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Cosmic Rays: Open Problems

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Cosmic Rays: Open Problems Cosmic Rays: Open Problems and Perspectives for the future Paolo Lipari MAPS Perugia 20th october 2009 COSMIC RAYS A (nearly) one century old problem.... Victor Hess before the balloon flight of 1912 Discovery of Cosmic Rays Beginning of High Energy Astrophysics Review article in 1931 of Karl Darrow (Millikan collaborator). ``Data and nature of Cosmic Rays'' Physicists with their frail machines have gone to high mountain ponds in the Sierras and in the Andes, to the distant wildernesses about the Earth Magnetic poles; they have scooped out cavities in Alpine glaciers, they have lifted hundredweights of lead to the tops of peaks above the snow line, they have cruised the arctic and the tropical oceans, they have descended into tunnels and deep mines, they have ascended into the sky in airplanes and balloons .... Leading theory for CR in 1931: Georges-Henry Lemaitre New York Times article: Prof. Albert Einstein has given his scientifc blessing to the ingenious theory proposed by Abbe' Georges Lemaitre that cosmic rays are birth cries of the universe and the radiations from the superradioactive primeval matter that existed when the universe was young. COMPTON (1933) Cosmic Rays are CHARGED LATITUDE EFFECT GEOMAGNETIC EFFECTS Latitude efect → Charged particles East-West efect → Positively charged particles An understanding of the Origin of Cosmic Ray is still incomplete. Most fundamental problems are still open. Which are the sources ? What are the production mechanisms ? ..... An understanding of the Origin of Cosmic Ray is still incomplete. Most fundamental problems are still open. Which are the sources ? What are the production mechanisms ? ..... … but... The perspectives to arrive soon to clear Answers to (many) of these fundamental questions are (in my opinion) excellent. CR measurements (ground, space) Gamma Astronomy + multi=wavelength observations [neutrino astronomy] Cosmic Rays Spectrum Features: 1. Knee [1a. 2nd Knee] 2. Ankle 3. High Energy bending Remarkably smooth (nearly featureless) Energy Spectrum. UHECR Flux * E3 representation. Knee 2nd Knee “suppression” Ankle UHECR Flux * E3 representation. HIRES Tibet ARRAY Data points depend on the interaction model used for modeling the showers AUGER QGSJET-1 QGSJET-2 SIBYLL We do not have a convincing, unique explanation for ANY of the main features observed in the CR spectrum: (knee, 2nd knee, ankle, high energy suppression). [Diferent explanations for the (astro)physics that produces these features have been advanced (with diferent degrees of plausibility)] Clarifcation of these issues is a main goal for future studies Proton, electron, positron, antiproton Spectra Diferent spectra For diferent Particle types High Energy Astrophysical Source: ObJect (or an “event”) that produces (and for some time contains) relativistic particles p e± Escaping particles: CR High Energy Astrophysical Source: ObJect (or an “event”) that produces (and for some time contains) relativistic particles Unavoidable p Photon emission ! e± Escaping particles: CR “Hadronic Emission” “Leptonic Emission” “The HIGH ENERGY UNIVERSE” WHERE are high energy particles produced? which sources (source classes) WHICH physical mechanism produce them? [Fermi mechanism ? Pulsars? ...] HOW do they get from the production point to the observation point [Magnetic feld, energy loss, ....] GAMMA ASTRONOMY [multi-wavelength photon observations] Several diferent type of astrophysical obJects emit high energy photons (GeV, TeV). Therefore contain populations of high energy relativistic charged particles. [And therefore can accelerate them with “non-thermal mechanisms”] Are these relativistic particles electrons/positrons or protons and Nuclei? “Leptonic versus Hadronic radiation mechanism” Which (if any) of these high energy sources produce the particles we observe at the Earth ? GAMMA ASTRONOMY [multi-wavelength photon observations] Several diferent type of astrophysical obJects emit high energy photons (GeV, TeV). Therefore contain populations of high energy relativistic charged particles. [And therefore can accelerate them with “non-thermal mechanisms”] Are these relativistic particles electrons/positrons or protons and Nuclei? “Leptonic versus Hadronic radiation mechanism” Which (if any) of these high energy sources produce the particles we observe at the Earth (Neutrino Astronomy ?!) Agile Fermi Hess Magic GALACTIC EXTRAGALACTIC Cosmic Rays We live in a “bubble” flled with Cosmic Rays. A “magnetic bottle” where the CR density is enhanced by magnetic trapping. B Extra-galactic space is flled by a much more tenuous gas of cosmic rays inJected during the entire history of the Universe. This “extragalactic population” emerges only at sufciently high