arXiv subjects

Vitor de Souza

Publications and source records attributed to Vitor de Souza.

At least 37 records · Page 2Linked to original sources

arXiv2022

Cosmic-ray measurements with an array of Cherenkov telescopes using reconstruction of longitudinal profiles of air showers

We present a method to reconstruct the longitudinal profile of electrons in showers using Cherenkov telescopes. We show how the Cherenkov light collected by an array of telescopes can be transformed into the number of electrons as a function of atmospheric depth. This method is validated using air shower and simplified telescope simulations. The reconstruction of the depth in which the shower has the maximum number of electrons ($\mathrm{X_{max}}$) opens the possibility of cosmic ray composition studies with Cherenkov telescopes in the energy range from 10 to 100 TeV. A resolution of less than 16 $\mathrm{g/cm^{2}}$ in the $\mathrm{X_{max}}$ reconstruction is obtained.

astro-ph.HE↗

arXiv2021

Magnetically Induced Anisotropies in the Arrival Directions of Ultra-high-energy Cosmic Rays from Nearby Radio Galaxies

Detailed simulations of the arrival directions of ultra-high energy cosmic rays are performed under the assumption of strong and structured extragalactic magnetic field (EGMF) models. Particles leaving Centaurus A, Virgo A, and Fornax A are propagated to Earth, and the simulated anisotropic signal is compared to the dipole and hotspots published by the Pierre Auger and Telescope Array Collaborations. The dominance of the EGMF structure on the arrival directions of events generated in local sources is shown. The absence of events from the Virgo A direction is related to the strong deviation caused by the EGMF. Evidence that these three sources contribute to an excess of events in the direction of the three detected hotspots is presented. Under the EGMF considered here, M82 is shown to have no contribution to the hotspot measured by the Telescope Array Observatory.

astro-ph.HE↗

arXiv2021

On the parametrization of the distributions of depth of shower maximum of ultra-high energy extensive air showers

The distribution of depth in which a cosmic ray air shower reaches its maximum number of particles ($X_{max}$) is studied and parametrized. Three functions are studied for proton, carbon, silicon, and iron primary particles with energies ranging from $10^{17}$ eV to $10^{20}$ eV for three hadronic interaction models: EPOS-LHC, QGSJetII.04, and Sibyll2.3c. The function which best describes the $X_{max}$ distribution of a mixed composition is also studied. A very large number of simulated showers and a detailed analysis procedure were used to guarantee negligible effects of undersampling and of fitting in the final results. For the first time, a comparison of several functions is presented under the same assumption with the intention of selecting the best functional form to describe the $X_{max}$ distribution. The Generalized Gumbel distribution is shown to be the best option for a general description of all cases. The Log-normal distribution is also a good choice for some cases while the Exponentially Modified Gaussian distribution has shown to be the worst choice in almost all cases studied. All three functions are parametrized as a function of energy and primary mass.

astro-ph.HE↗

arXiv2021

Probing UHECR production in Centaurus A using secondary neutrinos and gamma-rays

In this paper, the production of neutrinos and photons by ultra high energy cosmic rays (UHECR) interacting with the extragalactic background radiation is studied. Centaurus A is assumed as the prime source of UHECR and the possibility to identify this source by detecting the secondary neutrinos and photons produced in the propagation of UHECR is investigated. Fifteen astrophysical models regarding three extragalactic magnetic fields (EGMF) and five composition abundances are simulated. The flux and arrival direction of neutrinos and photons are investigated. It is shown that the detection of a signal from Cen A with statistical significance is achievable by current observatories in a few years and by proposed experiments in the near future. The dependence of the results on the models is also presented.

astro-ph.HE↗

arXiv2021

Effect of the uncertainty in the hadronic interaction models on the estimation of the sensitivity of the Cherenkov Telescope Array

