Universidade de S~aoPaulo Instituto de Astronomia, Geof´ısicae Ci^enciasAtmosf´ericas Departamento de Astronomia Raniere Maciel de Menezes Exploring the extreme Universe with the Fermi Large Area Telescope S~aoPaulo 2020 Raniere Maciel de Menezes Exploring the extreme Universe with the Fermi Large Area Telescope Thesis presented to the Departament of As- tronomy of the University of S~aoPaulo in par- tial fulfillment of the requirements for the de- gree of Doctor of Philosophy In the subject of: Astronomy & Astrophysics Advisor: Professor Rodrigo Nemmen S~aoPaulo 2020 To the memory of Carl Sagan, whose efforts in spreading science were able to reach me 10 years after his death, in the depths of the sert~aopernambucano. Acknowledgements \Onde for, ande direito" said my old father when I left home on my way to Italy. I could not andar direito if it wasn't for you, mommy and Ra´ı. Thank you. I would like also to thank my aunts, uncles and cousins for all the fondness and support during my whole life. A special thanks to Lica, whose hard work literally makes my life easier. I am also really grateful for what I have learned from my friends in Pesqueira. Whenever I remember our old times playing football, fishing and exploring those wild hills, the only feeling coming to my mind is the untranslatable Portuguese word saudade. Regarding the school, I would like to thank my teachers of Math and Geography, Jos´eLopes and Maria da Paz. Even today I still remember important details from your lessons. A big thanks to my dear friends Carol, Cl^o,Landau, Lind~ao,Luc~ao,Mario, Nat´alia,Rafa Bandeira, Rafa Rechiche and Vit~ao.You are among the smartest and kindest people I have ever met. For me, you are the real joy that I found across the event horizon of the university. Thank you very much to my office mates Andr´e,Henrique, Mirian, Pablo and Rog´erio.After so many years sitting by your side, I feel like you are a branch of my family in S~aoPaulo. Thank you to my research teammates Artur, Fabito, Gus, Ivanildo, Malu Gubolin and Roberta. We have been sailing in the same spaceship for many years. Thank you for holding up the sails during the solar storms and for the many funny moments. A special thanks to Querido Jhon and Leo Santos, the best observational astronomers I have ever met. It would be a pleasure and an honor for me if we could work together someday. Grazie mille a tutti gli amici che ho trovato in Italia, soprattutto ad Alessandro, Ana, Ezequiel, Gian Luca, Giovanni, Harold, Leonardo, Marta, Romana, Valentina Cesare, Valentina Missaglia, Vittoria e Domenico Maisano. Peccato, perch`estavo bene in loro compagnia. My sincere thanks to Prof. Rama and the staff at the OAM. The project Noite com as Estrelas is by far the best science outreach project I have ever seen, and being part of this team was one of the most significant things for me during my stay in S~aoPaulo. A great thanks to the staff at the Department of Astronomy, especially to Marco and Ulisses, who helped me several times with every kind of software and hardware issues. My most sincere thanks to the Fermi-LAT Collaboration, in particular to Liz Hays, Bindu Rani and Teddy Cheung. Thank you for the wonderful opportunities and especially for helping me to improve the level of my science. A kind thanks to Prof. Zulema, who has been accompanying my work and giving me important advice for the last four years. And finally, a very special thanks to those who taught me how to science, especially to Fl´avioAguiar, Francesco Massaro, Raffaele D'Abrusco, and Rodrigo Nemmen. This thesis was supported by CNPq (Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico) and FAPESP (Funda¸c~aode Amparo `aPesquisa do Estado de S~aoPaulo) under grants 2016/25484-9 and 2018/24801-6. Esta tese foi escrita em LATEX com a classe IAGTESE, para teses e disserta¸c~oesdo IAG. \Quando eu te encarei frente a frente, n~aovi o meu rosto. Chamei de mau gosto o que vi, de mau gosto, mau gosto. E´ que Narciso acha feio o que n~ao´eespelho." Caetano Veloso \Veloso, o Sol n~ao´et~aobonito pra quem vem do Norte e vai viver na rua." Belchior Abstract High-energy astrophysical processes commonly happen in the surroundings of compact objects, like neutron stars and black holes. In such environments, relativistic particles can emit γ-rays and also scatter low-energy photons up to the γ-ray domain. With the launch of the Fermi Large Area Telescope (LAT) in 2008, a new window was open to the γ-ray sky, allowing us to observe a huge variety of astrophysical phenomena never seen before. Fermi-LAT's remarkable sensitivity in the 100 MeV { 500 GeV energy band allowed for the detection of more than 5000 γ-ray sources and this number is still growing. In