Rayos cósmicos de energía extrema

Los rayos cósmicos de energía extrema son partículas subatómicas que llegan a la tierra con energías mayores que ~1018 eV. Su flujo es muy reducido (menos de 1 rayo cósmico por kilómetro cuadrado y por siglo) y son detectados a través de la cascada de partículas que se produce tras el impacto del rayo cósmico con uno de los átomos de la atmósfera. La mera existencia de estas partículas pone a prueba nuestro conocimiento sobre el comportamiento de la materia a energías que se encuentran varios órdenes de magnitud por encima de las que pueden alcanzarse en los mayores aceleradores de partículas construidos por el hombre. A pesar de que en las últimas décadas se han logrado enormes avances tanto teóricos como experimentales en el estudio de los rayos cósmicos, preguntas fundamentales sobre estas partículas de energía extrema continúan aún sin respuesta cierta: ¿cuáles son sus fuentes?, ¿existe una energía máxima?, ¿cuál es su espectro de energías?, ¿existe una transición entre un origen galáctico y otro extra-galáctico?, ¿cuál es su composición?

Grand, A Giant Radio Array For Neutrino Detection: Objectives, Design And Current Status

The Giant Radio Array for Neutrino Detection (GRAND) aims to answer one of the most pressing open questions in astrophysics: what is the origin of ultra-highenergy cosmic rays (UHECRs)?. It will do so indirectly: UHECRs make secondary UHE neutrinos which encode information about the properties of UHECRs and their sources. GRAND is designed to discover UHE neutrinos even under pessimistic predictions of their flux, reaching a sensitivity of 6x 10−9 GeV.cm −2 .s −1 .sr −1 around 109 GeV . It will do so by using 20 sub-arrays of 10 000 radio antennas forming a total detector area of 200 000 km 2 , making it the largest air-shower detector ever built. With this sensitivity, GRAND will discover cosmogenic neutrinos in 3 years of operation, even in disfavorable scenarios. Because of its subdegree angular resolution, GRAND will also search for point sources of UHE neutrinos, both steady and transient. Moreover, GRAND will be a valuable instrument for astronomy and cosmology, allowing for the discovery and followup of large numbers of radio transients - fast radio bursts, giant radio pulses - and studies of the epoch of reionization. In this contribution we will present briefly some of the science goals, detection strategy, construction plans and current status of the GRAND project.Instituto de Física La Plat

Física de astropartículas de alta energía : Refinamiento de técnicas de simulación y reconstrucción de rayos cósmicos en el Observatorio Pierre Auger

