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Vitor de Souza

Publications and source records attributed to Vitor de Souza.

At least 55 records · Page 3Linked to original sources
arXiv2017

Measurements of the depth of maximum of air-shower profiles at the Pierre Auger Observatory and their composition implications

Air-showers measured by the Pierre Auger Observatory were analyzed in order to extract the depth of maximum (Xmax).The results allow the analysis of the Xmax distributions as a function of energy ($> 10^{17.8}$ eV). The Xmax distributions, their mean and standard deviation are analyzed with the help of shower simulations with the aim of interpreting the mass composition. The mean and standard deviation were used to derive and its variance as a function of energy. The fraction of four components (p, He, N and Fe) were fit to the Xmax distributions. Regardless of the hadronic model used the data is better described by a mix of light, intermediate and heavy primaries. Also, independent of the hadronic models, a decrease of the proton flux with energy is observed. No significant contribution of iron nuclei is derived in the entire energy range studied.

astro-ph.HE↗
arXiv2016

Interpretation of measurements of the number of muons in extensive air shower experiments

In this paper we analyze the energy evolution of the muon content of air showers between $10^{18.4}$ and $10^{19.6}$ eV to be able to determine the most likely mass composition scenario from future number of muons measurements. The energy and primary mass evolution of the number of muons is studied based on the Heitler-Matthews model and Monte Carlo simulation of the air shower. A simple model to describe the evolution of the first and second moments of number of muons distributions is proposed and validated. An analysis approach based on the comparison between this model's predictions and data to discriminate among a set of composition scenarios is presented and tested with simulations. It is shown that the composition scenarios can be potentially discriminated under the conditions imposed by the method. The discrimination power of the proposed analysis is stable under systematic changes of the absolute number of muons from model predictions and on the scale of the reconstructed energy.

astro-ph.HE↗
arXiv2016

Measurement of the Radiation Energy in the Radio Signal of Extensive Air Showers as a Universal Estimator of Cosmic-Ray Energy

We measure the energy emitted by extensive air showers in the form of radio emission in the frequency range from 30 to 80 MHz. Exploiting the accurate energy scale of the Pierre Auger Observatory, we obtain a radiation energy of 15.8 \pm 0.7 (stat) \pm 6.7 (sys) MeV for cosmic rays with an energy of 1 EeV arriving perpendicularly to a geomagnetic field of 0.24 G, scaling quadratically with the cosmic-ray energy. A comparison with predictions from state-of-the-art first-principle calculations shows agreement with our measurement. The radiation energy provides direct access to the calorimetric energy in the electromagnetic cascade of extensive air showers. Comparison with our result thus allows the direct calibration of any cosmic-ray radio detector against the well-established energy scale of the Pierre Auger Observatory.

astro-ph.HE↗
arXiv2016

Energy Estimation of Cosmic Rays with the Engineering Radio Array of the Pierre Auger Observatory

The Auger Engineering Radio Array (AERA) is part of the Pierre Auger Observatory and is used to detect the radio emission of cosmic-ray air showers. These observations are compared to the data of the surface detector stations of the Observatory, which provide well-calibrated information on the cosmic-ray energies and arrival directions. The response of the radio stations in the 30 to 80 MHz regime has been thoroughly calibrated to enable the reconstruction of the incoming electric field. For the latter, the energy deposit per area is determined from the radio pulses at each observer position and is interpolated using a two-dimensional function that takes into account signal asymmetries due to interference between the geomagnetic and charge-excess emission components. The spatial integral over the signal distribution gives a direct measurement of the energy transferred from the primary cosmic ray into radio emission in the AERA frequency range. We measure 15.8 MeV of radiation energy for a 1 EeV air shower arriving perpendicularly to the geomagnetic field. This radiation energy -- corrected for geometrical effects -- is used as a cosmic-ray energy estimator. Performing an absolute energy calibration against the surface-detector information, we observe that this radio-energy estimator scales quadratically with the cosmic-ray energy as expected for coherent emission. We find an energy resolution of the radio reconstruction of 22% for the data set and 17% for a high-quality subset containing only events with at least five radio stations with signal.

