Multi-Dimensional Simulations of the Expanding Supernova Remnant of SN 1987A

The expanding remnant from SN 1987A is an excellent laboratory for investigating the physics of supernovae explosions. There is still a large number of outstanding questions, such as the reason for the asymmetric radio morphology, the structure of the pre-supernova environment, and the efficiency of particle acceleration at the supernova shock. We explore these questions using three-dimensional simulations of the expanding remnant between days 820 and 10,000 after the supernova. We combine a hydrodynamical simulation with semi-analytic treatments of diffusive shock acceleration and magnetic field amplification to derive radio emission as part of an inverse problem. Simulations show that an asymmetric explosion, combined with magnetic field amplification at the expanding shock, is able to replicate the persistent one-sided radio morphology of the remnant. We use an asymmetric Truelove & McKee progenitor with an envelope mass of 10 M-circle dot and an energy of 1.5 x 10(44) J. A termination shock in the progenitor's stellar wind at a distance of 0 ''.43-0 ''.51 provides a good fit to the turn on of radio emission around day 1200. For the H II region, a minimum distance of 0 ''.63 +/- 0 ''.01 and maximum particle number density of (7.11 +/- 1.78) x 10(7) m(-3) produces a good fit to the evolving average radius and velocity of the expanding shocks from day 2000 to day 7000 after explosion. The model predicts a noticeable reduction, and possibly a temporary reversal, in the asymmetric radio morphology of the remnant after day 7000, when the forward shock left the eastern lobe of the equatorial ring.

The TARANIS laser : A multi-terawatt system for laser plasma physics

The Terawatt Apparatus for Relativistic And Non-linear Interdisciplinary Science (TARANIS), installed in the Centre for Plasma Physics at the Queen's University Belfast, supports a wide ranging science program, including laser-driven particle acceleration, X-ray lasers and high energy density physics experiments. We present (1) an overview of the laser facility, (2) results of preliminary investigations on proton acceleration, laser action at 13.9 nm and Kα sources and (3) speculation on future experiments using these extreme sources.

Alfven waves as a driving mechanism in stellar winds

Alfven waves have been invoked as an important mechanism of particle acceleration in stellar winds of cool stars. After their identification in the solar wind they started to be studied in winds of stars located in different regions of the FIR diagram. We discuss here some characteristics of these waves and we present a direct application in the acceleration of late-type stellar winds. (C) 2009 COSPAR. Published by Elsevier Ltd. All rights reserved.

MAGNETIC FIELD EFFECTS NEAR THE LAUNCHING REGION OF ASTROPHYSICAL JETS

One of the fundamental properties of astrophysical magnetic fields is their ability to change topology through reconnection and in doing so, to release magnetic energy, sometimes violently. In this work, we review recent results on the role of magnetic reconnection and associated heating and particle acceleration in jet/accretion disk systems, namely young stellar objects (YSOs), microquasars, and active galactic nuclei (AGNs).

Velocity-dependent KAM islands

We revisit the non-dissipative time-dependent annular billiard and we consider the chaotic dynamics in two planes of conjugate variables in order to describe the behavior of the growth, or saturation, of the mean velocity of an ensemble of particles. We observed that the changes in the 4-d phase space occur without changing any parameter. They occur depending on where the initial conditions start. The emerging KAM islands interfere in the behavior of the particle dynamics especially in the Fermi acceleration mechanism. We show that Fermi acceleration can be suppressed, without dissipation, even considering the non-dissipative energy context. (C) 2011 Published by Elsevier Ltd.

