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Campos Magnéticos Em Afterglows De Gamma-Ray Bursts

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Campos Magnéticos Em Afterglows De Gamma-Ray Bursts Universidade de S˜ao Paulo Instituto de Astronomia, Geof´ısica e Ciˆencias Atmosf´ericas Departamento de Astronomia Gustavo Rocha da Silva Campos Magn´eticos em Afterglows de Gamma-Ray Bursts S˜ao Paulo 2009 Gustavo Rocha da Silva Campos Magn´eticos em Afterglows de Gamma-Ray Bursts Disserta¸c˜ao apresentada ao Departamento de Astronomia do Instituto de Astronomia, Geof´ısica e Ciˆencias Atmosf´ericas da Universidade de S˜ao Paulo como parte dos requisitos para a obten¸c˜ao do t´ıtulo de Mestre em Ciˆencias. Area´ de Concentra¸c˜ao: Astronomia Orientador(a): Prof. Dr. Reuven Opher S˜ao Paulo 2009 Para o Papai, a Mam˜ae, Tati, Clara e Ro, que me ensinaram algo que os sentidos, ainda que ampliados, n˜ao podem capturar. Agradecimentos Certamente esta ´ea p´agina mais dif´ıcil desta disserta¸c˜ao e ao mesmo tempo a mais agrad´avel. Mais dif´ıcil porque as palavras de afeto devem fazer jus ao sentimento e a lembran¸ca, e mais agrad´avel pois essa ´ea sensa¸c˜ao de lembrar das pessoas aqui citadas. Por motivos hist´oricos, agrade¸co inicialmente `as pessoas que muito me influenciaram no interesse pela pesquisa, assim como na minha escolha pelo mestrado em astronomia: Professora Vera Jatenco-Pereira, Aline Vidotto e Diego Falceta-Gon¸calves. A convivˆencia com vocˆes foi sempre animadora e me incentivou a caminhar adiante. Agrade¸co ao meu orientador Reuven Opher, por me auxiliar a enxergar a simplici- dade nos trabalhos mais indecifr´aveis. A capacidade de s´ıntese e de argumenta¸c˜ao sempre foi motivo de inspira¸c˜ao e continuar´asendo uma meta a ser alcan¸cada profissionalmente. Agrade¸co tamb´em aos amigos do grupo Ana, Ulisses, Luiz Felippe (companheiro de via- gens) e Rafael. Aos professores do IAG, que auxiliaram direta ou indiretamente na minha forma¸c˜ao e pesquisa, desde a gradua¸c˜ao. Em particular ao Jorge Horvath que sempre se mostrou particularmente interessado no meu trabalho e acess´ıvel para a troca de id´eias. Espero que ele passe a torcer pela sele¸c˜ao brasileira em breve. Aos meus pais, cujas palavras de agradecimento s˜ao insuficientes para demonstrar senti- mento de tal natureza. Gra¸cas a eles cresci aprendendo a buscar e cultivar conhecimento e valores eternos. As` minhas muito amadas irm˜as Clara e Thais com quem tudo compartilho. A` Roberta, pelo quanto a sua compreens˜ao diminuiu o peso de me privar da convivˆencia durante a elabora¸c˜ao deste trabalho. Este trabalho ´eem muito dedicado a vocˆee ao quanto n´os crescemos juntos. Entre as minhas maiores conquistas nos ´ultimos dois anos, relaciono os meus amigos no IAG que propiciaram `aminha experiˆencia na p´os-gradua¸c˜ao uma oportunidade ´unica de convivˆencia, al´em de muita cafe´ına. Inicialmente colegas e agora amigos Thiago Triumpho (futuro milion´ario), Thiago Almeida, Bruno Dias, Vin´ıcius Busti, Tatiana Lagan´a, Oscar Cavichia (R´a), Fernanda e Tatiana (chuiquititas). Ao Marcus Vin´ıcius (TF) companheiro de todas as conversas. Com carinho tamb´em agrade¸co a oportunidade de dividir sala com duas pessoas e amigos fant´asticos que foram o Alessandro e o Mairan, que tornaram o meu ambiente de trabalho t˜ao agrad´avel quanto a minha pr´opria casa. Outro que n˜ao posso deixar de citar, ainda que n˜ao no IAG ´eo Max Ujevic, amizade ilustre que veio de brinde com o mestrado. Agrade¸co a Mirr Corporation Iluminatti Society, por preencher minha mente com ima- gina¸c˜ao, vocˆes certamente s˜ao os amigos mais presentes na minha vida. Aos meus amigos F´abio, Gustavo e Thiago (Kibe), com quem a convivˆencia durante anos permitiu que a minha mente vagasse por lugares que n˜ao teria acesso sozinha. Vocˆes certamente influen- ciaram as minhas escolhas mais acertadas, incluindo a astrof´ısica. Ao Marcos e Ulisses funcion´arios do IAG, de quem sempre pude contar com muita aten¸c˜ao e boa vontade. Ao` pessoal da secretaria, principalmente `aMarina minha amiga, que com certeza me ajudou sempre mais do que o necess´ario. Agradecimentos aos amigos que mesmo sem o nome aqui participam de diversos mo- mentos importantes na minha vida. Ao CNPQ pela bolsa concedida durante toda a realiza¸c˜ao da disserta¸c˜ao, assim como a CAPES/PROEX pelo aux´ılio na participa¸c˜ao de eventos cient´ıficos. A Esta tese/disserta¸c˜ao foi escrita em LTEX com a classe IAGTESE, para teses e disserta¸c˜oes do IAG. “Para que a pesquisa continue florescendo no campo fragmentado e complexo do saber, escolho a palavra c´eu. C´eu convoca, um pouco em desordem, as almas viajantes e as tecnologias galopantes. Nos ´ultimos vinte anos aprendemos mais do c´eu