Pos(ICRC2015)008 ∗ -Ray Observations of Snrs and the Origin of the Knee in the Galactic CR Spectrum

PoS(ICRC2015)008 http://pos.sissa.it/ ∗ -ray observations of SNRs and the origin of the knee in the Galactic CR spectrum. γ -ray telescopes has been collecting evidence that Galactic CRs are accelerated in the γ [email protected] Speaker. The origin of cosmic rays (CRs)Hess has in puzzled 1912. scientists since the Indow pioneering the on discovery the last by Victor processes decade, regulating however,acceleration astrophysical modern via collisionless supercomputers first-principles plasmas, kinetic have allowing simulations. openedray the a and study At new of the win- CR same time, a new-generation of X- blast waves of supernova remnantstions (SNRs). of non-relativistic shocks, I in presentfield which state-of-the-art amplification ion are particle-in-cells and studied simula- electron in acceleration detailtheoretical as efficiency and a and observational counterparts function magnetic of of these thediffusive findings, shock shock comparing parameters. acceleration them I theory with then predictions and discussespecially of with outline the some multi-wavelength major observations open of questions, such younginferred as from SNRs. the possible causes I of theFinally, steep CR I spectra put such a theoreticalorder to understanding bridge in the relation gap with between CR acceleration in propagation sources in and the measurements Galaxy of in CRs at Earth. ∗ Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c