astro-ph.HE↗
arXiv2016

Ultra high energy cosmic rays and possible signature of black strings

Ultra high energy cosmic rays (UHECRs) probably originate in extreme conditions in which extra dimension effects might be important. In this paper we calculate the correction in black hole accretion mechanisms due to extra dimension effects in the static and rotating cases. A parametrization of the external Kerr horizons in both cases is presented and analysed. We use previous calculations of upper limits on the UHECR flux to set limits on the UHECR production efficiency of nine sources. The upper limit on the UHECR luminosity calculation is based on GeV-TeV gamma-ray measurements. The total luminosity due to the accretion mechanism is compared to the upper limit on UHECRs. The dependence of the UHECR production efficiency upper limit on black hole mass is also presented and discussed

hep-th↗
arXiv2015

Measurement of the cosmic ray spectrum above $4{\times}10^{18}$ eV using inclined events detected with the Pierre Auger Observatory

A measurement of the cosmic-ray spectrum for energies exceeding $4{\times}10^{18}$ eV is presented, which is based on the analysis of showers with zenith angles greater than $60^{\circ}$ detected with the Pierre Auger Observatory between 1 January 2004 and 31 December 2013. The measured spectrum confirms a flux suppression at the highest energies. Above $5.3{\times}10^{18}$ eV, the "ankle", the flux can be described by a power law $E^{-γ}$ with index $γ=2.70 \pm 0.02 \,\text{(stat)} \pm 0.1\,\text{(sys)}$ followed by a smooth suppression region. For the energy ($E_\text{s}$) at which the spectral flux has fallen to one-half of its extrapolated value in the absence of suppression, we find $E_\text{s}=(5.12\pm0.25\,\text{(stat)}^{+1.0}_{-1.2}\,\text{(sys)}){\times}10^{19}$ eV.

astro-ph.HE↗
arXiv2015

An improved limit to the diffuse flux of ultra-high energy neutrinos from the Pierre Auger Observatory

Neutrinos in the cosmic ray flux with energies near 1 EeV and above are detectable with the Surface Detector array of the Pierre Auger Observatory. We report here on searches through Auger data from 1 January 2004 until 20 June 2013. No neutrino candidates were found, yielding a limit to the diffuse flux of ultra-high energy neutrinos that challenges the Waxman-Bahcall bound predictions. Neutrino identification is attempted using the broad time-structure of the signals expected in the SD stations, and is efficiently done for neutrinos of all flavors interacting in the atmosphere at large zenith angles, as well as for "Earth-skimming" neutrino interactions in the case of tau neutrinos. In this paper the searches for downward-going neutrinos in the zenith angle bins $60^\circ-75^\circ$ and $75^\circ-90^\circ$ as well as for upward-going neutrinos, are combined to give a single limit. The $90\%$ C.L. single-flavor limit to the diffuse flux of ultra-high energy neutrinos with an $E^{-2}$ spectrum in the energy range $1.0 \times 10^{17}$ eV - $2.5 \times 10^{19}$ eV is $E_ν^2 dN_ν/dE_ν< 6.4 \times 10^{-9}~ {\rm GeV~ cm^{-2}~ s^{-1}~ sr^{-1}}$.

astro-ph.HE↗
arXiv2015

Cosmic rays: the spectrum and chemical composition from $10^{10}$ to $10^{20}$ eV

The production of energetic particles in the universe remains one of the great mysteries of modern science. The mechanisms of acceleration in astrophysical sources and the details about the propagation through the galactic and extragalactic media are still to be defined. In recent years, the cosmic ray flux has been measured with high precision in the energy range from \energy{10} to \energyEV{20.5} by several experiments using different techniques. In some energy ranges, it has been possible to determine the flux of individual elements (hydrogen to iron nuclei). This paper explores an astrophysical scenario in which only our Galaxy and the radio galaxy Cen A produce all particles measured on Earth in the energy range from \energy{10} to \energyEV{20.5}. Data from AMS-02, CREAM, KASCADE, KASCADE-Grande and the Pierre Auger Observatories are considered. The model developed here is able to describe the total and individual particle flux of all experiments considered. It is shown that the theory used here is able to describe the smooth transition from space-based to ground-based measurements. The flux of each element as determined by KASCADE and KASCADE-Grande and the mass sensitivity parameter \xmax measured by the Pierre Auger Observatory above \energyEV{18} are also explored within the framework of the model. The transition from \energy{16} to \energyEV{18} is carefully analyzed. It is shown that the data measured in this energy range suggest the existence of an extra component of cosmic rays yet to be understood.

astro-ph.HE↗
arXiv2013

Comparison of the moments of the Xmax distribution predicted by different cosmic ray shower simulation models