this thesis, we use the unprecedented Fermi-LAT capabilities in two different ways. In the first half of the thesis, we explore the γ-ray emission from globular clusters and low-luminosity active galactic nuclei (LLAGNs), where both classes are poorly understood in the high-energy do- main. By using roughly 10 years of γ-ray observations, we are able to uncover some aspects of the dynamical formation of millisecond pulsars (MSPs) in globular clusters, especially on how stellar close encounters in the cores of globular clusters can form and/or disrupt compact mass-transferring binary systems, which could potentially evolve into γ-ray emit- ting MSPs. With a similar analysis, we explore the mechanisms behind the γ-ray emission of LLAGNs, this time also comparing hadronic and leptonic models for high-energy pro- cesses happening in the black hole surrounding hot accretion flow. In the second half of the thesis, we use Fermi-LAT data to associate γ-ray sources with their optical-infrared coun- 0 terparts. Although the Fermi-LAT spatial resolution, typically . 5 , is the highest ever achieved for a γ-ray space-based telescope, it is not enough to avoid problems with source association. In fact, 25% of the γ-ray sources listed in the last Fermi-LAT catalog have no obvious low-energy counterpart. To face this problem, we work on three distinct, direct and indirect, counterpart identification methods. At first, we use the correlations between mid-infrared colors and γ-ray emission to better select and characterize the counterpart candidates of Fermi-LAT blazars. We then develop an alternative method of counterpart association by accounting for the mid-infrared colors and radio loudness when computing the association probability with the likelihood ratio method. Finally, we perform a direct search for counterparts by doing an optical spectroscopic follow up of a few tens of sources. The results from these approaches are very promising, especially on the association and selection of future targets for spectroscopic follow up of Fermi-LAT blazars. Resumo Processos f´ısicosde altas energias comumente ocorrem nas redondezas de objetos as- tron^omicoscompactos, como estrelas de n^eutronse buracos negros. Nesses ambientes extre- mos, part´ıculassubat^omicasviajando a velocidades relativ´ısticass~aocapazes de emitir raios γ, bem como ricochetear f´otonsde baixas energias, tornando-os muito mais energ´eticos. Com o lan¸camento do Fermi Large Area Telescope (LAT) em 2008, a observa¸c~ao de novos fen^omenosastrof´ısicostornou-se poss´ıvel. A sensitividade sem precedentes do Fermi-LAT na banda de 100 MeV { 500 GeV permitiu a detec¸c~aode mais de 5000 fontes de raios γ ao longo de mais de uma d´ecadade opera¸c~oes,n´umeroeste que continua crescendo. Nesta tese de doutorado, n´osusamos o potencial do Fermi-LAT de duas maneiras diferentes. Na primeira metade da tese, exploramos a emiss~aode raios γ de aglomerados globula- res e de n´ucleosativos de gal´axiascom baixa luminosidade (LLAGNs), ambas as classes ainda pouco compreendidas no dom´ınio dos raios γ. Usando aproximadamente 10 anos de observa¸c~oes,n´osconseguimos desvendar alguns dos aspectos din^amicosque levam `a forma¸c~aode pulsares de milissegundo (MSPs) em aglomerados globulares, e especialmente como encontros estelares nos n´ucleosdesses aglomerados podem levar `aforma¸c~aoe/ou desmembramento de sistemas bin´arioscompactos que, dependendo de sua configura¸c~ao, podem evoluir para MSPs emissores de raios γ. Partindo de uma an´alisesimilar, n´os exploramos os mecanismos de produ¸c~aode raios γ em LLAGNs, mas dessa vez testando modelos hadr^onicose lept^onicosna tentativa de discriminar quais processos astrof´ısicos de altas energias geram os raios γ que observamos nesses ambientes. Na segunda metade da tese, n´osusamos os dados do Fermi-LAT para associar fontes de raios γ `assuas con- trapartidas no infravermelho. Apesar dos excelentes avan¸cosdo Fermi-LAT em rela¸c~ao aos seus predecessores, sua resolu¸c~aon~ao´ealta o suficiente para evitar o problema de associa¸c~aode contrapartidas de baixas energias. De fato, cerca de 25% das fontes lista- das no ´ultimocat´alogodo Fermi-LAT n~aopossuem uma contrapartida ´obvia em baixas energias. Para lidar com esse problema, n´ostrabalhamos com tr^esm´etodos distintos de identifica¸c~aode contrapartidas. Primeiro, usamos correla¸c~oesentre cores no infravermelho e emiss~aode raios γ para melhor caracterizar os candidatos a contrapartida de blazares do Fermi-LAT. N´osent~aodesenvolvemos um m´etodo estat´ısticoalternativo de associa¸c~ao de contrapartidas, onde levamos em conta cores no infravermelho e o comportamento no r´adiodas poss´ıveis contrapartidas de fontes de raios γ.