The 34th International Cosmic Ray Conference, 30 July- 6 August, 2015 The Hague, The Netherlands Damiano Caprioli Cosmic-ray Acceleration and Propagation Princeton University 4 Ivy Ln. - Princeton, NJE-mail: 08544 - USA PoS(ICRC2015)008 ). 65 . 4 2 ]. − 3 ] and E 8 , along with 3 Damiano Caprioli GeV. This suggests 8 in SNR blast waves is -ray observations (§ 10 γ ∼ ). The most recent findings 2.1 ]). Recently, PAMELA [ , § 7 , 6 , 5 . 1 , 4 -ray telescopes have opened new windows γ SNR paradigm GeV, which are likely accelerated in our Galaxy, 8 2 diffusive shock acceleration International Cosmic Ray Conference held in The 10 th . ]. Once combined with the local spectra measured by 2 ray emission from molecular clouds (MCs) in the Gould Belt [ , − 1 γ ), the all-particle CR spectrum steepens from about GeV, with the remarkable regularity of a power-law with spec- knee 11 is the nuclear charge, with the change in slope due to the convolution GeV (the 6 Z 10 × 5 measurement comes from the I discuss the bridge between the non-thermal SNR phenomenology and the CR fluxes ≈ , where 3. Below a few tens of GeV the CR spectrum is modulated by the solar wind, which 5 ] revealed an additional feature in the H and He spectra, i.e., a quite abrupt flattening 9 ∼ knee knee and its chemical composition becomes increasingly heavy up to E indirect 1 . ZE 3 An The CR spectrum measured at Earth spans more than ten orders of magnitude in energy, from At The quest for the sources of cosmic rays (CRs) has involved several generations of observers In the last few decades, state-of-the-art X-ray and -dependent cutoffs of different species (see, e.g., [ − 1 Z E of about 0.14 (AMS-02) and 0.22 (PAMELA) in spectral slope around 200 GeV/nucleon. Above their connection to the non-thermalFinally, phenomenology in § of SNRs, especially to tral index has a screening effect onfirst Galactic man-made CRs. object Yet, that the has VoyagerCR I left spectrum spacecraft, the of which heliosphere, electrons, in has H, 2013PAMELA directly and and became measured AMS-02, the He the this [ pristine additional informationtion, interstellar and will in allow turn to the better spectrum understand of solar low-energy modula- Galactic CRs fractions of GeV up to about 10 obtained with kinetic simulations of non-relativistic shock waves are outlined in § measured at Earth, and in particularfrom the their current sources and uncertainties in in the the self-confinement escape of of energetic accelerated particles. particles 2. The (almost) universal spectrum of cosmic rays a homogenous class of sourcesup in to which CR are accelerated via a rigidity-dependent mechanism AMS-02 [ of and theorists for more than a century. The 34 CR Acceleration and Propagation 1. Introduction on the non-thermal universe, providing us unprecedentedcandidate high-resolution sources, images joining and the spectra radio of telescopes CR thatrelativistic already electrons in in the Galactic ’50s have objects revealed such theadvent as presence of supernova of modern remnants supercomputers (SNRs). has At allowed the numericaltools same plasma time, for simulations the to studying become the prominent which complex is interplay at the between basis energetic ofof particles acceleration the in and direct collisionless detection electromagnetic plasmas. of I fields, CRsand briefly with critically summarize energies review the the current long-standing status ideathe that mechanism responsible for their acceleration ( Hague, in which about 1,300 contributions werea presented, quest. is Unraveling just the the physical most mechanisms recentparticles responsible milestone for in in the such the acceleration universe of has the traditionally fastestEarth, moved massive along also three by paths: means direct offrom detection balloons, astrophysical of objects; CR spacecraft, and fluxes and theoretical at satellites; interpretation of observation such of a non-thermal wealth emission of data. PoS(ICRC2015)008 3 per cen- GeV. -ray bursts 6 γ − ], 10 1 ] for an earlier × Damiano Caprioli 22 ≈ 20 ]). However, the GV have a gyro- , 5 8 28 SN 21 ∼ 10 ]). The very presence R & 14 ], though see [ 19 ] for a recent review). ([ 29 ], and ATIC-2 [ 13 ]. Addressing the nature of the transition from in Galactic accelerators. is determined by the deterioration of Galactic ]), which means that the dependence of acceler- 26 , 3 11 ], BESS [ knee Hillas criterion 30 below the proton knee reported by KASCADE- knee 25 E E 12 ] applied to SNR blast waves has what it takes to be ∼ 30 [ pc that exceeds the size of the Galactic disk, which sug- ], finding that the light (H+He) component shows a gradual 1 − 17 ) , in which a simple, global (and hopefully elegant) paradigm G µ / ], TRACER [ Despite the spread in the environmental parameters intrinsic in any B 3 PeV [ 11 )( , ∼ 10 GeV precision era 6 ]). The ARGO–YBJ experiment has measured the chemical composition of 10 ]. Their energetic argument is still valid today, even if in their pioneering paper 16 × 27 Z 30TeV and ( / ∼ SN explosions were associated to CR acceleration for the first time by Baade and E ] for a thorough discussion of such “anomalies”, as well as for the implications of the ], while confirming the canonical value for the all-particle knee of ' First-order Fermi acceleration 15 18 L ], and newly-born millisecond pulsars [ r The last few years have reserved some surprises also for what concerns the nature of the knee, In the quest for the actual CR sources, the Such deviations from straight power-laws and rigidity scalings are clear examples that CR 24 , 23 tury, Galactic SN explosions can account forof the the luminosity ejecta of CRs kinetic below energy the is knee if channeled about into 5–15% accelerated particles (see, e.g., [ formulation) can rule out objectsnecessary that to lack accelerate particles the up minimum to magnetic a field given strength energy. and CRs system with size rigidities [ 2.1 The SNR paradigm Energetics. Zwicky in 1934 [ appeal of SNe as CR sourcesstellar is winds not may merely provide limited an to adequate an energetic energy argument, reservoir since (see in [ principle also they argued for an extra-galactic origin of all the CRs. Assuming a rate of gests their sources to be extra-galactic objects, such as active galactic nuclei [ ation and propagation on rigidity onlyrefer may to be [ questioned at this level of accuracy. The reader can change of slope above 700Grande TeV, a [ factor of radius Galactic to extra-galactic CRs is beyond thewhether scope CRs of can this review, be but accelerated it at is indeed least important up to to check Universal power-law spectra. class of astrophysical objects,celeration the mechanism regularity returning of a universal the power-law CRmust spectrum, and be spectrum that preserved below such by the a propagation power-law kneearise in nature requires the from an Galaxy. the ac- The “imperfections” spectralsources. of features such outlined above universal may models either or reflect the diverse taxonomy of CR which is usually interpreted astheir due sources to the (another intrinsic possibility maximum being rigidity that that particles can achieveCRs in between peculiar spectra of positrons and antiprotons measured by PAMELA and AMS-02. confinement, e.g., [ for the origin and thecorrections to transport account of for a energetic richer particles phenomenology. needs to be complemented with first-order physics has entered its CR Acceleration and Propagation 300 GeV/nucleon these slopes are consistent with theier results nuclei of (e.g., previous CREAM experiments, [ also for heav- of such a spectral breakacceleration suggests or a at first-order the correction transport to stage.with the Also, respect the “universal” to H CR He spectrum slope, is and either steeper heavier at by elements the about (e.g., 0.1 [ in spectral slope PoS(ICRC2015)008 . 