arXiv subjects

Vitor de Souza

Publications and source records attributed to Vitor de Souza.

At least 19 recordsLinked to original sources

arXiv2026

Relativistic Magnetohydrodynamics from Myers-Pospelov Lorentz-Violating Electrodynamics

We derive the equations of relativistic magnetohydrodynamics from Lorentz-violating Myers-Pospelov electrodynamics. Starting from the modified fermionic and electromagnetic equations of motion, we employ the covariant Wigner formalism to establish the semiclassical spectral constraints and distribution function, which, combined with Noether's theorem, yield the energy-momentum tensor, particle current, and corresponding conservation laws in the presence of Lorentz violation. The resulting hydrodynamic description contains Lorentz-violating corrections to the thermodynamic quantities and to the fluid currents. In the purely timelike sector, these corrections can be absorbed into effective thermodynamic quantities, preserving the isotropic structure of the fluid equations. In contrast, spacelike backgrounds introduce explicit contributions along the preferred direction, which cannot be absorbed into the standard hydrodynamic variables and give rise to anisotropic forces and torques. These effects lead to departures from standard relativistic magnetohydrodynamics and provide a direct macroscopic manifestation of the Lorentz-violating background. Our results provide a first-principles derivation of Lorentz-violating relativistic magnetohydrodynamics and establish a framework for investigating the macroscopic consequences of Lorentz violation in magnetized relativistic fluids.

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arXiv2026

Probing Dark Photon Dark Matter with CTAO

The dark photon is a hypothetical gauge boson arising in extensions of the Standard Model, and has emerged as a compelling dark matter candidate. As dark photon dark matter (DPDM), it can interact with electromagnetic fields via kinetic mixing, and the inelastic scattering process $γA^{\prime} \to e^+ e^-$ becomes kinematically allowed for gamma rays above a characteristic energy threshold. This interaction imprints unique spectral attenuation features at very-high-energies (VHE), offering an observational probe of DPDM models. Using the Cherenkov Telescope Array Observatory (CTAO) Instrument Response Functions (IRFs), we simulate observations of VHE sources and forecast novel sensitivities to the kinetic mixing parameter for the photon-dark photon scattering process. Our study focuses on three key astrophysical targets: the Crab Nebula and the blazars Markarian 421 and Markarian 501. Additionally, we investigate the impact of dark matter spikes around black holes on the upper limits. For the dark matter spike scenario considered in this work, CTAO observations could probe the DPDM parameter space down to a mixing parameter of $\varepsilon \sim 10^{-8}$ for masses around $m_{A^{\prime}} \sim 10^{-1}\,\textrm{eV}$ through high-energy spectral attenuation, at a $95\%$ confidence level.

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arXiv2026

Suppression of boosted relic neutrinos by photon backgrounds during ultra-high-energy cosmic ray propagation

Constraining the cosmic neutrino background (C$ν$B) represents a major experimental challenge in cosmology. Recent studies have suggested that relic neutrinos boosted by ultra-high-energy cosmic rays (UHECRs) may generate observable diffuse neutrino fluxes. Previous estimates have not effectively propagated the primary cosmic rays, often neglecting crucial energy losses and the unavoidable, competing interactions with diffuse photon backgrounds. Here we revisit these expectations using a realistic Monte Carlo propagation framework. This approach allows us to consistently incorporate cosmic ray energy losses, nuclear photodisintegration, and production of secondary neutrinos. We show that interactions with diffuse photon backgrounds strongly suppress the boosted relic neutrino flux predicted in simplified propagation scenarios. Furthermore, we demonstrate that to produce any observable suppression on the UHECR energy spectrum at Earth, or for the boosted C$ν$B component to become comparable to the cosmogenic neutrino flux, the C$ν$B density must be enhanced by a factor, the so-called overdensity, of extreme magnitude ($η\gtrsim 10^{8}$).

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arXiv2026

Multiwavelength study of non-thermal emission in the Swift J1834.9-0846/W41 region

We investigate the origin of non-thermal emission from the Swift J1834-0846/W41 region by modeling its broadband spectral energy distribution from radio to TeV energies within leptonic and lepto-hadronic frameworks using Markov Chain Monte Carlo sampling. Motivated by morphological studies of HESS J1834-087 suggesting a two-component TeV structure, we explore a single extended source scenario and a configuration comprising a central point-like component embedded within extended emission. Purely leptonic models are disfavored in both scenarios by unrealistically low magnetic field strengths, whereas lepto-hadronic solutions yield field intensities and non-thermal energy budgets consistent with an evolved supernova remnant undergoing efficient cosmic-ray acceleration. In the two-component scenario, hadronic interactions dominate the extended TeV emission from W41, while the central excess is well described by a leptonic magnetar wind nebula powered by Swift J1834-0846, implying a short initial spin period of $P_0 \lesssim 0.2$ s. Simulated observations with the Cherenkov Telescope Array Observatory show that 30 h exposures will discriminate between the two morphological configurations and extend spectral measurements beyond $\sim$10 TeV.

