Browsing by Author "Assis, Pedro"
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- A search for point sources of EeV photonsPublication . Pierre Auger collaboration (484 authors); Abreu, Pedro; Andringa, Sofia; Assis, Pedro; Brogueira, Pedro; Cazon, Lorenzo; Conceição, Ruben; Diogo, Francisco; Espadanal, Joao; Gonçalves, Patrícia; Oliveira, Micael; Pimenta, Mário; Santo, Catarina E; Santos, Eva; Sarmento, Raul; Tomé, BernardoMeasurements of air showers made using the hybrid technique developed with the fluorescence and surface detectors of the Pierre Auger Observatory allow a sensitive search for point sources of EeV photons anywhere in the exposed sky. A multivariate analysis reduces the background of hadronic cosmic rays. The search is sensitive to a declination band from –85° to +20°, in an energy range from 10(17.3) eV to 10(18.5) eV. No photon point source has been detected. An upper limit on the photon flux has been derived for every direction. The mean value of the energy flux limit that results from this, assuming a photon spectral index of –2, is 0.06 eV cm(–)(2) s(–)(1), and no celestial direction exceeds 0.25 eV cm(–)(2) s(–)(1). These upper limits constrain scenarios in which EeV cosmic ray protons are emitted by non-transient sources in the Galaxy.
- A Targeted Search for Point Sources of EeV NeutronsPublication . Pierre Auger collaboration (496 authors); Abreu, Pedro; Andringa, Sofia; Assis, Pedro; Brogueira, Pedro; Cazon, Lorenzo; Conceição, Ruben; Diogo, Francisco; Espadanal, Joao; Gonçalves, Patrícia; Oliveira, Micael; Pimenta, Mário; Santo, Catarina E; Santos, Eva; Sarmento, Raul; Tomé, BernardoA flux of neutrons from an astrophysical source in the Galaxy can be detected in the Pierre Auger Observatory as an excess of cosmic-ray air showers arriving from the direction of the source. To avoid the statistical penalty for making many trials, classes of objects are tested in combinations as nine 'target sets,' in addition to the search for a neutron flux from the Galactic center or from the Galactic plane. Within a target set, each candidate source is weighted in proportion to its electromagnetic flux, its exposure to the Auger Observatory, and its flux attenuation factor due to neutron decay. These searches do not find evidence for a neutron flux from any class of candidate sources. Tabulated results give the combined p-value for each class, with and without the weights, and also the flux upper limit for the most significant candidate source within each class. These limits on fluxes of neutrons significantly constrain models of EeV proton emission from non-transient discrete sources in the Galaxy.
- A targeted search for point sources of EeV photons with the Pierre Auger ObservatoryPublication . Pierre Auger collaboration (399 authors); Abreu, Pedro; Andringa, Sofia; Assis, Pedro; Barreira Luz, Ricardo Jorge; Blanco, Alberto; Cazon, Lorenzo; Conceição, Ruben; Diogo, Francisco; Espadanal, João; Lopes, Luis; Pimenta, Mário; Santos, Eva; Sarmento, Raul; Tomé, BernardoSimultaneous measurements of air showers with the fluorescence and surface detectors of the Pierre Auger Observatory allow a sensitive search for EeV photon point sources. Several Galactic and extragalactic candidate objects are grouped in classes to reduce the statistical penalty of many trials from that of a blind search and are analyzed for a significant excess above the background expectation. The presented search does not find any evidence for photon emission at candidate sources, and combined p-values for every class are reported. Particle and energy flux upper limits are given for selected candidate sources. These limits significantly constrain predictions of EeV proton emission models from non-transient Galactic and nearby extragalactic sources, as illustrated for the particular case of the Galactic center region.
