Microquasar jets and their interaction with the medium: theoretical studies and observations with the MAGIC telescope

[eng] The present thesis focuses on the investigation of some of the high energy processes ocurring in microquasars. The matter content and the power transported in the jets of microquasars are studied, the effects that they have on the surrounding medium are quantified, and their expected capability to emit at high end very high energies is constrained. Since these are quite broad objectives, we have treated them separatedly. The thesis is therefore divided in two distinct parts, preceeded by a short overview of the microquasar phenomenology and a characterization of the physical mechanisms that will be needed further on (Chapters 1 to 3). Although we note that other processes may occur in microquasars, our description is however limited to those directly related to the problems faced in some specific scenarios. In Part I (Chapters 4 and 5) we explore the interactions that microquasar jets have with their environments. In Chapter 4 the interactions of microquasar jets with the wind and radiation fields of the companinon star are explored. We investigate the radiative consequences of the presence of relativistic hadrons in the jets. We study the primary hadronic emission in the gamma-ray domain, and we then focus on the secondary particles generated in proton-proton interactions. We analyse how the detection of the emission of these secondaries could point to the presence of hadrons in the jet. We consider a high-mass system with a constant energy injection rate in a circular orbit, but our results can be extended to more realistic scenarios. After inspection of the obtained results, we conclude that the non-thermal radiation output from secondaries alone cannot account for the radio and X-ray flux levels observed in microquasars. The existence of a primary lepton population seems therefore to be required. In Chapter 5 we examine the impact of microquasar jets with the interstellar medium (ISM). Extended emitting structures are thought to be formed when the jets are effectively decelerated by the surrounding gas. The situation is analogous to that found in AGNs, where radio-lobes and hot spots are frequently observed at the jet termination regions. The jet composition is difficult to constrain in these studies. Both primary hadronic and e+e− plasmas can provoke similar effects in their surroundings. Our model can provide an estimate of the total power carried in the jets when the source age and the medium properties are known. More importantly, these studies are independent from other approaches in which the jet kinetic power is derived directly from its radiative output (in radio, assuming minimum energy arguments and rough equipartition of the magnetic field; at higher energies, by using the observed flux and assuming a certain constant fraction of jet power converted to non-thermal power). They provide therefore constraints to be applied in all cases where jet/ISM interactions occur. It must be noted in this sense that an increasing number of microquasars do indeed show evidences of such interactions. In Part II (Chatpers 6 to 8) we present the results of the search of VHE gammaray emission from microquasars. We make use of the exceptional capabilities of the MAGIC telescope for this purpose. The Cherenkov Imaging technique has become the most successful method to detect gamma-ray emission from ∼ 100 GeV up to 10 TeV. Four well-known microquasars have been observed by MAGIC, namely Cyg X-1, Cyg X-3, GRS 1915+105 and SS 433. No evidence of steady signal have been found, whereas an evidence of detection of a flare was found only from Cyg X- 1 in September 2006. Particular source conditions have been used to trigger the observations. They were based on the presence of the source in the low-hard state in which radio jets are thought to be present, and in state transitions, when strong radio/X-ray flares can be produced. The MAGIC upper limits are compared to recent theoretical models predicting gamma-rays from the observed sources. We study in detail several physical mechanisms in the light of the collected results in both hadronic and leptonic scenarios. Conclusions derived from the attained results are included separatedly in each Part, and we close this thesis with some concluding remarks of the main outcomes obtained, which are presented in Chapter 9.[cat] Aquesta tesi recull un treball d’investigació centrat en la natura dels “jets” dels microquàsars, la seva interacció amb l’entorn i la capacitat d’emetre raigs gamma de molt alta energia. Per a aconseguir prediccions concretes ha estat necessari estudiar la física dels jets en detall: l’energia que transporten, la matèria que els constitueix, el medi que els envolta o en el qual es propaguen (camps de matèria, magnètics i de radiació), així com els processos d’acceleració que s’hi donen. El context astrofísic actual situa els microquàsars en un encreuament on diverses disciplines hi centren l’atenció, des de l’estudi dels sistemes binaris amb una companya massiva que esdevé un objecte compacte (una estrella de neutrons o un forat negre) fins la comprensió dels orígens dels raigs còsmics que arriben a la terra des de totes les direccions