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arXiv2026

Modeling individual nearby radio galaxies as ultra-high-energy cosmic-ray accelerators

Nearby radio galaxies are among the most promising candidates for the acceleration of ultra-high-energy cosmic rays (UHECRs). In this work, we develop a physically motivated, source-resolved framework to quantify the contribution of the three nearest FR-I radio galaxies$-$Centaurus A, Virgo A, and Fornax A to the UHECR flux measured by the Pierre Auger Observatory. Acceleration spectra derived from detailed jet-acceleration models are combined with numerical simulations of extragalactic propagation, while the more distant radio-galaxy population is treated as a continuous background. By fitting exclusively the measured UHECR energy spectrum, we determine the relative contribution of each source and constrain the fraction of jet power converted into UHECR luminosity. We find that a small number of nearby radio galaxies can account for the highest-energy UHECR flux with acceleration efficiencies of order $10^{-3}-10^{-2}$, while the background contribution remains subdominant. The resulting scenarios yield mass-composition trends broadly consistent with observations and predict distinct levels of secondary neutrino fluxes. These results demonstrate that physically grounded, source-specific modeling of nearby radio galaxies provides a viable and predictive explanation for the origin of the highest-energy cosmic rays.

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arXiv2025

Effects of Lorentz invariance violation on charged particles and photon production in astrophysical sources

We investigate the impact of Lorentz invariance violation (LIV) on radiation processes in astrophysical sources, focusing on synchrotron and inverse Compton interactions. We derive modified expressions for radiated power and photon energy under LIV assumptions and incorporate them into first-order Fermi acceleration models. Our analysis reveals energy thresholds beyond which LIV, within a kinematic framework, significantly alters particle dynamics and photon spectra, introducing non-physical divergences that highlight limitations in perturbative approaches. We model synchrotron self-Compton (SSC) emission in the presence of LIV and assess its consequences for photon fluxes from blazars, including Markarian 501 and the BL Lac population. LIV introduces distinct high-energy emission regions that deviate from standard expectations. Comparisons with observational data, particularly upper limits from the Pierre Auger Observatory, suggest that future multi-messenger observations, together with the full analysis of particle's trajectories, could constrain LIV parameters through the non-detection of such excesses.

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arXiv2025

Constraining Effective Field Theories for dark matter candidates annihilating into gamma-ray lines with CTAO

Gamma-ray lines constitute a smoking gun signature for annihilating dark matter particles. Imaging Atmospheric Cherenkov Telescopes and satellites have searched for such signals but null results have been reported thus far. We take advantage of the expected gamma-ray flux sensitivity of the Cherenkov Telescope Array Observatory (CTAO) toward the direction of the Galactic Centre and Dwarf Galaxies and its exquisite energy resolution to derive upper limits on fermionic and scalar dark matter annihilations into gamma-ray lines. We consider the lowest-order effective operators for scalar and fermion dark matter, and derive limits on the energy scale using the recent CTAO projected sensitivity. Putting our findings into perspective with existing limits from direct and indirect detection experiments, we conclude that CTAO will either play a complementary role or be a discovery channel for dark matter signals.

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arXiv2025

Recent advances on multimessenger astrophysics: Centaurus A, GW 170817, and KM3-230213A

We review recent advances in multimessenger astrophysics, with particular emphasis on Centaurus A and on two pivotal events: the gravitational-wave detection GW 170817 and the ultra-high-energy neutrino KM3-230213A. Centaurus A is the prototype multimessenger source. The GW 170817 event, arising from a binary neutron star merger, marked a transformative moment in astronomy through the joint observation of gravitational waves and a broad spectrum of electromagnetic signals. The KM3-230213A neutrino, detected by the KM3NeT Collaboration, is the most energetic neutrino observed to date and poses significant challenges for current models, given its tension with null results from IceCube and the Pierre Auger Observatory. We assess astrophysical interpretations, including galactic, cosmogenic, and transient extragalactic sources, as well as implications for cosmic-ray acceleration. These cases underscore the scientific potential of high-energy multimessenger events in probing both the extreme universe and new physics.