Reconnection current sheet structure in a turbulent medium

In the presence of turbulence, magnetic field lines lose their dynamical identity and particles entrained on field lines diffuse through space at a rate determined by the amplitude of the turbulence. In previous work (Lazarian and Vishniac, 1999; Kowal et al., 2009; Eyink et al., 2011) we showed that this leads to reconnection speeds which are independent of resistivity. In particular, in Kowal et al. (2009) we showed that numerical simulations were consistent with the predictions of this model. Here we examine the structure of the current sheet in simulations of turbulent reconnection. Laminar flows consistent with the Sweet-Parker reconnection model produce very thin and well ordered currents sheets. On the other hand, the simulations of Kowal et al. (2009) show a strongly disordered state even for relatively low levels of turbulence. Comparing data cubes with and without reconnection, we find that large scale field reversals are the cumulative effect of many individual eddies, each of which has magnetic properties which are not very different from turbulent eddies in a homogeneous background. This implies that the properties of stationary and homogeneous MHD turbulence are a reasonable guide to understanding turbulence during large scale magnetic reconnection events. In addition, dissipation and high energy particle acceleration during reconnection events take place over a macroscopic volume, rather than being confined to a narrow zone whose properties depend on microscopic transport coefficients.

MHD numerical simulations of colliding winds in massive binary systems - I. Thermal versus non-thermal radio emission

In the past few decades detailed observations of radio and X-ray emission from massive binary systems revealed a whole new physics present in such systems. Both thermal and non-thermal components of this emission indicate that most of the radiation at these bands originates in shocks. O and B-type stars and WolfRayet (WR) stars present supersonic and massive winds that, when colliding, emit largely due to the freefree radiation. The non-thermal radio and X-ray emissions are due to synchrotron and inverse Compton processes, respectively. In this case, magnetic fields are expected to play an important role in the emission distribution. In the past few years the modelling of the freefree and synchrotron emissions from massive binary systems have been based on purely hydrodynamical simulations, and ad hoc assumptions regarding the distribution of magnetic energy and the field geometry. In this work we provide the first full magnetohydrodynamic numerical simulations of windwind collision in massive binary systems. We study the freefree emission characterizing its dependence on the stellar and orbital parameters. We also study self-consistently the evolution of the magnetic field at the shock region, obtaining also the synchrotron energy distribution integrated along different lines of sight. We show that the magnetic field in the shocks is larger than that obtained when the proportionality between B and the plasma density is assumed. Also, we show that the role of the synchrotron emission relative to the total radio emission has been underestimated.

Reconnection studies under different types of turbulence driving

We study a model of fast magnetic reconnection in the presence of weak turbulence proposed by Lazarian and Vishniac (1999) using three-dimensional direct numerical simulations. The model has been already successfully tested in Kowal et al. (2009) confirming the dependencies of the reconnection speed V-rec on the turbulence injection power P-inj and the injection scale l(inj) expressed by a constraint V-rec similar to P(inj)(1/2)l(inj)(3/4)and no observed dependency on Ohmic resistivity. In Kowal et al. (2009), in order to drive turbulence, we injected velocity fluctuations in Fourier space with frequencies concentrated around k(inj) = 1/l(inj), as described in Alvelius (1999). In this paper, we extend our previous studies by comparing fast magnetic reconnection under different mechanisms of turbulence injection by introducing a new way of turbulence driving. The new method injects velocity or magnetic eddies with a specified amplitude and scale in random locations directly in real space. We provide exact relations between the eddy parameters and turbulent power and injection scale. We performed simulations with new forcing in order to study turbulent power and injection scale dependencies. The results show no discrepancy between models with two different methods of turbulence driving exposing the same scalings in both cases. This is in agreement with the Lazarian and Vishniac (1999) predictions. In addition, we performed a series of models with varying viscosity nu. Although Lazarian and Vishniac (1999) do not provide any prediction for this dependence, we report a weak relation between the reconnection speed with viscosity, V-rec similar to nu(-1/4).

CENTAURUS A: THE 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) to 3 x 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 PeV to near ZeV energy range. Data are available over this entire energy range from the KASCADE, KASCADE-Grande, and Pierre Auger Observatory experiments. 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 x 10(19) eV, the Greisen-Zatsepin-Kuz'min turnoff 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.