5 ∝ α 1, 3) . . 1 ) . 0 2 − (2.1) E 4 and ], one − ( E ≈ ), then s E c p gal 35 δ ∝ → / N ∝ ) ∝ r sh and is cru- 6 [ ) v E . E ( E 0 δ 10 GeV spend ( = N − − s N Damiano Caprioli E ∼ 3 M . ]), and (if . For monoatomic ∝ 0 r 40 , while the maximum ≈ gal , a few kpc) in the halo, 100 larger than in the τ ∼ 39 − ) one gets ( , 3.1.4 p H . 38 ∝ 4 , is the diffusion coefficient that − ) p 37 dp E ) with those measured at Earth, , / ( ]. ∝ α 36 1 − r gal 41 dE − 3 , r E D 5 is inferred to be − → ∝ p δ : if particles are relativistic ( 2 s Another pillar of the SNR paradigm is the p ∝ N dp ) ) ∝ p , where p ( ) ( E f f E 4 ( 2 energy) is discussed in § p 65. Since ; gal . ) π 2 D p 4 / ( ≈ 2 f The CR Galactic residence time can be estimated thanks = 2 H α p injection dE ≈ π + ) 4 ) δ E -ray emission (see, e.g., [ E ( γ ( 3 and strong shocks with sonic Mach number Be (only available to relatively low energies) and to the ratios of ) = N / ]). The equilibrium CR spectrum can in fact be written as gal 1, they are expected to accelerate CRs with spectra p 5 10 , but it is remarkable how a simple homogenous diffusive model for 9 τ ( , N = 4.3 s 35 γ ]. In such a diffusive shock acceleration (DSA), particles with gyroradii M 34 such as , 35, slightly steeper than the DSA prediction for strong shocks. Such a discrep- . 33 , which imposes are the shock velocity and the speed of sound, the compression ratio 2 ) , s − E c 32 ( , , while for non-relativistic particles ( 05 gal 2 . τ and 2 31 − below the knee [ E SN sh ≈ 65 . v yr in the Galaxy before escaping, significantly longer that the ballistic propagation time. If 2 ∝ R α 8 ) − The energy dependence of such primary/secondary ratios scales as ) E E 10 E radioactive clocks ( ( s ∝ Since SNR shocks have extent of such a universal power-law.the DSA is maximum scale-free, energy and of cannot the predictfor spectrum either entering of the the minimum the acceleration or accelerated process particles. ( The minimum energy required CRs are produced in the disk and diffusively escape at some distance Diffusive transport in the Milkyto Way. secondary to primary species such asby B/C, primary Li/C, CRs (Sc+V)/Fe, in which the return Galaxy.∼ the All grammage of traversed these measurements suggest that CRs with the Galactic residence time is parametrizes CR transport in the Galaxy, assumed homogeneous and isotropic. cial for connecting the( spectra injected at sources ( finds CR transport is able todiffuse simultaneously Galactic reproduce synchrotron and the measured CR secondary/primary ratios, the SNR magnetic fields and the maximum CR energy. energy attainable during the SNR lifetime dependsthe on shock, how which rapidly in particles turn can depends bemagnetic scattered on irregularities. across the amplitude and In theinterstellar SNRs spectrum medium of magnetic (ISM), upstream as fields it and is can downstream inferred be from the factors following of observational facts. 10 ancy will be discussed in § where The energy spectrum in turnN is N also the observed anisotropy in the arrival directions of CRs [ larger than the shock thickness can beenergy repeatedly as scattered if back and they forth were acrossand squeezed the the shock, between probability gaining of converging being walls. advected awayhydrodynamics Since from only, the both accelerated acceleration particles the region develop are energy power-law controlled gain distributions by whose per the spectral shock cycle index CR Acceleration and Propagation such a universal mechanism, aslate it ’70s has [ been put forward independently by several scientists in the is fully determined by the downstream/upstream density compression ratio, the differential momentum spectrum of accelerated particles reads gas with adiabatic index PoS(ICRC2015)008 ] ' 44 ) E ( gal D 10GeV at the ]. Such an evi- in young SNRs. . Damiano Caprioli 48 [ max knee E E ]). ]. would still be limited to 51 47 , max upstream G[ 50 E , µ ), 49 L r 200 3 / ≈ c in the downstream region via turbulent ' ]. Recent measurements in SN1006 [ B only 43 D , 42 5 , i.e., ]. G[ µ 46 1 mG [ . 50 below the observed knee. If Bohm diffusion were achieved in Bohm diffusion . ]; therefore, the ISM magnetic turbulence has way too little power at the 52 and would allow to achieve a very low maximum energy δ ], a factor of ] showed that the thickness of the rims is frequency-dependent, which allows to )] 54 45 , 53 GeV Z 3 Fitting the SNR synchrotron spectra from radio to X-rays typically reveals electrons to The lack of detection —within Chandra resolution— of X-rays in front of the forward ( X-ray hotspots in RX J1713.7–3946 show variability on a few year timescale, which may Young SNRs show thin non-thermal X-ray rims produced by multi-TeV electrons radiating / Can DSA be as efficient as 10–20%? What regulatesWhat such determines an the efficiency? fraction of ions and electrons thatHow is are injected B fields into amplified DSA? in SNRs? What controls theHow saturation do of CRs CR-driven instabilities? diffuse in self-generated fields, both inIs SNRs there and any in observational the evidence Galaxy? of DSA in SNRs? What determines the CR transport in the Galaxy? GeV [ E 5 As presented in the previous section, the SNR paradigm seems to check most, if not all, of iv. iii. i. ii. The most intriguing aspect of such a stupendous amplification of the pre-shock magnetic field [ • • • • • • 28 10 the requirements to be the ultimateseveral theory theoretical for assumptions, Galactic CR which acceleration.mulation, have that In accompanied reality, have it never the been encompasses model corroboratedobservational since by signatures. first-principles its Some calculations of very and/or the by original questions unequivocal crucial for- to the problem are: 2.2 What is missing in the standard SNR paradigm? the amplified magnetic fields, DSA would allow to reach energies as large as shock of SN1006 suggests that field amplification must occur in the and in Tycho [ dence challenges the scenarios in whichdynamo B processes is triggered amplified by upstream inhomogeneities (e.g., [ end of the Sedov stage [ scales resonant with CRs toations allow their were acceleration rearranged up in such to asmall the way knee. as that the Even the particle if mean gyroradius ISM free ( magnetic path fluctu- for pitch-angle scattering became as be fast-cooled above the criticalinstance, energy this at corresponds which to the an loss-time average downstream equals field the of SNR age; in Tycho, for in magnetic fields as large as a few hundred assess the relative importance of magneticmagnetic field fields damping are and indeed amplified radiative losses beyond simple and compression. to conclude that is that it iscelerated likely particles, due in to a the non-linearbulence, plasma chain and that then instabilities transfers back driven to energy bytion. from the the particles the The super-Alfvénic by CRs typical streaming enhancing to ISM their of the magnetic diffusion magnetic ac- and fluctuations tur- favoring correspond rapid to energiza- a diffusion coefficient of CR Acceleration and Propagation require localized magnetic fields of 10 . PoS(ICRC2015)008 p 2 ω / c dHybrid ]. ✝ , velocities the Alfvén p ✁ ✁ 55 ✡ by iteratively ω ✁ ✌ / mn c 20, showing a non- Damiano Caprioli π ]. 4 ]. The progress in = 64 √ M , 62 / is the velocity of the ✁✁ 0 ☞ s ab initio 63 B v ✄ if not otherwise specified). = ✁ A M v ✁ ✡✡ 2, where / the ion density, charge, and mass). 2 s , with A m mv v ✁✁ ☎ ☛ / ✁ while resolving the ion skin depth ≡ sh and v . In the inset, the momentum spectrum is multiplied p , sh ]. Particularly promising is also the coupling e sh ω E ❪ ≡ , E ✟ / ✁ 61 n ✎ ✍ ❝ 2 c ✡ A , A ❬ ✦ & ✁ ✞❊❊ M 6 60 M (both are indicated by E ✟ ]. Lengths are measured in units of ,