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arXiv2025

History of UHECR production in Centaurus A

The origin of the UHECR continues to puzzle, however, an excess of detection in the direction of the radio galaxy Centaurus A (Cen A) raises the possibility of this object being the first UHECR source identifiable. Cen A is known to be currently active, and also exhibits known past episodes of high activity. In this work, we investigate whether the known activity episodes in Cen A may be related to the excess events in the \textit{Centaurus region}. Analysing the energy of the events and the overall mass composition of UHECR, we report that an activity in the last $\sim30$ Myr is necessary to explain the excess of events. This period perfectly fits with the timescale where the transition regions and the Giant Lobes must be energized, as revealed by radio and $γ$ ray observations.

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arXiv2025

The origin of the very-high-energy radiation along the jet of Centaurus A

As the closest known active galactic nucleus, Centaurus A (Cen A) provides a rich environment for astrophysical exploration. It has been observed across wavelengths from radio to gamma rays, and indications of ongoing particle acceleration have been found on different scales. Recent measurements of very-high-energy (VHE) gamma-rays ($>240$ GeV) by the HESS observatory have inferred the presence of ultra-relativistic electrons along Cen A's jet, yet the underlying acceleration mechanism remains uncertain. Various authors have proposed that jet substructures, known as knots, may serve as efficient particle accelerators. In this study, we investigate the hypothesis that knots are the particle acceleration sites along Cen A's jets. We focus on stationary knots, and assume that they result from interactions between the jet and the stellar winds of powerful stars. By combining relativistic hydrodynamic simulations and shock acceleration theory with the radio and X-ray data, we compare theoretical predictions with morphological and spectral data from different knots. We estimate the maximum electron energy and the resulting VHE gamma-ray emission. Our findings suggest that electrons accelerated at the knots are responsible for the gamma-ray spectrum detected in the VHE band.

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arXiv2025

Investigating the effect of hadronic models on IACT images

The predictions of hadronic interaction models for cosmic-ray induced air showers contain inherent uncertainties due to limitations of available accelerator data. This leads to differences in shower simulations using each of those models. Many studies have been carried out to track those differences by investigating the shower development or the particle content. In this work, we propose a new approach to search for discrepancies and similarities between the models, via the IACT images resulting from the observations of hadronic air showers. We use simulations of H.E.S.S. as a show-case scenario and, by investigating variables of the camera images, we find potential indicators to highlight differences between models. Number of pixels, Hillas image size, and density showed the largest difference between the models. We then further explore the (in)compatibility of the models by combining all the variables and using Boosted Decision Trees. For protons, a significant difference in the classifier output is found for EPOS-LHC when compared to both QGSJET-II04 and Sybill 2.3d. For helium and nitrogen, QGSJET-II04 is shown to be the outlier case. No significant differences are found for silicon and iron. The distribution of (in)compatibility between the models in the phase space of reconstructed shower parameters shows that a targeted search can be fruitful, with showers with reconstructed energies of a few TeV and reconstructed core closer to the large telescope presenting the largest power of separation. An investigation of the distribution of first interaction parameters has shown that EPOS-LHC and QGSJET-II04 result in significantly different distributions of multiplicity and height of first interaction for protons and elasticity and fraction of energy carried by neutral pions for helium and nitrogen.

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arXiv2025

Probing late-time annihilations of oscillating asymmetric dark matter via rotation curves of galaxies

In this paper, we explore the Oscillating Asymmetric Dark Matter (OADM) model to address the core-cusp problem, aiming to resolve the discrepancy between the predictions of the $Λ\rm{CDM}$ cosmological model and the observed dark matter profiles in dwarf spheroidal galaxies. The reactivation of dark matter annihilation during the structure formation epoch is possible if there is a small Majorana mass term that breaks the conservation of dark matter particle number, leading to oscillations between dark matter and its antiparticle. We analyzed the effects of the annihilation mechanism in the galaxy rotation curves of the SPARC and LITTLE THINGS catalogs. We searched for the characteristics of the OADM model which best describes the data. Our results show that the OADM model can successfully turn originally cusp-type halos into core-type ones according to our data sample.

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arXiv2025

Prospects for the detection of Dark Matter with Long-lived Mediators in the Sun using the Southern Wide-field Gamma-ray Observatory

The operation of the next generation of gamma-ray observatories will lead to a great advance in dark matter searches. In this paper, we use the hidden sectors hypothesis within the so-called secluded models to calculate the capabilities of the Southern Wide-field Gamma-ray Observatory (SWGO) to detect gamma-ray signatures produced by dark matter particles concentrated in the Sun. We assume the dark matter particle annihilates into metastable mediators which decay into $γγ$, $e^+e^-$, $τ^+τ^-$, and $\bar{b}b$ outside the Sun. We found that the SWGO will be able to probe a spin-dependent cross-section of about $10^{-46}$ cm$^2$ for dark matter masses smaller than 5 TeV. This result shows an unprecedented sensitivity surpassing the current instruments by more than one order of magnitude.