- An Indication of anisotropy in arrival directions of ultra-high-energy cosmic rays through comparison to the flux pattern of extragalactic gamma-ray sourcesPublication . Pierre Auger collaboration (392 authors); Abreu, Pedro; Andringa, Sofia; Assis, Pedro; Barreira Luz, Ricardo Jorge; Blanco, Alberto; Cazon, Lorenzo; Conceição, Ruben; Diogo, Francisco; Espadanal, João; Lopes, Luis; Pimenta, Mário; Riehn, Felix; Santos, Eva; Sarmento, Raul; Tomé, BernardoA new analysis of the data set from the Pierre Auger Observatory provides evidence for anisotropy in the arrival directions of ultra-high-energy cosmic rays on an intermediate angular scale, which is indicative of excess arrivals from strong, nearby sources. The data consist of 5514 events above with zenith angles up to 80° recorded before 2017 April 30. Sky models have been created for two distinct populations of extragalactic gamma-ray emitters: active galactic nuclei from the second catalog of hard Fermi-LAT sources (2FHL) and starburst galaxies from a sample that was examined with Fermi-LAT. Flux-limited samples, which include all types of galaxies from the Swift-BAT and 2MASS surveys, have been investigated for comparison. The sky model of cosmic-ray density constructed using each catalog has two free parameters, the fraction of events correlating with astrophysical objects, and an angular scale characterizing the clustering of cosmic rays around extragalactic sources. A maximum-likelihood ratio test is used to evaluate the best values of these parameters and to quantify the strength of each model by contrast with isotropy. It is found that the starburst model fits the data better than the hypothesis of isotropy with a statistical significance of 4.0σ, the highest value of the test statistic being for energies above . The three alternative models are favored against isotropy with 2.7σ–3.2σ significance. The origin of the indicated deviation from isotropy is examined and prospects for more sensitive future studies are discussed.
- Antennas for the Detection of Radio Emission Pulses from Cosmic-RayPublication . Pierre Auger collaboration (515 authors); Abreu, Pedro; Andringa, Sofia; Assis, Pedro; Brogueira, Pedro; Cazon, Lorenzo; Conceição, Ruben; Diogo, Francisco; Espadanal, Joao; Gonçalves, Patrícia; Pimenta, Mário; Santo, Catarina E; Santos, Eva; Tomé, BernardoThe Pierre Auger Observatory is exploring the potential of the radio detection technique to study extensive air showers induced by ultra-high energy cosmic rays. The Auger Engineering Radio Array (AERA) addresses both technological and scientific aspects of the radio technique. A first phase of AERA has been operating since September 2010 with detector stations observing radio signals at frequencies between 30 and 80 MHz. In this paper we present comparative studies to identify and optimize the antenna design for the final configuration of AERA consisting of 160 individual radio detector stations. The transient nature of the air shower signal requires a detailed description of the antenna sensor. As the ultra-wideband reception of pulses is not widely discussed in antenna literature, we review the relevant antenna characteristics and enhance theoretical considerations towards the impulse response of antennas including polarization effects and multiple signal reflections. On the basis of the vector effective length we study the transient response characteristics of three candidate antennas in the time domain. Observing the variation of the continuous galactic background intensity we rank the antennas with respect to the noise level added to the galactic signal.
- Autonomous RPCs for a Cosmic Ray ground arrayPublication . Conceição, Ruben; Assis, Pedro; Blanco, Alberto; Carolino, Nuno; Cunha, Orlando; Dobrigkeit, Carola; Ferreira, Miguel; Fonte, Paulo; Lopes, Luis; Luz, Ricardo; Martins, Victor Barbosa; Mendes, Luis; Pereira, Américo; Pimenta, Mário; Sarmento, Raul; Shellard, Ronald; de Souza, Vitor; Tomé, BernardoWe 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.
- Azimuthal Asymmetry in the Risetime of the Surface Detector Signals of the Pierre Auger ObservatoryPublication . Pierre Auger collaboration (434 authors); Abreu, Pedro; Andringa, Sofia; Assis, Pedro; Blanco, Alberto; Cazon, Lorenzo; Conceição, Ruben; Diogo, Francisco; Espadanal, Joao; Lopes, Luis; Pimenta, Mário; Santos, Eva; Sarmento, Raul; Tomé, BernardoThe azimuthal asymmetry in the risetime of signals in Auger surface detector stations is a source of information on shower development. The azimuthal asymmetry is due to a combination of the longitudinal evolution of the shower and geometrical effects related to the angles of incidence of the particles into the detectors. The magnitude of the effect depends upon the zenith angle and state of development of the shower and thus provides a novel observable, (secθ)max, sensitive to the mass composition of cosmic rays above 3×1018 eV. By comparing measurements with predictions from shower simulations, we find for both of our adopted models of hadronic physics (QGSJETII-04 and EPOS-LHC) an indication that the mean cosmic-ray mass increases slowly with energy, as has been inferred from other studies. However, the mass estimates are dependent on the shower model and on the range of distance from the shower core selected. Thus the method has uncovered further deficiencies in our understanding of shower modeling that must be resolved before the mass composition can be inferred from (secθ)max.