s " 55 ✆ 55 M ✽✒✓❚ ]. ✁ L approach, in which electrons are considered as a mass- ✧ ✁✁ r 57 ☞ ✏✑ ✙ sh ✘ ✗ v ♣✕ ♠✖ / " " v , and energies to hybrid ✁ A ≈ v ▼✏①✇❡❧ ✏♥ ✐ ❧ ✡✡ sh v ✂☎ /

✁ ✔ ! " " ✔ ✔ ✔ " D " " " " ], and still model shock formation, ion acceleration, and plasma insta- ✛ ✜ ✚ ✚ ✢ ✹ ✁✁ ☛ 56 is the ion plasma frequency and m ✁ ✂✄ ✡ / 2 ✁ ✂✝ ✆ ✂☎ ✂✄ ☎ ]), but their application to astrophysical shocks has been quite limited. SNR shocks ✁ ✁ ✁ ✁ ✁ ne , assumed to be comparable with ❢ ✠ ✭ ✠ ✮ ✶ ✳ ✺ 59 π A Time evolution of the post-shock ion energy spectrum for a parallel shock with 4 , M p 58 to emphasize the agreement with DSA prediction at strong shocks. The downstream temperature is reduced by Yet, very recently, a comprehensive analysis of ion acceleration has been performed via large Large 2D and 3D hybrid simulations have been performed with the Newtonian code To give an idea, time and length scales accessible to hybrid simulations on modern supercomputers are comparable To address most of these questions it is necessary to model the non-linear interplay between = 4 2 ] where the shock is set up as outlined in [ p p 20% with respect to standard jump conditions, because of the energy going into accelerated particles [ ω 65 ( 2D/3D hybrid simulations of strong shocks [ with the physical scales of the Earth’s bow shock [ velocity) Mach numbers, whichlength makes of accelerated it ions computationally challenging to capture the diffusion modeling non-relativistic shocks via first-principlessimulations simulations showing is simultaneous finally acceleration attested of by both ions the and first electrons PIC [ 3.1 Hybrid simulations: Ion acceleration [ upstream fluid in the downstreamnumber frame. The shock strength is expressed by the Alfvénic Mach are characterized by large sonic and Alfvénic ( of the hybrid technique with a MHD description of the background plasma [ energetic particles and the electromagnetictrophysical fields, plasmas which are is typically veryteractions collisionless, hard rather i.e., to than their tackle by dynamics analytically. binary is collisions, As- mediated and by can collective be in- fruitfully modeled 3. Collisionless Shocks: Kinetic Simulations moving particles on a grid accordingtromagnetic fields. to the In Lorentz order force tosimulations, and mitigate self-consistently one the adjusting may high the computational revert elec- to cost of the such particle-in-cells (PIC) bilities self-consistently. Hybrid simulations have(e.g., been [ extensively used for heliospheric shocks normalized to the Alfvén speed Figure 1: less neutralizing fluid [ CR Acceleration and Propagation thermal tail that stems out of the thermal distribution for by ∼ PoS(ICRC2015)008 ! ! " ! " !" !" !" #""" #""" #""" , see = 45 = 50 = 60 = 80 = 0 = 20 = 30 = 45 = 50 = 60 = 80 = 0 = 20 = 30 = 45 = 50 = 60 = 80 = 0 = 20 = 30 = 0 = 20 = 30 = 45 = 50 = 60 = 80 ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ ϑ " ]. 55

! !" !" !" " ]; such a spec- "#"" "#"" "#"" Damiano Caprioli ! 55

) 10%) is achieved for , instead, shows the 1 − c ! ! !

2 ω & 20 [ !" !" !" " cr ] ] ] = ξ = 198 p p p sh sh sh sh t ω ω ω c/ c/ c/ )]( M [ [ [ """" """" """"

Post-shock particle spectra for E/E E/E E/E E/E E x x x ( " " " " Ef [ ]). On top of these instabilities, !" " !" !" 10 ]. 70 [ Log resonant streaming instability 55 [ ! 30, with the upstream (downstream)

ϑ = ] have been able, for the ﬁrst time, to ! ! ! !#"" !#"" !#"" Right Panel:

. M " !" !" !" 61 , M = 10 = 30 = 50 = 5 " 60

M M M M , between the shock normal and the background

! ]).

55 7 ! ! " " # ! ! " ! ! " ! 15% of the shock kinetic energy is converted into ! ! ! ! " ! " ϑ

!" " !" !"

" ! " ! " ! "

!""" !""" !"""

sh 10 10 sh 10

E/E ( Log ( Log ) E/E (

!" !" !" !" !" !" !" !" !" Log !" !" !" !" !" !" !" " " " " " " !" !"