In this paper we study the depth at which a cosmic ray shower reaches its maximum (\Xmax) as predicted by Monte Carlo simulation. The use of \Xmax in the determination of the primary particle mass can only be done by comparing the measured values with simulation predictions. For this reason it is important to study the differences between the available simulation models. We have done a study of the first and second moments of the \Xmax distribution using the \Corsika and \Conex programs. The study was done with high statistics in the energy range from $10^{17}$ to \energy{20.4}. We focus our analysis in the different implementations of the hadronic interaction models \Sibyll and \Qgsjet in \Corsika and \Conex. We show that the predictions of the \meanXmax and \sigmaXmax depend slightly on the combination of simulation program and hadronic interaction model. Although these differences are small, they are not negligible in some cases (up to 5 g/cm$^2$ for the worse case) and they should be considered as a systematic uncertainty of the model predictions for \meanXmax and \sigmaXmax. We have included a table with the suggested systematic uncertainties for the model predictions. Finally, we present a parametrization of the \Xmax distribution as a function of mass and energy according to the models \Sibyll and \Qgsjet, and showed an example of its application to obtain the predicted \Xmax distributions from cosmic ray propagation models.

astro-ph.HE↗
arXiv2012

Centaurus A: the one extragalactic source of cosmic rays with energies above the knee

The origin of cosmic rays at all energies is still uncertain. In this paper we present and explore an astrophysical scenario to produce cosmic rays with energy ranging from below $10^{15}$ o $3 \times 10^{20}$ eV. We show here that just our Galaxy and the radio galaxy Cen A, each with their own galactic cosmic ray particles, but with those from the radio galaxy pushed up in energy by a relativistic shock in the jet emanating from the active black hole, are sufficient to describe the most recent data in the energy range PeV to near ZeV. Data are available over this entire energy range from the experiments KASCADE, KASCADE-Grande and Pierre Auger Observatory. The energy spectrum calculated here correctly reproduces the measured spectrum beyond the knee, and contrary to widely held expectations, no other extragalactic source population is required to explain the data, even at energies far below the general cutoff expected at $6 \times 10^{19}$ eV, the Greisen-Zatsepin-Kuzmin turn-off due to interaction with the cosmological microwave background. We present several predictions for the source population, the cosmic ray composition and the propagation to Earth which can be tested in the near future.

astro-ph.HE↗
arXiv2010

Ultra-High Energy Cosmic Rays from Centaurus A: Jet Interaction with Gaseous Shells

Ultra high energy cosmic rays (UHECRs), with energies above ~6 x 10^19 eV, seem to show a weak correlation with the distribution of matter relatively near to us in the universe. It has earlier been proposed that UHECRs could be accelerated in either the nucleus or the outer lobes of the nearby radio galaxy Cen A. We show that UHECR production at a spatially intermediate location about 15 kpc northeast from the nucleus, where the jet emerging from the nucleus is observed to strike a large star-forming shell of gas, is a plausible alternative. A relativistic jet is capable of accelerating lower-energy heavy seed cosmic rays (CRs) to UHECRs on timescales comparable to the time it takes the jet to pierce the large gaseous cloud. In this model many CRs arising from a starburst, with a composition enhanced in heavy elements near the knee region around PeV, are boosted to ultra-high energies by the relativistic shock of a newly oriented jet. This model matches the overall spectrum shown by the Auger data and also makes a prediction for the chemical composition as a function of particle energy. We thus predict an observable anisotropy in the composition at high energy in the sense that lighter nuclei should preferentially be seen toward the general direction of Cen A. Taking into consideration the magnetic field models for the Galactic disk and a Galactic magnetic wind, this scenario may resolve the discrepancy between HiRes and Auger results concerning the chemical composition of UHECRs.

astro-ph.HE↗
arXiv2007

Effects of the Energy Error Distribution of Fluorescence Telescopes on the UHECR energy spectrum

The measurement of the ultra high energy cosmic ray (UHECR) spectrum is strongly affected by uncertainties on the reconstructed energy. The determination of the presence or absence of the GZK cutoff and its position in the energy spectrum depends not only on high statistics but also on the shape of the energy error distribution. Here we determine the energy error distribution for fluorescence telescopes, based on a Monte Carlo simulation. The HiRes and Auger fluorescence telescopes are simulated in detail. We analyze the UHECR spectrum convolved with this energy error distribution. We compare this spectrum with one convolved with a lognormal error distribution as well as with a Gaussian error distribution. We show that the energy error distribution for fluorescence detectors can not be represented by these known distributions. We conclude that the convolved energy spectrum will be smeared but not enough to affect the GZK cutoff detection. This conclusion stands for both HiRes and Auger fluorescence telescopes. This result differs from the effect of the energy error distribution obtained with ground detectors and reinforces the importance of the fluorescence energy measurement. We also investigate the effect of possible fluorescence yield measurement errors in the energy spectrum.