- Bounds on the density of sources of ultra-high energy cosmic rays from the Pierre Auger ObservatoryPublication . Pierre Auger collaboration (512 authors); Abreu, Pedro; Andringa, Sofia; Assis, Pedro; Brogueira, Pedro; Cazon, Lorenzo; Conceição, Ruben; Diogo, Francisco; Espadanal, Joao; Gonçalves, Patrícia; Oliveira, Micael; Pimenta, Mário; Santo, Catarina E; Santos, Eva; Tomé, BernardoWe derive lower bounds on the density of sources of ultra-high energy cosmic rays from the lack of significant clustering in the arrival directions of the highest energy events detected at the Pierre Auger Observatory. The density of uniformly distributed sources of equal intrinsic intensity was found to be larger than ~ (0.06−5) × 10(−)(4) Mpc(−)(3) at 95% CL, depending on the magnitude of the magnetic deflections. Similar bounds, in the range (0.2−7) × 10(−)(4) Mpc(−)(3), were obtained for sources following the local matter distribution.
- Calibration of the logarithmic-periodic dipole antenna (LPDA) radio stations at the Pierre Auger Observatory using an octocopterPublication . Pierre Auger collaboration (404 authors); Abreu, Pedro; Andringa, Sofia; Assis, Pedro; Barreira Luz, Ricardo Jorge; Blanco, Alberto; Cazon, Lorenzo; Conceição, Ruben; Diogo, Francisco; Espadanal, João; Lopes, Luis; Pimenta, Mário; Santos, Eva; Sarmento, Raul; Tomé, BernardoAn in-situ calibration of a logarithmic periodic dipole antenna with a frequency coverage of 30 MHz to 80 MHz is performed. Such antennas are part of a radio station system used for detection of cosmic ray induced air showers at the Engineering Radio Array of the Pierre Auger Observatory, the so-called Auger Engineering Radio Array (AERA) . The directional and frequency characteristics of the broadband antenna are investigated using a remotely piloted aircraft carrying a small transmitting antenna. The antenna sensitivity is described by the vector effective length relating the measured voltage with the electric-field components perpendicular to the incoming signal direction. The horizontal and meridional components are determined with an overall uncertainty of 7.4(+0.9)(−)(0.3)% and 10.3(+2.8)(−)(1.7)% respectively. The measurement is used to correct a simulated response of the frequency and directional response of the antenna. In addition, the influence of the ground conductivity and permittivity on the antenna response is simulated. Both have a negligible influence given the ground conditions measured at the detector site. The overall uncertainties of the vector effective length components result in an uncertainty of 8.8(+2.1)(−)(1.3)% in the square root of the energy fluence for incoming signal directions with zenith angles smaller than 60°.
- Combined fit of spectrum and composition data as measured by the Pierre Auger ObservatoryPublication . Pierre Auger collaboration (399 authors); Abreu, Pedro; Andringa, Sofia; Assis, Pedro; Barreira Luz, Ricardo Jorge; Blanco, Alberto; Cazon, Lorenzo; Conceição, Ruben; Diogo, Francisco; Espadanal, João; Lopes, Luis; Pimenta, Mário; Santos, Eva; Sarmento, Raul; Tomé, BernardoWe present a combined fit of a simple astrophysical model of UHECR sources to both the energy spectrum and mass composition data measured by the Pierre Auger Observatory. The fit has been performed for energies above 5 ⋅ 10(18) eV, i.e. the region of the all-particle spectrum above the so-called 'ankle' feature. The astrophysical model we adopted consists of identical sources uniformly distributed in a comoving volume, where nuclei are accelerated through a rigidity-dependent mechanism. The fit results suggest sources characterized by relatively low maximum injection energies, hard spectra and heavy chemical composition. We also show that uncertainties about physical quantities relevant to UHECR propagation and shower development have a non-negligible impact on the fit results.