Gamma-ray and Neutrino Signatures of Galactic Cosmic-ray Accelerators PDF
Preview Gamma-ray and Neutrino Signatures of Galactic Cosmic-ray Accelerators
Springer Theses Recognizing Outstanding Ph.D. Research Silvia Celli Gamma-ray and Neutrino Signatures of Galactic Cosmic-ray Accelerators Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists. Theses are accepted into the series by invited nomination only and must fulfill all of the following criteria (cid:129) They must be written in good English. (cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, Engineeringandrelatedinterdisciplinary fields such asMaterials,Nanoscience, Chemical Engineering, Complex Systems and Biophysics. (cid:129) The work reported in the thesis must represent a significant scientific advance. (cid:129) Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis must be gained from the respective copyright holder. (cid:129) They must have been examined and passed during the 12 months prior to nomination. (cid:129) Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. (cid:129) The theses should have a clearly defined structure including an introduction accessible to scientists not expert in that particular field. More information about this series at http://www.springer.com/series/8790 Silvia Celli Gamma-ray and Neutrino Signatures of Galactic Cosmic-ray Accelerators Doctoral Thesis accepted by ’ the Gran Sasso Science Institute, L Aquila, Italy 123 Author Supervisor Dr. Silvia Celli Prof. Felix Aharonian Department ofPhysics HighEnergy Astrophysics Gran SassoScienceInstitute MaxPlanckInstitute for NuclearPhysics L’Aquila, Italy Heidelberg, Germany ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN978-3-030-33123-8 ISBN978-3-030-33124-5 (eBook) https://doi.org/10.1007/978-3-030-33124-5 ©SpringerNatureSwitzerlandAG2019 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfrom therelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained hereinorforanyerrorsoromissionsthatmayhavebeenmade.Thepublisherremainsneutralwithregard tojurisdictionalclaimsinpublishedmapsandinstitutionalaffiliations. ThisSpringerimprintispublishedbytheregisteredcompanySpringerNatureSwitzerlandAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland To my parents, whom I admire for the education, nurture and support they devoted to me. To Francesco, for his persistent dedication and love. ’ Supervisor s Foreword The current paradigm of Galactic cosmic rays assumes that supernova remnants (SNRs)—theresultsofgiganticexplosionsofstars—areresponsibleforthelocally measuredfluxesofcosmicraysatenergiesbelow1015eV.Fordecades,thisbelief has been based on phenomenological arguments and theoretical meditations. The kinetic energy contained in shock waves of young SNRs is one of the strongest arguments supporting SNRs as the major factories of Galactic cosmic rays. Also, the so-called diffusive shock acceleration has been established as a viable mecha- nism for the effective acceleration of particles in young SNRs. Over the last decade, we have seen remarkable progress towards the under- standing of the origin of cosmic rays in two directions. The first one concerns the precisionandhighqualityofmeasurementsofprimaryandsecondarycosmicrays. Althoughtheseresultscanbeinterpretedintheframeworkofthe‘standardmodel’, the detected ‘excess’ of antiparticles (positrons and antiprotons) requires non-negligible modification of the current paradigm of Galactic cosmic rays. Still, several key issues regarding, in particular, the sites and sources of cosmic-ray productionare not fully resolved and identified. A real breakthrough in this regard is expected from gamma-ray observations. The recent discoveries of gamma-ray emission at high and very high energies from many young and middle-aged SNRs confirm,ingeneral,theearlytheoreticalpredictions,butatthesametimeraisenew questions and challenges. Advanced and new detailed calculations concerning the acceleration and escape of particles in SNRs and their interactions leading to the gamma-ray and neutrino production become ‘hot’ topics in the field. SomeofthesetopicsconstitutedthebasisofthePh.D.thesisofDr.SilviaCelli. ShehascompletedasolidPh.D.thesis,performedonahighprofessionallevel.The problems discussed and studied in the thesis are related to (1) the acceleration, propagation,andradiationofparticlesinsupernovaremnants;(2)veryhigh-energy neutrinos from the Galactic Center; (3) the potential of the next-generation gamma-ray and neutrino detectors, CTA and KM3NeT, for the study of extended non-thermal astrophysical sources in the Galaxy. Here, she has demonstrated excellentcomputational skills anda deepunderstanding ofthe underlying physics. The obtained results and conclusions are based on extensive analytical and vii viii Supervisor’sForeword numerical simulations. They are formulated and presented in transparent and convenient forms that can be readily used in the interpretations of gamma-ray and neutrino observations as well as in confident predictions for future measurements. This concerns especially the identification of nature (‘hadronic or leptonic?’) of gamma-ray emission from young and middle-aged supernova remnants and the search for cosmic-ray PeVatrons. Heidelberg, Germany Prof. Felix Aharonian July 2019 Preface Supernova remnants are believed to be the major contributors to the observed Galactic