energy. Galactic Sources InJection Q(E,x) Containement D(E/Z, x) Difusion Escape Galactic Sources InJection Q(E,x) Lifetime Containement Of a particle D(E/Z, x) Difusion Escape Galactic Sources InJection Q(E,x) Lifetime Containement Of a particle D(E/Z, x) Difusion (high energy) Extra-galactic CR crossing the Galaxy. Piece of extragalactic space “MilkyWay-like sources” Milky Way Galaxy Piece of extragalactic space: Non MilkyWay-like sources AGN Milky Way Galaxy Measurement of the CR density SMC Dominant contribution to the GeV photon emission: CR density in the Large and Small Magellanic Clouds much smaller than in our Milky Way Eγ > 100 MeV Fermi all sky Map Galactic and Extra-Galactic components One general way to state the problem of the cosmic ray fuxes: For each particle type: p, e-, e+, He, Li, Be, B, … , Fe, .... Compute this integral: Source of CR particles with energy E 0 at time t (in the past) 0 at position r 0 (Measurable) Density of CR particles With energy E, at the present time “Propagation “here” (r ) obs Probability” Propagation as difusion Sources “Primary Particles” Directly produced in astrophysical ObJects (like SNR, GRB). For primary particles (protons, electrons): Ensemble of “quasi-instantaneous” [SNR, Pulsars] point sources? Universal Shape of The Spectrum ? Time-averaged, Space-smoothed source: Stationary. Space dependence, refects Galaxy structure “Secondary Particles:” Continuous (time, space) InJection In the volume Of the Galaxy Source spectrum of secondary has a the shape determined by the primary spectrum. (and known physics). Source of Relativistic particles From Dark Matter annihilation: Annihilation DM density “Branching Cross section Ratios” in Diferent Annihilation channels MILKY WAY sun Fundamental propagation equation: Source Difusion Energy Loss ... additional terms: interaction decay convection reacceleration Fundamental propagation equation: Source Difusion Energy Loss Fundamental propagation equation: 0 Source Difusion Energy Loss Stationary Solution Escape as “absorbing boundary condition” The difusion coefcient become infnity at the boundary Galaxy as a cylinder PROTON SPECTRUM InJection Spectrum Escape [difusion description] Interaction [small efect] Energy Loss [negligible] Toy Model: “Spherical Galaxy” : Exact solution : “Concentrical spheres Galaxy” Two models Same CR inJection Larger/smaller halo Galaxy modeled “Slab Galaxy” as an infnite “slab” 1-D problem Creation of particles difusion with escape “normal” difusion” Time and space origin of the protons (in the “slab galaxy” toy model) Homogeneous Stationary inJection Dashed line: exponential of same average Stationary sources (no time dependence) Isotropic Difusion description Good approximation: Factorization of the energy dependence: For the source. For the difusion coefcient Cosmic Ray Nuclear Composition Cosmic Ray Nuclear Composition Overabundance of Li, Be, B Sub-iron elements Spallation efect: Column density Confnement time Injection of Secondary Nuclei: A = Primary Nucleus (C, N, O) A' = “Secondary Nucleus” (Li, Be, B) E = Energy per nucleon: Injection of Secondary Nuclei: A = Primary Nucleus (C, N, O) A' = “Secondary Nucleus” (Li, Be, B) E = Energy per nucleon: Diferent Spectrum! Primary Nucleus Secondary Nucleus Primary/Secondary Primary Nucleus Secondary Nucleus Primary/Secondary From CREAM tau(E) ~ E-0.6 What are the SOURCES of COSMIC RAYS? ENERGETICS Where can one fnd the power to create the cosmic rays ? Non-thermal DYNAMICS Non-equilibrium How is the energy transformed “Violent” into ultra-relativistic particles phenomena Power Provided by SN is sufcient with a conversion efciency of 15-20 % in relativistic particles POWERING THE GALACTIC COSMIC RAYS CAS A (1667) SuperNovae types Chandra X-Ray images SN1006 Tycho Cas A FERMI acceleration mechanism Collision with Moving Plasma Clouds in the Galaxy Unshocked material at rest Shock Piston Front 2 The Energy Spectrum of Particle accelerated near Shock Waves has a UNIVERSAL FORM In agreement with observations. SuperNova Discovered in 1996 by the Roentgen Satellite RX J1713.7-3946 (Rosat) Foreground star Point Source (Neutron Star) 1st observation of RX J1713.7-3946 AD 393 A guest star appeared within the asterism Wei during 2nd lunar month of the 18th year of the Tai-Yuan reign period (february 27-march 28 AD 393), and disappeared during the 9th lunar month (october 22 - november 19) HESS Telescope Observations with TeV photons Comparison with ROSAT observation ENERGY Spectrum Hess estimate Essentially compatible with the Ortodoxy (10% conversion of SN kinetic energy into relativisic particles) G23.3-0.3 VELA JUNIOR W41 Have we proved that SNR are the source of the bulk of the Galactic Cosmic Rays ? Important “Hints” But conclusion Still Controversial. Need additional Data. Cherenkov telescopes, FERMI (Glast)
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