astro-ph↗
arXiv2005

The shower size parameter as estimator of extensive air shower energy in fluorescence telescopes

The fluorescence technique has been successfully used to detect ultrahigh energy cosmic rays by indirect measurements. The underlying idea is that the number of charged particles in the atmospheric shower, i.e, its longitudinal profile, can be extracted from the amount of emitted nitrogen fluorescence light. However the influence of shower fluctuations and the very possible presence of different nuclear species in the primary cosmic ray spectrum makes the estimate of the shower energy from the fluorescence data analysis a difficult task. We investigate the potential of shower size at maximum depth as estimator of shower energy. The detection of the fluorescence light is simulated in detail and the reconstruction biases are carefully analyzed. We extend our calculations to both HiRes and EUSO experiments. This approach has shown some advantages to the reconstruction of the energy when compared to the standard analysis procedure.

astro-ph↗
arXiv2005

Acceptance of fluorescence detectors and its implication in energy spectrum inference at the highest energies

Along the years HiRes and AGASA experiments have explored the fluorescence and the ground array experimental techniques to measure extensive air showers, being both essential to investigate the ultra-high energy cosmic rays. However, such Collaborations have published contradictory energy spectra for energies above the GZK cut-off. In this article, we investigate the acceptance of fluorescence telescopes to different primary particles at the highest energies. Using CORSIKA and CONEX shower simulations without and with the new pre-showering scheme, which allows photons to interact in the Earth magnetic field, we estimate the aperture of the HiRes-I telescope for gammas, iron nuclei and protons primaries as a function of the number of simulated events and primary energy. We also investigate the possibility that systematic differences in shower development for hadrons and gammas could mask or distort vital features of the cosmic ray energy spectrum at energies above the photo-pion production threshold. The impact of these effects on the true acceptance of a fluorescence detector is analyzed in the context of top-down production models.

astro-ph↗
arXiv2005

Acceptance of fluorescence detectors for photons and its implication in energy spectrum inference at the highest energies

The HiRes and AGASA Collaborations have published contradictory energy spectra for energies above the GZK cut-off. In this article, we investigate the acceptance of fluorescence telescopes to different primary particles at the highest energies. Using CORSIKA shower simulations without and with the new pre-showering scheme, which allows photons to interact in the Earth magnetic field, we estimate the aperture of the HiRes I telescope for gammas and protons primaries. We calculate the dependence of the telescope sensitivity to primary particle identity. We also investigate the possibility that systematic differences in shower development for hadrons and gammas could mask or distort vital features of the cosmic ray energy spectrum at energies above the photo-pion production threshold. The impact of these effects on the true acceptance of a fluorescence detector is analyzed in the context of top-down production models.

astro-ph↗
arXiv2005

Alternative energy estimation from the shower lateral distribution function

The surface detector technique has been successfully used to detect cosmic ray showers for several decades. Scintillators or Cerenkov water tanks can be used to measure the number of particles and/or the energy density at a given depth in the atmosphere and reconstruct the primary particle properties. It has been shown that the experiment configuration and the resolution in reconstructing the core position determine a distance to the shower axis in which the lateral distribution function (LDF) of particles shows the least variation with respect to different primary particles type, simulation models and specific shapes of the LDF. Therefore, the signal at this distance (600 m for Haverah Park and 1000 m for Auger Observatory) has shown to be a good estimator of the shower energy. Revisiting the above technique, we show that a range of distances to the shower axis, instead of one single point, can be used as estimator of the shower energy. A comparison is done for the Auger Observatory configuration and the new estimator proposed here is shown to be a good and robust alternative to the standard single point procedure.

astro-ph↗
arXiv2005

Analysis of shower size as estimator of extensive air shower energy

The fluorescence technique has been successfully used to detect ultrahigh energy cosmic rays by indirect measurements. The underlying idea is that the number of charged particles in the atmospheric shower, i.e, its longitudinal profile, can be extracted from the amount of emitted nitrogen fluorescence light. However the influence of shower fluctuations and the very possible presence of different nuclear species in the primary cosmic ray spectrum make the estimate of the shower energy from the fluorescence data analysis a difficult task. We investigate the potential of shower size at maximum depth as estimator of shower energy. The detection of the fluorescence light is simulated in detail and the reconstruction biases are carefully analyzed. We extend our calculations to both Auger and EUSO experiments. This kind of approach is of particular interest for showers that are not fully contained inside the field of view of the detector.

astro-ph↗