Untersuchungen zu Mechanismen der Laser-Teilchenbeschleunigung

Auf dem Gebiet der Teilchenbeschleunigung mittels Hochintensitäts-Lasern wurden in der letzten Dekade viele erfolgreiche Entwicklungen hin zu immer höheren Energien und größeren Teilchenzahlen veröffentlicht. In den meisten Fällen wurde der sogenannte TNSA-Prozess (engl. Target-Normal-Sheath-Acceleration (TNSA)) untersucht. Bei diesem Prozess erfolgt die Beschleunigung in dem an der Oberfläche durch Ladungstrennung erzeugten Potential. Ein kaum vermeidbares Problem ist hierbei das resultierende breite Energie-Spektrum der beschleunigten Teilchen. Diese Situation konnte in den letzten Jahren zwar verbessert, aber nicht vollständig gelöst werden. Für Intensitäten größer 10^(20..21) W/cm^2 sagen theoretische Modellrechnungen eine auf dem Lichtdruck basierende Beschleunigung (engl. Radiation-Pressure-Acceleration (RPA)) mit deutlich eingegrenztem, fast monoenergetischem Spektrum voraus. Im Rahmen dieser Arbeit wurde ein Experiment zur Untersuchung dieses Prozesses bei Intensitäten von einigen 10^19 W/cm^2 durchgeführt. Dazu wurden zunächst spezielle Targets entwickelt und als Patent angemeldet, welche den Experimentbedingungen angepasst sind. Durch die Auslegung des experimentellen Aufbaus und der Diagnostiken auf hohe Repetitionsraten, in Verbindung mit einem geeigneten Lasersystem, konnte auf Basis einer Statistik von mehreren Tausend Schüssen ein großer Parameterraum untersucht werden. Untersucht wurden unter anderem die Abhängigkeit von Targetmaterial und Dicke, Intensität, Laserpolarisation und Vorplasmabedingungen. Aus den gewonnenen Daten und Vergleichen mit 2-dimensionalen numerischen Simulationen konnte ein Modell des Beschleunigungsprozesses aufgestellt und durch Vergleich mit den experimentellen Ergebnissen geprüft werden. Dabei wurden klare Indizien für die Existenz eines neuen, nicht feldinduzierten, Beschleunigungsprozesses gefunden. Darüber hinaus wurde zur Polarisationsbeeinflussung ein optisches System entwickelt, das ausschließlich mit reflexiven Elementen arbeitet. Damit konnten viele Nachteile bestehender, auf Verzögerungsplatten beruhender Elemente vermieden, und die Anwendbarkeit bei hohen Laserenergien erreicht werden.

Radio relics and magnetic fields in galaxy clusters

Radio relics are diffuse synchrotron sources generally located in the peripheries of galaxy clusters in merging state. According to the current leading scenario, relics trace gigantic cosmological shock waves that cross the intra-cluster medium where particle acceleration occurs. The relic/shock connection is supported by several observational facts, including the spatial coincidence between relics and shocks found in the X-rays. Under the assumptions that particles are accelerated at the shock front and are subsequently deposited and then age downstream of the shock, Markevitch et al. (2005) proposed a method to constrain the magnetic field strength in radio relics. Measuring the thickness of radio relics at different frequencies allows to derive combined constraints on the velocity of the downstream flow and on the magnetic field, which in turns determines particle aging. We elaborate this idea to infer first constraints on magnetic fields in cluster outskirts. We consider three models of particle aging and develop a geometric model to take into account the contribution to the relic transverse size due to the projection of the shock-surface on the plane of the sky. We selected three well studied radio relics in the clusters A 521, CIZA J2242.8+5301 and 1RXS J0603.3+4214. These relics have been chosen primarily because they are almost seen edge-on and because the Mach number of the shock that is associated with these relics is measured by X-ray observations, thus allowing to break the degeneracy between magnetic field and downstream velocity in the method. For the first two clusters, our method is consistent with a pure radiative aging model allowing us to derive constraints on the relics magnetic field strength. In the case of 1RXS J0603.3+4214 we find that particle life-times are consistent with a pure radiative aging model under some conditions, however we also collect evidences for downstream particle re-acceleration in the relic W-region and for a magnetic field decaying downstream in its E-region. Our estimates of the magnetic field strength in the relics in A 521 and CIZA J2242.8+5301 provide unique information on the field properties in cluster outskirts. The constraints derived for these relics, together with the lower limits to the magnetic field that we derived from the lack of inverse Compton X-ray emission from the sources, have been combined with the constraints from Faraday rotation studies of the Coma cluster. Overall results suggest that the spatial profile of the magnetic field energy density is broader than that of the thermal gas, implying that the ε_th /ε_B ratio decreases with cluster radius. Alternatively, radio relics could trace dynamically active regions where the magnetic field strength is biased high with respect to the average value in the cluster volume.