Imaging Atmospheric Cherenkov Telescopes (IACTs) are ground-based indirect detectors for cosmic gamma rays with energies above tens of GeV. The major backgrounds for gamma-ray observations in IACTs are cosmic-ray charged particles. The capability to reject these backgrounds is the most important factor determining the gamma-ray sensitivity of IACT systems. Monte Carlo simulations are used to estimate the residual background rates and sensitivity of the systems during the design and construction phase. Uncertainties in the modeling of high-energy hadronic interactions of cosmic rays with nuclei in the air propagate into the estimates of residual background rates and subsequently into the estimated instrument sensitivity. We investigate the influence of the difference in the current hadronic interaction models on the estimated gamma-ray sensitivity of the Cherenkov Telescope Array using four interaction models (QGSJET-II-03, QGSJET-II-04, EPOS-LHC, and SIBYLL2.3c) implemented in the air shower simulation tool CORSIKA. Variations in background rates of up to a factor 2 with respect to QGSJET-II-03 are observed between the models, mainly due to differences in the $π^0$ production spectrum. These lead to ~30% differences in the estimated gamma-ray sensitivity in the 1 - 30 TeV region, assuming a 50-hour observation of a gamma-ray point-like source. The presented results also show that IACTs have a significant capability in the verification of hadronic interaction models.

astro-ph.IM↗

arXiv2021

Ultrahigh-energy dipole and beyond

Recent data from the Pierre Auger Observatory has revealed the presence of a large-scale dipole in the arrival direction distribution of ultra-high energy cosmic rays (UHECR). In this work, we build up an understanding of the diffusive origin of such a dipolar behavior as well as its dependency on energy and astrophysical source assumptions such as extragalactic magnetic field strength and cosmic ray composition. We present a novel analytical approach for calculating the angular distribution of CR coming from a single source and discuss the regimes in which the steady-state dipole result is expected. We also present a semianalytical method for calculating the evolution with energy of the resultant dipole for an ensemble of sources. We show that a local source allows for a strong growth of the dipole with energy over a large energy range. The possibility of a transition from a dipolar to non-dipolar regime at the highest energies and its implications for the source density, magnetic field intensity, and cosmic ray composition are discussed.

astro-ph.HE↗

arXiv2021

Parametrization of the angular distribution of Cherenkov light in air showers

The Cherenkov light produced in air showers largely contributes to the signal observed in ground-based gamma-ray and cosmic-ray observatories. Yet, no description of this phenomenon is available covering both regions of small and large angles to the shower axis. To fill this gap, a parametrization of the angular distribution of Cherenkov photons is performed in terms of a physically-motivated parametric function. Model parameters are constrained using simulated gamma-ray and proton showers with energies in the TeV to EeV region. As a result, a new parametrization is obtained that improves the precision of previous works. Results presented here can be used in the reconstruction of showers with imaging Cherenkov telescopes as well as in the reconstruction of shower profiles with fluorescence detectors.

astro-ph.HE↗

arXiv2020

Revisiting the distance to the nearest UHECR source: Effects of extra-galactic magnetic fields

We update the constraints on the location of the nearest UHECR source. By analyzing recent data from the Pierre Auger Observatory using state-of-the-art CR propagation models, we reaffirm the need of local sources with a distance less than 25-100 Mpc, depending on mass composition. A new fast semi-analytical method for the propagation of UHECR in environments with turbulent magnetic fields is developed. The onset of an enhancement and a low-energy magnetic horizon of cosmic rays from sources located within a particular distance range is demonstrated. We investigate the distance to the nearest source, taking into account these magnetic field effects. The results obtained highlight the robustness of our constrained distances to the nearest source.

astro-ph.HE↗

arXiv2019

Searching for Dark Matter in the Galactic Halo with a Wide Field of View TeV Gamma-ray Observatory in the Southern Hemisphere

Despite mounting evidence that dark matter (DM) exists in the Universe, its fundamental nature remains unknown. We present sensitivity estimates to detect DM particles with a future very-high-energy ($\gtrsim$ TeV) wide field-of-view gamma-ray observatory in the Southern Hemisphere. This observatory would search for gamma rays from the annihilation or decay of DM particles in the Galactic halo. With a wide field of view, both the Galactic Center and a large fraction of the Galactic halo will be detectable with unprecedented sensitivity to DM in the mass range of $\sim$500 GeV to $\sim$2 PeV. These results, combined with those from other present and future gamma-ray observatories, will likely probe the thermal relic annihilation cross section of Weakly Interacting Massive Particles for all masses from $\sim$80 TeV down to the GeV range in most annihilation channels.

astro-ph.HE↗

arXiv2019

Depth of maximum of air-shower profiles: testing the compatibility of measurements performed at the Pierre Auger Observatory and the Telescope Array experiment