, 15% at strong, quasi-parallel shocks, and drops for ) E ( Ef ) E ( Ef ) E ( Ef ) E ( Ef ≈ as a function of shock strength and inclination. The 72 cr & for a parallel (perpendicular) shock. , regardless of , ξ cr ◦ 80 ) . The largest acceleration efﬁciency ( ξ ◦ 71 ], in particular with the generation of magnetic turbulence at 45 M 90 M=30 M=50 M= 5 M=10 & ( 33 70 ◦ ϑ 0 shows of the downstream energy density in non-thermal particles as a function ] for a comparison of different approaches to the problem). 50 and different inclinations; the DSA non-thermal tail vanishes for 60 = 2 cr 68 = ξ ϑ 50 M non-resonant hybrid, NRH, instability 40 (deg) Fraction ϑ ; therefore, 30 0 B shows the structure of a parallel shock with 20 ). In this case, a fraction of 3 ] for reviews, and [ Left Panel: 2.1 67 , independently of the shock Mach number. The right panel of Figure 10 ◦ ]). More recently, Bell pointed out that non-resonant, short-wavelength modes may grow , 50 and different shock obliquities, as in the legend. The black dashed line represents the downstream 45 33 Since the initial formulation of the DSA theory, particle acceleration has been predicted to be The left panel of Figure Figure The kinetic simulations presented in refs. [ 66 0 , 5 0

15 10 = & Efficiency (%) Efficiency shows the ion spectrum in the downstream of a parallel shock with 69 to the right (left). Inamplify the the shock initial precursor, magnetic a field cloud by of a non-thermal factor particles of drives a a few, also current leading able to to the formation of underdense associated with plasma instabilities [ 3.1.2 Magnetic Field Amplification scales comparable to the gyroradii[ of the accelerated particles ( faster than resonant ones ( acceleration efficiency can beϑ as high as quasi-perpendicular shocks, where ions gain a factorshow of the few in same energy, dependence at of most. the Also acceleration 3D simulations efficiency on CR Acceleration and Propagation 1 trum develops a non-thermal tailby whose accelerated extent ions) (corresponding increases to withtion the time (Eq. maximum and energy whose achieved slope agrees perfectly with the DSA predic- ion spectra for shocks with which excite modes parallelmodes to are the expected to background grow, magnetic too (e.g., field, [ some transverse and filamentary Figure 2: of shock inclinations and Mach numbers, 3.1.1 Ab-initio DSA energetic ions, and theHugoniot post-shock jump temperature conditions. is Such accordingly aof reduced modification efficient is with CR an respect acceleration exquisite to manifestation andsee Rankine– of is [ the usually back-reaction accounted for in models of non-linear DSA (NLDSA, Maxwellian. Note how the non-thermal power-law tail develops only at low-inclination shocks [ strong, parallel shocks, and drops for M The shock inclination is defined by the angle demonstrate that DSA acceleration at non-relativistic strong shocks can indeed be efficient. Figure magnetic field PoS(ICRC2015)008 , ] ∗ L . 5 0 −5 5 0 −5 ]. r 60 0 ; in / 72 ) 7 for B 1

p / 6500 6500 ( & ≈ B L r δ / 6000 6000 1 max ). k 3 ∼ 5500 5500 ) Damiano Caprioli p 5000 5000 ( k ). Such a rich structure is z ) ) 4500 4500 1 1 − − c c 5 to factors of B ω ω ] ] : density; total magnetic ﬁeld , p p y . c/ω c/ω = 500 = 500 [ [ 4000 4000 t t B x x ( ( , y x x M B B B 3500 3500 ]. The reader can refer to [ 78 Left panels 3000 3000 1 and both wave polarizations are 30. . 2500 2500 = 0 B M / 2000 2000 B δ

0 0 1500 1500 500 500

1000 1000

p p ] c/ω [ y ] c/ω [ y 10, 8 20 15 10 5 . M

6500 6000 : components of the magnetic field ( ]. The two methods return consistent results, as shown in the ] while the non-resonant one can grow up to non-linear levels 5500 61 77 20 the NRH instability grows significantly faster than the resonant and ranges from factors of a few for 5000 1 [ & M ∼ Right panels ) 1 4500 − c 0 √ M ω ] B p ∝ / =500 c/ω [ 4000 t ( x B 0 tot δ B B ]). The typical size of the cavities is comparable with the gyroradius of the / 3500 B 71 phase space. δ 3000 ], exciting distinctive right-handed modes with wavelength much smaller than the x p of the CRs driving the current. Then, in the non-linear stage, an inverse cascade in 76 ], fig. 5). For ∗ L − , 2500 r x ]) may become important even in the absence of large pre-existing density fluctuations. ]. Bohm diffusion is often heuristically extrapolated into the regime of strong field 60 Output of a 2D hybrid simulation of a parallel shock with 75 74 33 , 2000 [ , 4 50 ([ 71 CRs are scattered in pitch angle by waves with resonant wavenumbers Global hybrid simulations allow to reconstruct CR diffusion in different regions of the shock, Magnetic field generation depends on the presence of diffuse ions, hence it is more prominent The propagation of the shock through such an inhomogeneous medium leads to the forma- 50

− 0 1500 , space progressively channels magnetic energy into modes with increasingly small wavenumber 500 &