A new analog trigger system for the Cherenkov Telescope array

La astronomía de rayos γ estudia las partículas más energéticas que llegan a la Tierra desde el espacio. Estos rayos γ no se generan mediante procesos térmicos en simples estrellas, sino mediante mecanismos de aceleración de partículas en objetos celestes como núcleos de galaxias activos, púlsares, supernovas, o posibles procesos de aniquilación de materia oscura. Los rayos γ procedentes de estos objetos y sus características proporcionan una valiosa información con la que los científicos tratan de comprender los procesos físicos que ocurren en ellos y desarrollar modelos teóricos que describan su funcionamiento con fidelidad. El problema de observar rayos γ es que son absorbidos por las capas altas de la atmósfera y no llegan a la superficie (de lo contrario, la Tierra será inhabitable). De este modo, sólo hay dos formas de observar rayos γ embarcar detectores en satélites, u observar los efectos secundarios que los rayos γ producen en la atmósfera. Cuando un rayo γ llega a la atmósfera, interacciona con las partículas del aire y genera un par electrón - positrón, con mucha energía. Estas partículas secundarias generan a su vez más partículas secundarias cada vez menos energéticas. Estas partículas, mientras aún tienen energía suficiente para viajar más rápido que la velocidad de la luz en el aire, producen una radiación luminosa azulada conocida como radiación Cherenkov durante unos pocos nanosegundos. Desde la superficie de la Tierra, algunos telescopios especiales, conocidos como telescopios Cherenkov o IACTs (Imaging Atmospheric Cherenkov Telescopes), son capaces de detectar la radiación Cherenkov e incluso de tomar imágenes de la forma de la cascada Cherenkov. A partir de estas imágenes es posible conocer las principales características del rayo γ original, y con suficientes rayos se pueden deducir características importantes del objeto que los emitió, a cientos de años luz de distancia. Sin embargo, detectar cascadas Cherenkov procedentes de rayos γ no es nada fácil. Las cascadas generadas por fotones γ de bajas energías emiten pocos fotones, y durante pocos nanosegundos, y las correspondientes a rayos γ de alta energía, si bien producen más electrones y duran más, son más improbables conforme mayor es su energía. Esto produce dos líneas de desarrollo de telescopios Cherenkov: Para observar cascadas de bajas energías son necesarios grandes reflectores que recuperen muchos fotones de los pocos que tienen estas cascadas. Por el contrario, las cascadas de altas energías se pueden detectar con telescopios pequeños, pero conviene cubrir con ellos una superficie grande en el suelo para aumentar el número de eventos detectados. Con el objetivo de mejorar la sensibilidad de los telescopios Cherenkov actuales, en el rango de energía alto (> 10 TeV), medio (100 GeV - 10 TeV) y bajo (10 GeV - 100 GeV), nació el proyecto CTA (Cherenkov Telescope Array). Este proyecto en el que participan más de 27 países, pretende construir un observatorio en cada hemisferio, cada uno de los cuales contará con 4 telescopios grandes (LSTs), unos 30 medianos (MSTs) y hasta 70 pequeños (SSTs). Con un array así, se conseguirán dos objetivos. En primer lugar, al aumentar drásticamente el área de colección respecto a los IACTs actuales, se detectarán más rayos γ en todos los rangos de energía. En segundo lugar, cuando una misma cascada Cherenkov es observada por varios telescopios a la vez, es posible analizarla con mucha más precisión gracias a las técnicas estereoscópicas. La presente tesis recoge varios desarrollos técnicos realizados como aportación a los telescopios medianos y grandes de CTA, concretamente al sistema de trigger. Al ser las cascadas Cherenkov tan breves, los sistemas que digitalizan y leen los datos de cada píxel tienen que funcionar a frecuencias muy altas (≈1 GHz), lo que hace inviable que funcionen de forma continua, ya que la cantidad de datos guardada será inmanejable. En su lugar, las señales analógicas se muestrean, guardando las muestras analógicas en un buffer circular de unos pocos µs. Mientras las señales se mantienen en el buffer, el sistema de trigger hace un análisis rápido de las señales recibidas, y decide si la imagen que hay en el buér corresponde a una cascada Cherenkov y merece ser guardada, o por el contrario puede ignorarse permitiendo que el buffer se sobreescriba. La decisión de si la imagen merece ser guardada o no, se basa en que las cascadas Cherenkov producen detecciones de fotones en píxeles cercanos y en tiempos muy próximos, a diferencia de los fotones de