Los rayos cósmicos de energía extrema son partículas subatómicas que llegan a la tierra con energías mayores a 1018 eV. Su flujo es muy reducido (menos de 1 rayo cósmico primario por kilómetro cuadrado y por siglo) y sólo pueden ser detectados a través de las cascadas de partículas que producen tras el impacto con un núcleo de la atmósfera. La determinación del origen, la composición y el espectro energético de los rayos cósmicos de energía extrema representa uno de los mayores retos para la física de astropartículas en la actualidad. Hace ya más de 15 años un grupo de científicos comenzó una colaboración internacional para hacer frente a este reto. Hoy, la Colaboración Pierre Auger reúne a más de 450 autores de 17 países y gestiona la construcción y la operación del observatorio de rayos cósmicos más grande del mundo. En la primera parte de esta Tesis (capítulos 1 y 2) presento una reseña de las características de los rayos cósmicos, el estado actual de conocimiento en este campo y las técnicas empleadas para su detecci ón, haciendo especial hincapié en el Observatorio Pierre Auger y en los resultados publicados por la Colaboración Pierre Auger, de la cual formo parte desde 2004. La detección de rayos cósmicos de energía extrema plantea un enorme desafío científico tecnológico, en el que existen aún muchos problemas que resolver y otros tantos que mejorar. En el inicio del doctorado me interioricé en las simulaciones por computadora de las cascadas de partículas, y un viaje de especialización a Roma me llevó a interesarme por los eventos recolectados por el detector de fluorescencia. En la segunda parte de esta Tesis (capítulos 3 y 4), reporto las mejoras logradas sobre diversos aspectos de los algoritmos utilizados para la simulación de eventos generados por rayos cósmicos de muy alta energía y sobre los algoritmos utilizados para la reconstrucción de los mismos a partir de los datos recolectados por el detector de fluorescencia. Estas mejoras brindan mayor precisión en las medidas experimentales y contribuyen a tener una mejor comprensión del funcionamiento del detector. En particular, en el capítulo 3 presento el desarrollo de un nuevo método de reconstrucci ón que hace uso del tamaño finito del spot óptico de los telescopios de fluorescencia y del tamaño finito de la imagen de la cascada, brindando una mejora importante en la calidad de la reconstrucción y avanzando en la mejor comprensión del funcionamiento de los detectores. En el capítulo 4 presento un estudio detallado del balance energético de las cascadas de partículas iniciadas por rayos cósmicos utilizando simulaciones Monte Carlo y cuál es el uso correcto de las mismas para la simulación y reconstrucción de eventos de fluorescencia. Mientras se finalizaba la construcción del Observatorio, comenzó a tomar forma la idea de disminuir su límite de detección para estudiar la zona del espectro de rayos cósmicos en la que se cree se encuentra la transición entre el origen galáctico y extragal áctico de los mismos. En la tercera parte de esta Tesis (capítulos 5 y 6) presento un nuevo software para la simulación de cascadas subterráneas que desarrolle para cubrir las necesidades de AMIGA, una de las mejoras planeadas para el Observatorio Pierre Auger que incluye la instalación de detectores de muones enterrados a 3m de profundidad. Así, en el capítulo 5 describo la estructura del software desarrollado y presento algunas de las pruebas realizadas para su validación y en el capítulo 6 presento los primeros resultados obtenidos para AMIGA. El software fue desarrollado buscando una gran versatilidad, de manera que pueda ser utilizado para tratar problemas planteados por otros experimentos. Algunos ejemplos de aplicación en otros escenarios, como la simulación de cascadas en hielo para la detección de neutrinos por la técnica de radio, son también presentados en ése capítulo. Para concluir, algunos comentarios generales sobre los resultados presentados en esta Tesis y las perspectivas que abren para futuros estudios fueron volcados en el capítulo 7.Facultad de Ciencias Exacta

Synthesis of radio signals from extensive air showers using previously computed microscopic simulations

The detection of extensive air showers (EAS) through their radio signal is becoming one of the most promising techniques for the study of Neutrinos and Cosmic rays at the highest energies. For the design, optimization and characterization of radio arrays, and of their associated reconstruction algorithms, tens of thousands of Monte Carlo simulations are needed. Current available simulation codes can take several days to compute the signals produced by a single shower, making it impossible to produce the required simulations in a reasonable amount of time, in a cost-effective and environmental-conscious way. In this article we present a method to synthesize the expected signals (the full time trace, not just the peak amplitude) at any point around the shower core, given a set of signals simulated in a finite number of antennas strategically located in a pattern that exploits the signature features of the radio wavefront. The method can be applied indistinctly to the electric field or to the antenna response to the electric field, in the three polarization directions. The synthesized signal can be used to evaluate trigger conditions, compute the fluence or reconstruct the shower incoming direction, allowing for the production of one single library of simulations that can be used and re-used for the characterization and optimization of radio arrays and their associated reconstruction methods, for a thousandth part of the otherwise required CPU time.Instituto de Física La Plat

Cosmic reionization by primordial cosmic rays

Context. After the so-called cosmic recombination, the expanding universe entered into a period of darkness since most of the matter was in a neutral state. About a billion years later, however, the intergalactic space was once again ionized. The process, known as the cosmic reionization, required the operation of mechanisms that are not well understood. Among other ionizing sources, Population III stars, mini-quasars, and X-ray emitting microquasars have been invoked. Aims. We propose that primordial cosmic rays, accelerated at the termination points of the jets of the first microquasars, may have contributed to the reionization of the intergalactic space as well. Methods. We quantify the ionization power of cosmic rays (electrons and protons) in the primordial intergalactic medium. This power is calculated using extensive particle cascade simulations. Results. We establish that, depending on the fraction of electrons to protons accelerated in the microquasar jets, cosmic rays should have contributed to the reionization of the primordial intergalactic medium as much as X-rays from microquasar accretion disks. If the primordial magnetic field was of the order of 10-17 G, as some models suggest, cosmic rays had an important role in ionizing the neutral material far beyond the birth places of the first stars.Facultad de Ciencias Astronómicas y Geofísica