1000

p ] ω c/ [ y − . The NRH instability eventually saturates when the maximally-growing mode is 49 1) for an Alfvénic∝ turbulence generated via resonant streaming instability by a CR distribution the regime of small deflectionspopular this choice process is can to be assume described the by Bohm a limit, diffusion which coefficient. is The obtained most (in the quasi-linear limit either by using an analyticalor procedure by based tracking on individual the particles extent [ of the CR distribution in the upstream M observed, consistently with the prediction offor quasi-linear a theory more [ detailed discussion of the wave spectra and the3.1.3 saturation of Particle the Diffusion two instabilities. amplification, but such a prescription used to lack a solid justification. one [ before the driving current is disrupted. For at quasi-parallel shocks. Simulations showshock that scales the as maximum amplification achieved in the fore- cavities filled with energeticfields particles (see and also surrounded [ by dense filaments with strong magnetic gyroradius k k which effectively scatters the current ions.urates This already is when the very reason why the resonant instability sat- Figure 3: highest-energy particles (a few hundred ion skin depths for the simulation in Figure tion of turbulent structures (viastirred, the Richtmyer–Meshkov stretched, instability), and in which further[ magnetic amplified. fields are In this case, amplification via turbulent dynamo (e.g., CR Acceleration and Propagation strength; ion entirely generated by instabilities driven by accelerated ions diffusing in the upstream (to the right in the figures) [ PoS(ICRC2015)008 , 1 & 2000 0 B / 1800 B ]). δ 1600 82 , ), demonstrates Damiano Caprioli 2 1400 53 , . The right panel of supra-thermal ions , which encounter a 81 1200 ] max via shock drift acceler- 1 − c E ω [ 1000 sh t E 800 10 ≈ thermal ions 600 inj E 400 . = 20 = 60 M M DSA E 200 is roughly proportional to the Bohm coeffi- ]. 0 ) 0

500 400 300 200 100

] )[ ( sh max that allows them to escape upstream. Only non- E t E ( D , which are reflected, energized via several cycles 9 ]. The sharp shock transition (few ion skin depths) inj E 83 ], it is also possible to calculate the minimum energy 3 15000 & 87 10 E ], where the scattering rate depends on the magnetic power : Time evolution of the maximum ion energy for parallel shocks with 79 , 33 ], figs. 6 and 7) peaks at relatively large wavelengths, comparable 2 10000 non-thermal ions 10 60 ] for more details). When magnetic field amplification occurs in the Right panel ] p ] 61 ), which sets the period for ion injection. c/ω sh [ 80. c 20), particle scattering is well described by the diffusion coefficient self- E , sh [ ]); iii) x ω E ]); because of such a reformation, the height of the barrier fluctuates on a 20 . − / 85 x (see [ π = 83 , M normalization depends on the level of magnetic field amplification 4 , 5000 1 M ∼ 86 : Diffusion coefficient, normalized to Bohm, immediately in front of the shock for hybrid simu- 10 85 , = 20 = 80 M M 84 ]). Such a scaling is determined by the fact that far upstream the spectrum of the overall 80 Left panel shows such an evolution, which is linear with time with a slope inversely proportional to 0 4 60, compared with the DSA prediction (dashed lines) [ 0 , 10 By generalizing the formalism of ref. [ Explaining the correlation between ion acceleration and shock obliquity requires understand- At any shock reformation cycle, about 25% of the incoming ions are reflected, but not all of The effective scattering rate is also imprinted in the time evolution of 0 −1 −1 1 2 1 0 20

10 10 10 10 10 10 10

B B /D ) E ( D /D ) E ( D = that reflection is necessary but not sufficient condition for DSA injection. ing the conditions necessary for thermalsimulations particles show to that be all injected the into ionspotential barrier DSA. that at High-resolution eventually their achieve hybrid first large shock energies encounter are [ reflected by the shock them enter DSA. Ions impinging on the shock may turn into: i) 3.1.4 A theory of ion injection “low” barrier too weak to reflect them and immediately cross downstream; ii) of SDA, and eventually achieve an energy thermal ions are really injected intoon the self-generated DSA process, turbulence— since to they get mustquasi-perpendicular rely shocks, back on which to diffusion in —possibly the simulations shock. do not The show DSA existence tails of (Figure supra-thermal ions at Larmor timescale ( excited magnetic turbulence ([ the measured diffusion coefficient (dashed lines), as expected for DSA (e.g., [ quasi-linear regime ( in resonant waves. For stronger shocks, instead, with the gyroradius of the highest-energy ions. Figure is associated with compression andpotential pressure that increase generates (overshoot), an and upstream-directed withreflection electric a field. of cross-shock electric impinging At quasi-parallel ions shocks,shock induces the (e.g., a coherent [ shock reformation about one gyroradius upstream of the which are initially reflected by agyrations “high” around barrier, the but shock, are achieving advected a downstream maximum energy during their first few Figure 4: lations of shocks with M left panel of Figure generated via resonant instability [ CR Acceleration and Propagation ation (SDA, e.g., [ cient and its (see also [ PoS(ICRC2015)008 . ) ◦ ◦ ]. ϑ 45 ( 30 83 ). inj ≈ = 3 SDA E

ϑ 3.1.4 ϑ − !" 2 Simulation Maxwellian Minimal model ≈ ] shows that at Damiano Caprioli 20, and N 63 ; ii) ion injection , ). ◦ ◦ = ! 5 30 !" ]. These findings can 55 M sh , , allowing the first self- . . 60 c 4 1 E/E . − ϑ ϑ 0 p . = ∝ ◦ " sh !" v Post-shock ion spectrum for a parallel ]; in particular, ref. [ 64 ], which accounts for shock reformation 1% of the incoming particles is injected. ! ! , (B) 0 corresponds to the first shock encounter. Ions 83 !" ∼ [ 63 = ! ! " Right Panel: N t ! " ! ! ! !