NSB (night sky background), que llegan aleatoriamente. Para detectar cascadas grandes es suficiente con comprobar que más de un cierto número de píxeles en una región hayan detectado más de un cierto número de fotones en una ventana de tiempo de algunos nanosegundos. Sin embargo, para detectar cascadas pequeñas es más conveniente tener en cuenta cuántos fotones han sido detectados en cada píxel (técnica conocida como sumtrigger). El sistema de trigger desarrollado en esta tesis pretende optimizar la sensibilidad a bajas energías, por lo que suma analógicamente las señales recibidas en cada píxel en una región de trigger y compara el resultado con un umbral directamente expresable en fotones detectados (fotoelectrones). El sistema diseñado permite utilizar regiones de trigger de tamaño seleccionable entre 14, 21 o 28 píxeles (2, 3, o 4 clusters de 7 píxeles cada uno), y con un alto grado de solapamiento entre ellas. De este modo, cualquier exceso de luz en una región compacta de 14, 21 o 28 píxeles es detectado y genera un pulso de trigger. En la versión más básica del sistema de trigger, este pulso se distribuye por toda la cámara de forma que todos los clusters sean leídos al mismo tiempo, independientemente de su posición en la cámara, a través de un delicado sistema de distribución. De este modo, el sistema de trigger guarda una imagen completa de la cámara cada vez que se supera el número de fotones establecido como umbral en una región de trigger. Sin embargo, esta forma de operar tiene dos inconvenientes principales. En primer lugar, la cascada casi siempre ocupa sólo una pequeña zona de la cámara, por lo que se guardan muchos píxeles sin información alguna. Cuando se tienen muchos telescopios como será el caso de CTA, la cantidad de información inútil almacenada por este motivo puede ser muy considerable. Por otro lado, cada trigger supone guardar unos pocos nanosegundos alrededor del instante de disparo. Sin embargo, en el caso de cascadas grandes la duración de las mismas puede ser bastante mayor, perdiéndose parte de la información debido al truncamiento temporal. Para resolver ambos problemas se ha propuesto un esquema de trigger y lectura basado en dos umbrales. El umbral alto decide si hay un evento en la cámara y, en caso positivo, sólo las regiones de trigger que superan el nivel bajo son leídas, durante un tiempo más largo. De este modo se evita guardar información de píxeles vacíos y las imágenes fijas de las cascadas se pueden convertir en pequeños \vídeos" que representen el desarrollo temporal de la cascada. Este nuevo esquema recibe el nombre de COLIBRI (Concept for an Optimized Local Image Building and Readout Infrastructure), y se ha descrito detalladamente en el capítulo 5. Un problema importante que afecta a los esquemas de sumtrigger como el que se presenta en esta tesis es que para sumar adecuadamente las señales provenientes de cada píxel, estas deben tardar lo mismo en llegar al sumador. Los fotomultiplicadores utilizados en cada píxel introducen diferentes retardos que deben compensarse para realizar las sumas adecuadamente. El efecto de estos retardos ha sido estudiado, y se ha desarrollado un sistema para compensarlos. Por último, el siguiente nivel de los sistemas de trigger para distinguir efectivamente las cascadas Cherenkov del NSB consiste en buscar triggers simultáneos (o en tiempos muy próximos) en telescopios vecinos. Con esta función, junto con otras de interfaz entre sistemas, se ha desarrollado un sistema denominado Trigger Interface Board (TIB). Este sistema consta de un módulo que irá montado en la cámara de cada LST o MST, y que estará conectado mediante fibras ópticas a los telescopios vecinos. Cuando un telescopio tiene un trigger local, este se envía a todos los vecinos conectados y viceversa, de modo que cada telescopio sabe si sus vecinos han dado trigger. Una vez compensadas las diferencias de retardo debidas a la propagación en las fibras ópticas y de los propios fotones Cherenkov en el aire dependiendo de la dirección de apuntamiento, se buscan coincidencias, y en el caso de que la condición de trigger se cumpla, se lee la cámara en cuestión, de forma sincronizada con el trigger local. Aunque todo el sistema de trigger es fruto de la colaboración entre varios grupos, fundamentalmente IFAE, CIEMAT, ICC-UB y UCM en España, con la ayuda de grupos franceses y japoneses, el núcleo de esta tesis son el Level 1 y la Trigger Interface Board, que son los dos sistemas en los que que el autor ha sido el ingeniero principal. Por este motivo, en la presente tesis se ha incluido abundante información