cosmic-ray flux, though indisputable observational pieces of evidence of such statement are still missing. A crucial aspect of the supernova remnant para- digm for the origin of Galactic cosmic rays is that particle acceleration, as due to diffusiveshockacceleration,requireseffectiveconfinementofparticlesintheshock regiontoletthemachieveenergiesuptotheso-calledknee,around (cid:1)1015(cid:3)1016 eV. However, the current theoretical description of cosmic-ray acceleration and propagationwithinandaroundsupernovaremnantssuffersfromcertainlimitations, which also affect the predictions on the shape of the energy spectra of secondary gamma rays and neutrinos. In particular, in this thesis, two relevant aspects of this theory are investigated: the particle acceleration at shocks propagating in clumpy non-homogeneous environments and the particle escaping process from the acceleration site. The standard diffusive shock acceleration model usually assumes thatshocksexpandintoideallyuniformenvironments,whileamorerealisticpicture should consider an inhomogeneous gas distribution where supernova remnants develop.Inthiswork,Iconductedadetailedstudy ontheparticleaccelerationand propagation through non-homogenous structures and its effect on the resulting secondary radiation. Regarding the particle escape from the acceleration site, I developed a phenomenological model to investigate this process and its impact on the gamma-ray emission from middle-aged supernova remnants, where particle escapeisexpectedtobeeffective.Iwillshowthatspectroscopicandmorphological studies of the gamma rays coming from both inside and immediately outside of those remnants can provide insight into the escaping process in general, and in particular,willshedlightontheirabilitytoactascosmic-rayPeVatrons.Sofar,the only hint of the presence of a PeVatron has been found in the Galactic Center region,whosenatureis,however,unclear.Undertheassumptionthattheobserved gamma-rayfluxoriginatesfromhadronicinteractions,Icalculatedtheexpectedflux ofmulti-TeVneutrinosinordertoinvestigateitsdetectabilitywithfuturekm3-scale neutrinotelescopes.Finally,acomparativeanalysisoftheperformancesofthetwo major upcoming detectors, namely CTA and KM3NeT, is presented in the context ix x Preface of future studies on the origin of Galactic cosmic rays through respectively gamma-ray and neutrino observations. The thesis is organized as follows: (cid:129) In Chap. 1, the supernova remnant paradigm for the origin of cosmic rays is introduced and a discussion concerning possible Galactic PeV accelerators is presented. As gamma rays and neutrinos constitute observational signatures of particle accelerationand propagation, areviewoftheir propertiesand detection techniques is provided. (cid:129) In Chap. 2, the propagation of accelerated particles within supernova remnants is investigated in the presence of strong shocks evolving through non-homogeneous media. A numerical approach to the particle transport under these conditions is here provided for the first time, conditions that represent realistic situations for the environments where sources as supernova remnants usually expand. Since dense molecular clumps constitute ideal targets for acceleratedprotons,enhanced gamma-rayandneutrino emissionsareexpected. The model is shown to provide an adequate description of the broadband gamma-rayemissionoftheGalacticsupernovaremnantRXJ1713.7-3946both in terms of total flux and spectral shape. (cid:129) InChap.3,aphenomenologicaldescriptionofparticleescapefrommiddle-aged supernova remnants is presented, and it represents the first attempt of studying thisprocesswithinthecontextofextendedsources.Aproperdescriptionofthis phenomenon is extremely relevant for the correct interpretation of the radiation spectrum observed in these sources, which reflects not only the acceleration mechanism and the interaction processes, but also the particle escape from the accelerationsite.Themodelisappliedtothreeinterestingmiddle-agedGalactic supernovaremnants,namelyIC443,W51C,andW28N. Amajorimplication of the presence of particle escape is represented by the possible production of high-energyradiationalsooutsideoftheremnantshock,characterizedbyavery peculiarbump-likeenergyspectrum.Thisfeatureisinterestingfromthepointof view of both gamma-ray and neutrino emissions, being experimentally con- nectedtopotentiallybackground-freeregions.Moreover,theescapingprocessis particularly relevant for a correct understanding of the cosmic-ray spectrum observedatEarthandtodisentanglethepropagationeffectsthroughtheGalaxy. (cid:129) In Chap. 4, a candidate source of PeV cosmic rays located at the center of the Galaxy is discussed. The Galactic Center, as recently observed in multi-TeV gammarays,showsacentralemissionwithspectralcut-offenergyatanenergy ofabout10TeV.Nonetheless,adiffuseemissionsurroundingthecentralsource shows no visible cut-off up to the energies currently probed by H.E.S.S.: the possibilityofanintenseinfraredradiationfieldabsorbinggammaraysfromthe central source is investigated for the first time. The detection of very high-energy neutrinos in angular correlation with the electromagnetic radiation would confirm the hadronic hypothesis for the origin of gamma rays. Hence, expectations from current and next-generation neutrino instruments are
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