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arXiv2024

Fermi acceleration under Lorentz invariance violation

In this paper, the acceleration of particles in astrophysical sources by the Fermi mechanism is revisited under the assumption of Lorentz invariance violation (LIV). We calculate the energy spectrum and the acceleration time of particles leaving the source as a function of the energy beyond which the Lorentz invariance violation becomes relevant. Lorentz invariance violation causes significant changes in the acceleration of particles by the first and second-order Fermi mechanisms. The energy spectrum of particles accelerated by first-order Fermi mechanism under LIV assumption shows a strong suppression for energies above the break. The calculations presented here complete the scenario for LIV searches with astroparticles by showing, for the first time, how the benchmark acceleration mechanisms (Fermi) are modified under LIV assumption.

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arXiv2023

Revisiting the implications of Liouville's theorem to the anisotropy of cosmic rays

We present a solution to Liouville's equation for an ensemble of charged particles propagating in magnetic fields. The solution is presented using an expansion in spherical harmonics of the phase space density, allowing a direct interpretation of the distribution of arrival directions of cosmic rays. The results are found for chosen conditions of variability and source distributions. We show there are two conditions for an initially isotropic flux of particles to remain isotropic while traveling through a magnetic field: isotropy and homogeneity of the sources. In case isotropically-distributed sources inject particles continuously in time, a transient magnetic induced dipole will appear. This dipole will vanish if the system reaches a steady-state. The formalism is used to analyze the data measured by the Pierre Auger Observatory, contributing to the understanding of the dependence of the dipole amplitude with energy and predicting the energy in which the quadrupole signal should be measured.

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arXiv2023

Constraining Gamma-ray Lines from Dark Matter Annihilation using Fermi-LAT and H.E.S.S. data

Using 14 years of Fermi-LAT data and 10 years of H.E.S.S. observations in the direction of the galactic center, we derive limits on gamma-ray lines originated from dark matter annihilations for fermionic and scalar fields. We describe the dark matter annihilation into $γγ$ or $γZ$ final states in terms of effective operators and place limits on the energy scale as a function of the dark matter mass taking into account the energy resolution of the instruments. For the Fermi-LAT data, we considered an NFW and a contracted NFW dark matter density profile, the latter being preferred by the Fermi GeV excess. For the H.E.S.S. observation, we used an NFW and Einasto profile. Fermi-LAT yields the most stringent constraints for dark matter masses below 300 GeV, whereas H.E.S.S. has the strongest ones for dark matter masses above 1 TeV. The telescopes share similar sensitivities for dark matter masses between 300 GeV and 1 TeV. We conclude that Fermi-LAT (H.E.S.S.) can probe energy scales up to $10(20)$~TeV for scalar and fermionic dark matter particles.

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arXiv2023

Uncertainties in measuring the dark matter signal from Milky Way satellites using Cherenkov telescopes

In this work, we present a modelling of the galactic sub-clumps based on statistical estimations of the full Milky Way satellite population. We introduce 10 substructure modellings (SM$_{i}$, i $\in$ {1, . . . , 10}) with the following varying parameters: a) subhalos inner profile, b) spatial distribution of subhalos, c) mass distribution of subhalos, d) total number of subhalos and e) concentration parameter. The sensitivity curves of CTA for sources in each model are calculated for the $τ^{+}τ^{-}$ and $b\bar{b}$ annihilation channels. With both detection of a signal (5$σ$) with the CTA and no signal observation, no model was effective in accessing the thermal values of <$σ$ v>. We analyse the systematic effects introduced by the substructures models.

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arXiv2022

Nearby active galactic nuclei and starburst galaxies as sources of the measured UHECRs anisotropy signal

The Pierre Auger and the Telescope Array observatories have measured independent and statistical significant anisotropy in the arrival direction of ultra-high-energy cosmic rays (UHECR). Three hotspot regions with relative excess of events and a dipole signal have been identified in different regions of the sky and energy ranges. In this paper, we investigate the conditions under which these anisotropy signal could be generated by nearby (<23 Mpc) active galactic nuclei (AGN) and/or starburst galaxies (SBG). We studied a wide range of possibilities including injected nuclei (p, He, N, Si, and Fe), three UHECR luminosity proxies and three extragalactic magnetic field models. The results shows that both local AGN and SBG are needed to describe all the anisotropy signal. The contribution of AGN to hotspots and to the generation of the dipole is dominant in most cases. SBG is required only to explain the hotspot measured by the Telescope Array Observatory.

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