce a $γ$-ray flux which degrades in energy en route to the Earth and contributes to the extragalactic $γ$-ray background measured by the Fermi satellite. In a quantitative multimessenger analysis, featuring minimalistic assumptions, we find a $\geq 3σ$ tension in the data, reaching $\sim 5σ$ for cosmic neutrinos extended down to $\sim 1$ TeV, interpreted as evidence for a population of hidden cosmic-ray accelerators.

hep-ph↗