Table Of ContentSpringer 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
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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)
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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
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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
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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
