Modeling and observations of relativistic outflows in high-energy binary systems

Abstract

Some binary systems consisting of a compact object, which can be either a neutron star or a black hole, and typically a non-degenerate companion star, have been shown to emit broadband radiation from radio up to gamma-ray frequencies. These systems are normally classified as X-ray or gamma-ray binaries, depending on the frequency at which their emission has its maximum. Unlike with stars, a big part of the observed emission cannot be explained by thermal radiation, and therefore non-thermal radiative processes need to be invoked. The interactions between the star and the compact object may result in the launching of outflows of plasma originating around the compact object position. These outflows can attain speeds close to the speed of light, and be an efficient site for acceleration of charged particles up to relativistic energies. A part of the non-thermal emission observed from X-ray and gamma-ray binaries comes precisely from the non-thermal radiative cooling of these accelerated particles. Additionally, when the companion star is very massive, it produces a strong stellar wind that interacts with the aforementioned relativistic outflows, modifying both their dynamical and radiative evolution.

The main theoretical objective of this thesis is the study the interactions between the outflows of X-ray and gamma- ray binary systems and the stellar wind of a massive companion star. For this purpose, we developed versatile semi- analytical models that give a complete view of these interactions for different kinds of systems. The results of the theoretical modeling include broadband spectral energy distributions and light curves that are directly comparable with the observational data. Radio sky maps are also obtained for the large-scale emission of the outflows. The latter allows to directly visualize the dynamical effect of the stellar wind in the outflow trajectory, which acquires a helical or spiral-like pattern. This modified trajectory gives rise to asymmetries in the light curves at different energy ranges, as well as changes in the spectral energy distributions mostly due to variations of angle-dependent processes influencing the outflow emission.

From the observational point of view, this thesis focuses on the analysis of the potential very high-energy gamma-ray emission above 100 GeV of the X-ray binary MAXI J1820+070, as seen by the MAGIC telescopes. The analysis is done through a custom software developed by MAGIC, which allows to reconstruct the arrival direction and energy of a gamma ray from the Cherenkov light emitted by the electromagnetic cascade that the gamma ray generates when it enters the atmosphere of the Earth. The observational results consist on a multiwavelength study of MAXI J1820+070 in the form of light curves and spectral energy distributions that use data from a number of telescopes at radio, optical, X-ray and gamma-ray frequencies. The source is not detected in gamma-rays above 100 MeV, and only flux upper limits can be given for those energies. Nevertheless, the obtained upper limits, together with the observed fluxes at other frequencies, are enough to constrain significantly the properties of a potential gamma-ray emitter in MAXI J1820+070.

In conclusion, this thesis deepens in our understanding of the interactions between the stellar wind and the outflows of high-energy binary systems. It shows that these interactions must be taken into account in order to properly characterize the subset of those binary systems hosting a massive companion star, in which a powerful stellar wind is present. In this thesis, it is also shown that observations in high-energy and very high-energy gamma rays of X-ray and gamma-ray binary systems allow to set meaningful limits to the outflow properties, even when the sources are not detected and only upper limits in the flux are obtained.

A la nostra Galàxia, existeixen sistemes binaris formats per un objecte compacte i una estrella companya típicament no degenerada que emeten a freqüències des de ràdio fins a raigs gamma. Les interaccions entre l'estrella i l'objecte compacte poden resultar en el llançament d'outflows de plasma originats al voltant de l'objecte compacte, els quals poden adquirir velocitats properes a la de la llum. Quan l'estrella companya és molt massiva, aquesta genera un fort vent estel·lar que interacciona amb els outflows i els afecta tant dinàmicament com radiativa.

El principal objectiu d'aquesta tesi des del punt de vista teòric és l'estudi de les interaccions entre els outflows de sistemes binaris i el vent estel·lar d'una companya massiva. Per això, hem desenvolupat una sèrie de models semi- analítics que donen una visió completa d'aquestes interaccions en diferents tipus d'escenaris. Els resultats de la modelització teòrica inclouen prediccions per a les distribucions espectrals d'energia i corbes de llum, així com mapes ràdio per a l'emissió a gran escala dels outflows.

En relació a la part observacional, aquesta tesi es focalitza en l'anàlisi de l'emissió potencial de raigs gamma de molt alta energia de la binària MAXI J1820+070, observada amb els telescopis MAGIC. Els resultats observacionals consisteixen en un estudi multifreqüència de la font de ràdia a raigs gamma. Tot i que la font no es detecta per sobre dels 100 MeV, els resultats obtinguts permeten limitar les propietats d'un emissor potencial de raigs gamma a MAXI J1820+070.

En conclusió, aquesta tesi profunditza en el coneixement de les interaccions entre el vent estel·lar i els outflows de sistemes binaris d'altes energies. També es conclou que les observacions en raigs gamma de sistemes binaris permeten limitar de manera important les propietats dels outflows, fins i tot quan les fonts no son detectades.