At the Pierre Auger Observatory and the Telescope Array, the measurements of depths of maximum of air-shower profiles, $X_\mathrm{max}$, are performed using direct observations of the longitudinal development of showers with the help of the fluorescence telescopes. Though the same detection technique is used at both installations, the straightforward comparison of the characteristics of the measured $X_\mathrm{max}$ distributions is not possible due to the different approaches to the analysis of the recorded events. In this work, the Auger-Telescope Array composition working group presents a technique to compare the $X_\mathrm{max}$ measurements from the Auger Observatory and the Telescope Array. Applying this technique the compatibility of the first two moments of the measured $X_\mathrm{max}$ distributions is qualitatively tested for energies $10^{18.2}~{\rm eV} < E < 10^{19.0}~{\rm eV}$ using the recently published Telescope Array data from the Black Rock Mesa and Long Ridge fluorescence detector stations. For a quantitative comparison, simulations of air showers with EPOS-LHC, folded with effects of the Telescope Array detector, are required along with the inclusion in the analysis of the systematic uncertainties in the measurements of $X_\mathrm{max}$ and the energies of the events.

astro-ph.HE↗

arXiv2019

Improved limits on Lorentz invariance violation from astrophysical gamma-ray sources

Lorentz invariance (LI) has a central role in science and its violation (LIV) at some high-energy scale has been related to possible solutions for several of the most intriguing puzzles in nature such as dark matter, dark energy, cosmic rays generation in extreme astrophysical objects and quantum gravity. We report on a search for LIV signal based on the propagation of gamma rays from astrophysical sources to Earth. An innovative data analysis is presented which allowed us to extract unprecedented information from the most updated data set composed of 111 energy spectra of 38 different sources measured by current gamma-ray observatories. No LIV signal was found, and we show that the data are best described by LI assumption. We derived limits for the LIV energy scale at least 3 times better than the ones currently available in the literature for subluminal signatures of LIV in high-energy gamma rays.

astro-ph.HE↗

arXiv2019

The optical depth including Lorentz invariance violation energy threshold shifts

Lorentz invariance violation (LIV) introduced as a generic modification to particle dispersion relations can change the photon energy threshold of pair-production, which modifies the expected gamma-ray flux from astrophysical sources. In this work, we review this phenomenon and explore its consequences through the derived effects in the optical depth. Then, by looking for subluminal LIV signatures in TeV gamma-ray spectra, we present stringent limits to the LIV energy scale at leading order n=1 and 2. And finally, we present the predicted flux of GZK-photons including LIV, in the astrophysical scenario which best describes UHECR data.

astro-ph.HE↗

arXiv2017

Limits on the Lorentz Invariance Violation from UHECR astrophysics

In this paper, Lorentz Invariance Violation (LIV) is introduced in the calculations of photon propagation in the Universe. LIV is considered in the photon sector and the mean free path of the $γγ\rightarrow e^{+} e^{-}$ interaction is calculated. The corresponding photon horizon including LIV effects is used to predict major changes in the propagation of photons with energy above $10^{18}$ eV. The flux of GZK photons on Earth considering LIV is calculated for several source models of ultra-high energy cosmic ray (UHECR). The predicted flux of GZK gamma-rays is compared to the new upper limits on the photon flux obtained by the Pierre Auger Observatory in order to impose upper limits on the LIV coefficients of order $n =$ 0, 1 and 2. The limits on the LIV coefficients derived here are more realistic than previous works and in some cases more restrictive. The analysis resulted in LIV upper limits in the photon sector of $δ_{γ,0}^{limit} \sim -10^{-20}$, $δ_{γ,1}^{limit} \sim -10^{-38} \; \mathrm{eV^{-1}}$ and $δ_{γ,2}^{limit} \sim -10^{-56} \; \mathrm{eV^{-2}}$ in the astrophysical scenario which best describes UHECR data.

astro-ph.HE↗

arXiv2017

Autonomous RPCs for a Cosmic Ray ground array

We report on the behaviour of Resistive Plate Chambers (RPC) developed for muon detection in ultra-high energy cosmic ray (UHECR) experiments. The RPCs were developed for the MARTA project and were tested on field conditions. These RPCs cover an area of $1.5 \times 1.2\,{m^2}$ and are instrumented with 64 pickup electrodes providing a segmentation better than $20\,$cm. By shielding the detector units with enough slant mass to absorb the electromagnetic component in the air showers, a clean measurement of the muon content is allowed, a concept to be implemented in a next generation of UHECR experiments. The operation of a ground array detector poses challenging demands, as the RPC must operate remotely under extreme environmental conditions, with limited budgets for power and minimal maintenance. The RPC, DAQ, High Voltage and monitoring systems are enclosed in an aluminium-sealed case, providing a compact and robust unit suited for outdoor environments, which can be easily deployed and connected. The RPCs developed at LIP-Coimbra are able to operate using a very low gas flux, which allows running them for few years with a small gas reservoir. Several prototypes have already been built and tested both in the laboratory and outdoors. We report on the most recent tests done in the field that show that the developed RPCs have operated in a stable way for more than 2 years in field conditions.