técnica relativa a estos sistemas. Existen actualmente importantes líneas de desarrollo futuras relativas tanto al trigger de la cámara (implementación en ASICs), como al trigger entre telescopios (trigger topológico), que darán lugar a interesantes mejoras sobre los diseños actuales durante los próximos años, y que con suerte serán de provecho para toda la comunidad científica participante en CTA. ABSTRACT -ray astronomy studies the most energetic particles arriving to the Earth from outer space. This -rays are not generated by thermal processes in mere stars, but by means of particle acceleration mechanisms in astronomical objects such as active galactic nuclei, pulsars, supernovas or as a result of dark matter annihilation processes. The γ rays coming from these objects and their characteristics provide with valuable information to the scientist which try to understand the underlying physical fundamentals of these objects, as well as to develop theoretical models able to describe them accurately. The problem when observing rays is that they are absorbed in the highest layers of the atmosphere, so they don't reach the Earth surface (otherwise the planet would be uninhabitable). Therefore, there are only two possible ways to observe γ rays: by using detectors on-board of satellites, or by observing their secondary effects in the atmosphere. When a γ ray reaches the atmosphere, it interacts with the particles in the air generating a highly energetic electron-positron pair. These secondary particles generate in turn more particles, with less energy each time. While these particles are still energetic enough to travel faster than the speed of light in the air, they produce a bluish radiation known as Cherenkov light during a few nanoseconds. From the Earth surface, some special telescopes known as Cherenkov telescopes or IACTs (Imaging Atmospheric Cherenkov Telescopes), are able to detect the Cherenkov light and even to take images of the Cherenkov showers. From these images it is possible to know the main parameters of the original -ray, and with some -rays it is possible to deduce important characteristics of the emitting object, hundreds of light-years away. However, detecting Cherenkov showers generated by γ rays is not a simple task. The showers generated by low energy -rays contain few photons and last few nanoseconds, while the ones corresponding to high energy -rays, having more photons and lasting more time, are much more unlikely. This results in two clearly differentiated development lines for IACTs: In order to detect low energy showers, big reflectors are required to collect as much photons as possible from the few ones that these showers have. On the contrary, small telescopes are able to detect high energy showers, but a large area in the ground should be covered to increase the number of detected events. With the aim to improve the sensitivity of current Cherenkov showers in the high (> 10 TeV), medium (100 GeV - 10 TeV) and low (10 GeV - 100 GeV) energy ranges, the CTA (Cherenkov Telescope Array) project was created. This project, with more than 27 participating countries, intends to build an observatory in each hemisphere, each one equipped with 4 large size telescopes (LSTs), around 30 middle size telescopes (MSTs) and up to 70 small size telescopes (SSTs). With such an array, two targets would be achieved. First, the drastic increment in the collection area with respect to current IACTs will lead to detect more -rays in all the energy ranges. Secondly, when a Cherenkov shower is observed by several telescopes at the same time, it is possible to analyze it much more accurately thanks to the stereoscopic techniques. The present thesis gathers several technical developments for the trigger system of the medium and large size telescopes of CTA. As the Cherenkov showers are so short, the digitization and readout systems corresponding to each pixel must work at very high frequencies (_ 1 GHz). This makes unfeasible to read data continuously, because the amount of data would be unmanageable. Instead, the analog signals are sampled, storing the analog samples in a temporal ring buffer able to store up to a few _s. While the signals remain in the buffer, the trigger system performs a fast analysis of the signals and decides if the image in the buffer corresponds to a Cherenkov shower and deserves to be stored, or on the contrary it can be ignored allowing the buffer to be overwritten. The decision of saving the image or not, is based on the fact that Cherenkov showers produce photon detections in close pixels during near times, in