do que em dois mil, e isso gra¸cas `auni˜ao da astronomia e da f´ısica. A astrof´ısica ´eo casamento da Terra e do c´eu no pensamento humano, da f´ısica, pr´atica de laborat´orio que consiste em extrair leis da mat´eria deste mundo, e da astronomia, que ´eum olhar dirigido para o inacess´ıvel. Sem a f´ısica, a astronomia n˜ao tem cabe¸ca, mas, sem a astronomia a f´ısica n˜ao tem asas.” Michel Cass´e “A verdade n˜ao faz sentido, a grandeza do mundo me encolhe. Aquilo que provavelmente pedi e finalmente tive, veio no entanto me deixar carente como uma crian¸ca que anda sozinha pela terra. T˜ao carente que s´oo amor de todo o universo por mim poderia me consolar e me cumular [...]” Clarice Lispector Resumo O objetivo da pesquisa descrita ao longo desta disserta¸c˜ao de mestrado, foi investigar o fenˆomeno e os problemas em aberto na astrof´ısica de Gamma-Ray Bursts e em particular explicar o campo magn´etico nos afterglows. O modelo de bola de fogo prediz que o espectro n˜ao t´ermico do afterglow ´edevido a radia¸c˜ao s´ıncroton, a qual requer fortes campos magn´eticos ( 1G na regi˜ao do afterglow), ∼ e part´ıculas aceleradas relativisticamente. Em modelos alternativos o campo magn´etico ´e ainda mais importante. O principal problema no que se refere ao campo magn´etico em afterglows est´arelacionado com o fato de que a regi˜ao de emiss˜ao ´emuito mais distante da fonte progenitora. Isto implica que o campo magn´etico gerado na fonte n˜ao ´esuficiente para explicar as observa¸c˜oes, sugerindo que o campo deve ser gerado na regi˜ao do choque. No entanto, o campo magn´etico no meio interestelar ´e 1µ G, o que requer forte amplifica¸c˜ao ∼ na regi˜ao do choque. Simula¸c˜oes num´ericas sugerem que a instabilidade Weibel possa amplificar o campo magn´etico, por´em instabilidades do plasma podem apenas gerar campo na ordem do skin depth, enquanto as observa¸c˜oes sugerem que o campo deve persistir em uma distˆancia de 109 skin depths. No presente trabalho sugerimos que as flutua¸c˜oes naturais do plasma preditas pelo Teorema da Flutua¸c˜ao-Dissipa¸c˜ao (FDT), podem explicar o campo semente gerado na regi˜ao do choque. Calculamos analiticamente o campo magn´etico e mostramos que com parˆametros t´ıpicos podemos gerar campo de at´e10−2 G. O campo gerado pelas flutua¸c˜oes depende essencialmente da densidade e temperatura do plasma, podendo explicar o campo. Mecanismos adicionais s˜ao necess´arios para amplificar o campo magn´etico. N´os sugerimos que a turbulˆencia pode ser este mecanismo, mas uma abordagem mais realista exigiria c´alculos computacionais eficientes. Abstract The objective of the research, summarized in this master’s thesis, is to investigate the phenomena and unsolved problems of Gamma-Ray Bursts and, in particular, to explain the magnetic fields in the Gamma-Ray Burst afterglows. The fireball model predicts that the non-thermal spectrum of the afterglow is synchroton radiation, which requires strong magnetic fields ( 1 G in the afterglow region) and relativistically accelerated ∼ particles. In alternative models the magnetic field is even more important. The main problem concerning magnetic fields in afterglow is that the emission region is located at a distance that is too far from the source. This implies that the magnetic field generated at the source is not enough to explain the observations, suggesting that the field must be generated in the shock region. However the magnetic field in the interstellar medium is 1µ G, which requires that the field is strongly amplified in the shock region. Numerical ∼ simulations suggest that Weibel instability could amplify the magnetic field, but plasma instabilities can only generate fields on the order of plasma skin depth, while observations suggests that the field must persist in a distance over 109 skin depths. In the present work we suggest that the plasma fluctuations predicted by the Fluctuation-Dissipation Theorem (FDT) can explain the seed field generated at the shock. We perform analytical calculations of the magnetic field and we show that with typical parameters we can generate a magnetic field of 10−2 G. The field generated by the FDT depends essentially of the density and the temperature of the plasma medium and can explain the large structure field. However an additional mechanism in necessary to amplify the magnetic field. We argue that turbulence can be the mechanism, but a realistic approach will require efficient numerical simulations. Lista de Figuras 1.1 Localiza¸c˜ao de todos os 2704 GRBs detectados pelo BATSE em coordenadas gal´acticas. O plano da gal´axia ´ea linha horizontal no meio dafigura. 23 1.2 Distribui¸c˜ao de 140 bursts como fun¸c˜ao do pico de contagem.
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