!" !" !" !" !" !" ) E ( Ef 25 . 10 0 $ ∼ and # shows ion and electron phase-space and spectra, as well as sh ]; for each ion, E 6 83 [ " 10 ◦ 2 ≈ ± ]), a comprehensive theory of electron injection is still missing. inj ◦ ! ] E 1 45 90 − c at the shock and induce electron reﬂection because of magnetic mirror- ω , [ π ' B 4, and the fraction of ions that escape upstream goes to zero quite rapidly. t/ 89

Injected ions ϑ SDA ions , Trajectories of test-particles impinging at random times on a periodically-reforming minimal model for ion injection & 88 and, since gyrating ions have a ﬁnite probability to encounter the barrier in the N ! , 10, as obtained in simulations and compared with the minimal model outlined in ref. [ 10 and ϑ ◦ requires SDA pre-energization. The number of cycles needed to achieve = = 60 is not too different from those inferred from multi-wavelength observations of young ◦ Advected ions 3 M M Left Panel: " ∼ 30 − ϑ (A) 10 & ] for more details): i) cold ions can be directly injected only if Electron acceleration feeds on the NRH modes excited by energetic ions, which increases the Ions do not conserve their magnetic moment since the shock is reforming on their Larmor time scale (§ The injection of electrons into DSA has traditionally been an outstanding problem since they Only the computationally-expensive PIC approach can study electron acceleration ab initio. needed to escape upstream of a shock with a given inclination. The main results are (see §3 . Non-resonant modes have the right polarization to effectively scatter electrons, too. Prelim- ≈ ! ϑ 83 3 " $ " $ " % !

! ! !