astro-ph.IM↗

arXiv2017

A Monte Carlo study of the relevance of fluorescence radiation in VHE gamma ray observations with Cherenkov telescopes

It is generally assumed that fluorescence radiation does not play a significant role in the performance of Cherenkov telescopes. However, this assumption is required to be verified using detailed Monte Carlo simulations. In order to do this, we have implemented the production and tracking of fluorescence radiation inside the CORSIKA code, and simulated gamma-ray induced showers in the very high energy range. The most accurate fluorescence-yield data available so far was used for this purpose. The distribution of both light components on the ground has been studied as a function of various parameters affecting the detection and reconstruction of gamma-ray showers such as the angular aperture. From these distributions, we determined the conditions under which fluorescence radiation becomes significant. These results will also be useful to estimate the corresponding systematic errors in Cherenkov telescope observations. The full simulation results have been cross-checked, on a small sample of events, against numerical calculations based on a one-dimension shower profile and found to be compatible with each other. Both tools can be used for further investigations, like studying the possibility to modify Cherenkov telescopes for the measurement of fluorescence induced by extensive air showers.

astro-ph.IM↗

arXiv2017

Astroparticle Physics Tests of Lorentz Invariance Violation

Testing Lorentz invariance is essential as it is one of the pillars of modern physics. Moreover, its violation is foreseen in several popular Quantum Gravity models. Several authors study the effects of Lorentz invariance violation (LIV) in the propagation of ultra-high energy cosmic rays. These particles are the most energetic events ever detected and therefore represent a promising framework to test LIV. In this work we present an analytic calculation of the in-elasticity for any $a+b \rightarrow c+d$ interaction using first order perturbation in the dispersion relation that violates Lorentz invariance. The inelasticity can be calculated by solving a third-order polynomial equation containing: a) the kinematics of the interaction, b) the LIV term for each particle and c) the geometry of the interaction. We use the inelasticity we calculate to investigate the proton propagation in the intergalactic media. The photopion production of the proton interaction with the CMB is taken into account using the inelasticity and the attenuation length in different LIV scenarios. We show how the allowed phase space for the photopion production changes when LIV is considered for the interaction. The calculations presented here are going to be extended in order to calculated the modified ultra-high energy cosmic rays spectrum and compare it to the data.

astro-ph.HE↗

arXiv2017

A new air-shower observable to constrain hadronic interaction models

The energy spectrum of muons at ground level in air showers are studied and a new observable is proposed to constrain hadronic interaction models used in air shower simulations. An asymmetric Gaussian function is proposed to describe the muon ground energy spectrum and its parameters are studied regarding primary particle, energy and hadronic interaction models. Based on two realistic measurements of the muon density at a given distance from the shower axis, a new observable (\rmu) is defined. Considering realistic values of detector resolutions and number of measured events, it is also shown \rmu can be successfully used to constrain low and high hadronic interaction models. The study is focused in the energy range between $10^{17.5}$ and $10^{18.0}$ eV because of the importance of this interval for particle physics and astrophysical models. The constraining power of the new observable is shown to be large within current experimental capabilities.

hep-ph↗

arXiv2017

The influence of the observatory latitude on the study of ultra high energy cosmic rays

Recent precision measurements of the Ultra High Energy Cosmic Rays (UHECR) arrival directions, spectrum and parameters related to the mass of the primary particle have been done by the HiRes, Pierre Auger and Telescope Array (TA) Observatories. In this paper, distributions of arrival directions of events in the nearby Universe are assumed to correlate with sources in the 2MASS Redshift Survey (2MRS), IRAS 1.2 Jy Survey, Palermo Swift-BAT and Swift-BAT catalogs, and the effect of the latitude of the observatory on the measurement of the energy spectrum and on the capability of measuring anisotropy is studied. The differences between given latitudes on the northern and southern hemispheres are quantified. It is shown that the latitude of the observatory: a) has an influence on the total flux measured and b) imposes an important limitation on the capability of measuring an anisotropic sky.

astro-ph.HE↗