contrast to the random arrival of the NSB phtotons. Checking if more than a certain number of pixels in a trigger region have detected more than a certain number of photons during a certain time window is enough to detect large showers. However, taking also into account how many photons have been detected in each pixel (sumtrigger technique) is more convenient to optimize the sensitivity to low energy showers. The developed trigger system presented in this thesis intends to optimize the sensitivity to low energy showers, so it performs the analog addition of the signals received in each pixel in the trigger region and compares the sum with a threshold which can be directly expressed as a number of detected photons (photoelectrons). The trigger system allows to select trigger regions of 14, 21, or 28 pixels (2, 3 or 4 clusters with 7 pixels each), and with extensive overlapping. In this way, every light increment inside a compact region of 14, 21 or 28 pixels is detected, and a trigger pulse is generated. In the most basic version of the trigger system, this pulse is just distributed throughout the camera in such a way that all the clusters are read at the same time, independently from their position in the camera, by means of a complex distribution system. Thus, the readout saves a complete camera image whenever the number of photoelectrons set as threshold is exceeded in a trigger region. However, this way of operating has two important drawbacks. First, the shower usually covers only a little part of the camera, so many pixels without relevant information are stored. When there are many telescopes as will be the case of CTA, the amount of useless stored information can be very high. On the other hand, with every trigger only some nanoseconds of information around the trigger time are stored. In the case of large showers, the duration of the shower can be quite larger, loosing information due to the temporal cut. With the aim to solve both limitations, a trigger and readout scheme based on two thresholds has been proposed. The high threshold decides if there is a relevant event in the camera, and in the positive case, only the trigger regions exceeding the low threshold are read, during a longer time. In this way, the information from empty pixels is not stored and the fixed images of the showers become to little \`videos" containing the temporal development of the shower. This new scheme is named COLIBRI (Concept for an Optimized Local Image Building and Readout Infrastructure), and it has been described in depth in chapter 5. An important problem affecting sumtrigger schemes like the one presented in this thesis is that in order to add the signals from each pixel properly, they must arrive at the same time. The photomultipliers used in each pixel introduce different delays which must be compensated to perform the additions properly. The effect of these delays has been analyzed, and a delay compensation system has been developed. The next trigger level consists of looking for simultaneous (or very near in time) triggers in neighbour telescopes. These function, together with others relating to interfacing different systems, have been developed in a system named Trigger Interface Board (TIB). This system is comprised of one module which will be placed inside the LSTs and MSTs cameras, and which will be connected to the neighbour telescopes through optical fibers. When a telescope receives a local trigger, it is resent to all the connected neighbours and vice-versa, so every telescope knows if its neighbours have been triggered. Once compensated the delay differences due to propagation in the optical fibers and in the air depending on the pointing direction, the TIB looks for coincidences, and in the case that the trigger condition is accomplished, the camera is read a fixed time after the local trigger arrived. Despite all the trigger system is the result of the cooperation of several groups, specially IFAE, Ciemat, ICC-UB and UCM in Spain, with some help from french and japanese groups, the Level 1 and the Trigger Interface Board constitute the core of this thesis, as they have been the two systems designed by the author of the thesis. For this reason, a large amount of technical information about these systems has been included. There are important future development lines regarding both the camera trigger (implementation in ASICS) and the stereo trigger (topological trigger), which will produce interesting improvements for the current designs during the following years, being useful for all the scientific community participating in CTA.