Maximum topological distance-based indices as molecular descriptors for QSPR 3 : Calculation of the hydrophobicity of polyaromatic hydrocarbons

Maximum topological distance-based indices are used together with standard ones to compute the hydrophobicity of polyaromatic hydrocarbons. Several variables and higher-order regression equations are computed and shown to be excellent predictors for the chosen physical-chemistry property. This new alternative offers advantages the usual manner of deriving global topological indices for QSPR. Some possible future extensions are pointed out.Centro de Química Inorgánic

Cosmic reionization by primordial cosmic rays

Context. After the so-called cosmic recombination, the expanding universe entered into a period of darkness since most of the matter was in a neutral state. About a billion years later, however, the intergalactic space was once again ionized. The process, known as the cosmic reionization, required the operation of mechanisms that are not well understood. Among other ionizing sources, Population III stars, mini-quasars, and X-ray emitting microquasars have been invoked. Aims. We propose that primordial cosmic rays, accelerated at the termination points of the jets of the first microquasars, may have contributed to the reionization of the intergalactic space as well. Methods. We quantify the ionization power of cosmic rays (electrons and protons) in the primordial intergalactic medium. This power is calculated using extensive particle cascade simulations. Results. We establish that, depending on the fraction of electrons to protons accelerated in the microquasar jets, cosmic rays should have contributed to the reionization of the primordial intergalactic medium as much as X-rays from microquasar accretion disks. If the primordial magnetic field was of the order of 10-17 G, as some models suggest, cosmic rays had an important role in ionizing the neutral material far beyond the birth places of the first stars.Facultad de Ciencias Astronómicas y Geofísica

Coherent Radiation from Extensive Air Showers in the Ultra-High Frequency Band

Using detailed Monte Carlo simulations we have characterized the features of the radio emission of inclined air showers in the Ultra-High Frequency band (300 MHz - 3 GHz). The Fourier-spectrum of the radiation is shown to have a sizable intensity well into the GHz frequency range. The emission is mainly due to transverse currents induced by the geomagnetic field and to the excess charge produced by the Askaryan effect. At these frequencies only a significantly reduced volume of the shower around the axis contributes coherently to the signal observed on the ground. The size of the coherently emitting volume depends on frequency, shower geometry and observer position, and is interpreted in terms of the relative time delays. At ground level, the maximum emission at high frequencies is concentrated in an elliptical ring-like region around the intersection of a Cherenkov cone with its vertex at shower maximum and the ground. The frequency spectrum of inclined showers when observed at positions that view shower maximum in the Cherenkov direction, is shown to be in broad agreement with the pulses detected by the Antarctic Impulsive Transient Antenna (ANITA) experiment, making the interpretation that they are due to Ultra-High Energy Cosmic Ray atmospheric showers consistent with our simulations. These results are also of great importance for experiments aiming to detect molecular bremsstrahlung radiation in the GHz range as they present an important background for its detection.Comment: 8 pages, 8 figure

Coherent Cherenkov radio pulses from hadronic showers up to EeV energies

The Cherenkov radio pulse emitted by hadronic showers of energies in the EeV range in ice is calculated for the first time using full three dimensional simulations of both shower development and the coherent radio pulse emitted as the excess charge develops in the shower. A Monte Carlo, ZHA ire S, has been developed for this purpose combining the high energy hadronic interaction capabilities of AIRES, and the dense media propagation capabilities of TIERRAS, with the precise low energy tracking and specific algorithms developed to calculate the radio emission in ZHS. A thinning technique is implemented to allow the simulation of radio pulses induced by showers up to 10 EeV in ice. The code is validated comparing the results for electromagnetic and hadronic showers to those obtained with GEANT4 and ZHS codes. The contribution to the pulse of other shower particles in addition to electrons and positrons, mainly protons, pions and muons, is found to be below 3% for 10 PeV and above proton induced showers. The characteristics of hadronic showers and the corresponding Cherenkov frequency spectra are compared with those from purely electromagnetic showers. The dependence of the spectra on shower energy and high-energy hadronic model is addressed and parameterizations for the radio emission in hadronic showers in ice are given for practical applications.Facultad de Ciencias Exacta

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 8 8-+ Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient's position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.La lista completa de autores que integran el documento puede consultarse en el archivo asociado a este ítem.Instituto de Física La Plat