Modeling nonthermal emission from stellar bow shocks (Research Note)

Context. Runaway O- and early B-type stars passing through the interstellar medium at supersonic velocities and characterized by strong stellar winds may produce bow shocks that can serve as particle acceleration sites. Previous theoretical models predict the production of high-energy photons by nonthermal radiative processes, but their efficiency is still debated. Aims. We aim to test and explain the possibility of emission from the bow shocks formed by runaway stars traveling through the interstellar medium by using previous theoretical models. Methods. We applied our model to AE Aurigae, the first reported star with an X-ray detected bow shock, to BD+43 3654, in which the observations failed in detecting high-energy emission, and to the transition phase of a supergiant star in the late stages of its life. Results. From our analysis, we confirm that the X-ray emission from the bow shock produced by AE Aurigae can be explained by inverse Compton processes involving the infrared photons of the heated dust. We also predict low high-energy flux emission from the bow shock produced by BD+43 3654, and the possibility of high-energy emission from the bow shock formed by a supergiant star during the transition phase from blue to red supergiant. Conclusions. Bow shocks formed by different types of runaway stars are revealed as a new possible source of high-energy photons in our neighborhood.

Modeling nonthermal emission from stellar bow shocks (Research Note)

Context. Runaway O- and early B-type stars passing through the interstellar medium at supersonic velocities and characterized by strong stellar winds may produce bow shocks that can serve as particle acceleration sites. Previous theoretical models predict the production of high-energy photons by nonthermal radiative processes, but their efficiency is still debated. Aims. We aim to test and explain the possibility of emission from the bow shocks formed by runaway stars traveling through the interstellar medium by using previous theoretical models. Methods. We applied our model to AE Aurigae, the first reported star with an X-ray detected bow shock, to BD+43 3654, in which the observations failed in detecting high-energy emission, and to the transition phase of a supergiant star in the late stages of its life. Results. From our analysis, we confirm that the X-ray emission from the bow shock produced by AE Aurigae can be explained by inverse Compton processes involving the infrared photons of the heated dust. We also predict low high-energy flux emission from the bow shock produced by BD+43 3654, and the possibility of high-energy emission from the bow shock formed by a supergiant star during the transition phase from blue to red supergiant. Conclusions. Bow shocks formed by different types of runaway stars are revealed as a new possible source of high-energy photons in our neighborhood.

Astrophysical parameters of LS 2883 and implications for the PSR B1259-63 gamma-ray binary

Only a few binary systems with compact objects display TeV emission. The physical properties of the companion stars represent basic input for understanding the physical mechanisms behind the particle acceleration, emission, and absorption processes in these so-called gamma-ray binaries. Here we present high-resolution and high signal-to-noise optical spectra of LS 2883, the Be star forming a gamma-ray binary with the young non-accreting pulsar PSR B1259-63, showing it to rotate faster and be significantly earlier and more luminous than previously thought. Analysis of the interstellar lines suggests that the system is located at the same distance as (and thus is likely a member of) Cen OB1. Taking the distance to the association, d = 2.3 kpc, and a color excess of E(B – V) = 0.85 for LS 2883 results in MV ≈ –4.4. Because of fast rotation, LS 2883 is oblate (R eq sime 9.7 R ☉ and R pole sime 8.1 R ☉) and presents a temperature gradient (T eq≈ 27,500 K, log g eq = 3.7; T pole≈ 34,000 K, log g pole = 4.1). If the star did not rotate, it would have parameters corresponding to a late O-type star. We estimate its luminosity at log(L */L ☉) sime 4.79 and its mass at M * ≈ 30 M ☉. The mass function then implies an inclination of the binary system i orb ≈ 23°, slightly smaller than previous estimates. We discuss the implications of these new astrophysical parameters of LS 2883 for the production of high-energy and very high-energy gamma rays in the PSR B1259-63/LS 2883 gamma-ray binary system. In particular, the stellar properties are very important for prediction of the line-like bulk Comptonization component from the unshocked ultrarelativistic pulsar wind.