c sh L sh ] / v = r [ x ) t ( X

ω − ep inj ing effective inclination of density and self-generated magnetic field profiles for a shock with have smaller gyroradii compared to ionsstead and pernicious the electrostatic to barrier electrons. crucial Despite for severalthese ion different issues reflection mechanisms (e.g., is have [ in- been proposed to address be encapsulated in a 3.2 PIC simulations: Electron acceleration quasi-parallel shocks both species develop the DSA power-law tail This year the first (1D) simulationsof of both non-relativistic electrons shocks and that ions show appeared simultaneous in acceleration the literature [ and reflection at thenon-thermal shock distributions, barrier, as and well as reproduces their the phase-space distribution fraction (see of Figure ions in the supra-thermal and SNRs and in Galactic CRs. Figure consistent measurement of the electron/proton ratioK in accelerated particles. The reported value of Figure 5: shock with may either not reflect (“Advected ions”), orescape upstream experience after SDA before a few ending reflections up (“Injected downstream ions”). (“SDA ions”), or CR Acceleration and Propagation shock with E of [ for cycles are needed to reach Above DSA-efficient shocks always converge to a configuration where an effective inclination increases with low state and be advected downstream,number of the cycles fraction is of exponentially ions suppressed; iii) that for can inclinations perform 30 an increasingly large is achieved because of the non-linear field amplification in the precursor [ PoS(ICRC2015)008 ep K ]). The ]. ]). Recent 101 72 97 ]. Finally, the ]. ], in qualitative 63 60 Damiano Caprioli , despite ions can , 95 ◦ [ 55 4 45 0 B & ϑ ]. ] and the review [ 94 , 100 , Evolution of the downstream momentum 93 99 100. Energetic protons and electrons diffuse = e m / –rays with Fermi-LAT favors values of p γ m Right Panel: phase space distributions, density profile (c), and transverse , and 11 x ◦ p 30 − x = ϑ , c ] show that the electrons energized in the shock foot (e.g., 1 . 0 = 1) typical of galaxy clusters [ ) show that the radio emission from the synchrotron-bright polar 7 sh & v ]) suggests either a mostly-perpendicular shock geometry or that the s M ]. These SNRs show a bilateral symmetry, found also in some older 103 , 96 A 102 1%; also the non-detection of M ] and the bright knots observed in RX J1713.7–3946 may be signatures of the Proton (a) and electron (b) & 98 cr ξ ep Caprioli et al., in prog. (d) for a shock with K B ]) can achieve velocities large enough to be injected even for Left Panel: ' e 63 ξ , Starlight polarization in the direction of SN1006 is consistent with this picture, too [B. Draine, priv. comm.] A natural question is whether such findings have any observational counterpart in SNRs or in On a much larger scale, the weak shocks in galaxy cluster can also be used to probe the 92 4 , Mpc) shocks (e.g., [ 91 ∼ not. Electron reflection and a hintfor of the a weak Fermi-like shocks process ( at oblique shocks has been reported also observations of SN1006 (Figure agreement with the trend ofTycho’s magnetic “stripes” field [ amplification seen in simulations [ radio emission is dominated by the oblique regions of the shock, where ion injection is disfavored. filamentation instability operating in the localized regions where the shock is parallel [ SNRs, that may correlate with the geometry of the backgroundcaps magnetic field has (e.g., a [ low degreefield; of such polarization, regions implying are the also inferred presence to of be a those strong parallel and to turbulent the magnetic large scale other non-relativistic shocks with knownmagnetic geometry. field is The of typical order coherence ofexceptions: length 10–50 pc, of SN1006, comparable the which with the lies Galactic radius significantlylength of above most should the SNRs, Galactic be with plane, two the notable whereSNR larger the in field than the coherence in Milky Way the [ disk, and G1.9+0.3, the youngest (and likely smallest) conditions conducive to electron and ion acceleration (e.g., [ ahead of the shock,distributions amplifying for the (a) protons upstream and magnetic (b) electrons. field. The dashed lines correspondinary to results thermal distributions [ [ 3.3 Comparisons with observations prominent radio emission of giant haloscies requires in of some cases large electronsignificantly acceleration larger efficien- than those inferred in( SNRs. The high degree of polarization of very extended Figure 6: [ CR Acceleration and Propagation component of PoS(ICRC2015)008 17 for 17 for 17 for y y ). y B B B 10 1 .Also,the .Also,the .Also,the ]; the 0 0 0 B ≈ B B 1 1 . 1 . . 95 3.1.2 0 0 0 ! ! ! cut z z z , -plane; the three panels B -plane; the three panels B -plane; the three panels B γ xy xy . . xy . ntheparallelcase,wherein E ntheparallelcase,wherein ntheparallelcase,wherein sition is marked by a plane of sition is marked by a plane of sition is marked by a plane of erent i erent i erent i ff ff ff . Any Galactic e Damiano Caprioli ] =0deg,whileitismainlyalong =0deg,whileitismainlyalong 80 deg (top to bottom). The iso-volume 80 deg (top to bottom). The iso-volume , =0deg,whileitismainlyalong 80 deg (top to bottom). The iso-volume , , ]. The quasi-parallel ϑ , ϑ ,whichliesinthe ,whichliesinthe ϑ 45 45 ,whichliesinthe 0 0 , 45 , 0 , B B 60 B max =0 =0 107 =0 ϑ ϑ , E ϑ , or of electrons with . cut 106 , γ , cut , E γ 7 )withinclinations )withinclinations E 8 8 )withinclinations 105 Acolorfigureisavailableintheonlinejournal Acolorfigureisavailableintheonlinejournal 8 ,inunitsoftheinitialfield ,inunitsoftheinitialfield Acolorfigureisavailableintheonlinejournal z z ,inunitsoftheinitialfield . Right panels show the self-generated z B B ∼ B 0 both upstream and downstream) for both upstream and downstream) for p B both upstream and downstream) for , ,andthequasi-perpendicularcase,whereintheupstream ,andthequasi-perpendicularcase,whereintheupstream 0 0 ,andthequasi-perpendicularcase,whereintheupstream 0 B B B ≈ ≈ max ≈ z z .Theamountofmagneticfieldamplificationisverydi .Theamountofmagneticfieldamplificationisverydi z B B p p .Theamountofmagneticfieldamplificationisverydi E ω ω B p 1, with the respective color code in the legends. The shock po 1, with the respective color code in the legends. The shock po ω c/ c/ 1, with the respective color code in the legends. The shock po ≤ ≤ c/ ≤ z z B B z =600 =600 erent 3D simulations (section erent 3D simulations (section o B o =600 ff x ff x ≤ ≤ erent 3D simulations (section o ff x ≤ 1 1 =45deg =80deg =0deg =45deg =80deg =0deg − − 1 for di for di =45deg =80deg =0deg ϑ ϑ ϑ ϑ ϑ ϑ − Simulations of ion acceleration at shocks: DSA efficiency Simulations of ion acceleration at shocks: DSA efficiency =0 for di 1 1 ϑ ϑ ϑ Simulations of ion acceleration at shocks: DSA efficiency − c − c 1 =45 � ω ω − c =80 ω � =45degcaseshowsintermediateproperties. =45degcaseshowsintermediateproperties. � 12 ϑ =45degcaseshowsintermediateproperties. ϑ =200 =200 t ϑ t =200 Self-generated component of the magnetic field, Self-generated component of the magnetic field, t Self-generated component of the magnetic field, 0 0 0 0 0 0 0 0 0 /B /B /B /B /B /B z z z z z z /B /B /B B B B B B B z z z =80deg.The =80deg.The B B B the upstream there are several regions with correspond to Figure 13. magnetic field exhibits large-scale turbulent structures ( ϑ rendering shows 10 levels of enhanced magnetic field, around the upstream there are several regions with correspond to Figure 13. magnetic field exhibits large-scale turbulent structures ( ϑ rendering shows 10 levels of enhanced magnetic field, around =80deg.The the upstream there are several regions with correspond to Figure 13. magnetic field exhibits large-scale turbulent structures ( ϑ rendering shows 10 levels of enhanced magnetic field, around decay). SNR spectra typically exhibit cutoffs around SN 1006: a parallel accelerator a parallel SN 1006: 0 TeV in the case of IC of CMB photons in the Thompson regime; if π rays, but they have not reported any SNR detection so far. For recent / − cut γ , γ 0 was at least twice its error were kept. E p B p –24– -ray astronomy, including SNR observations, see refs. [ γ The top left panel shows the thermal (red) and non-thermal (light blue) X-ray emission from SN1006, ] and refs. therein). Such a transition from an ion- to electron-foreshock modulated by 10TeV 150 TeV, the onset of the Klein–Nishina regime enforces of SN 1006 at 1.4 GHz. The resolution is 10 arcsecs. The p ∼ & 104 -ray observations of SNRs have been performed both via imaging atmospheric Cherenkov -ray emission may be either leptonic (relativistic bremsstrahlung and inverse-Compton scat- e e , , Finally, at the terrestrial bow shock, accelerated (diffuse) ions are only detected in the quasi- γ γ max max E parallel regions, while energetic electrons are(e.g., observed upstream [ also for more oblique geometries telescopes (IACTs, such as HESS, MAGIC,lites and VERITAS) (Fermi in and the TeV AGILE) energy in rangecan the and probe via GeV satel- band; multi-TeV also waterreviews on Cherenkov observatories (such as HAWC) tering, IC) or hadronic ( arrow shows the fiducial direction ofmagnetic the fields background for Galactic shocks magnetic ofregions field different show little inclinations, polarization obtained and in enhanced the synchrotron emissivity, hybrid consistent simulations with simulation in outputs. ref. [ 4. Gamma-ray observations of SNRs 4.1 Hunt for hadronic PeVatrons TeV, which implies the presence either ofE protons with the shock inclination is consistent with kinetic simulations without pre-existing turbulence (§ Figure 7: while the bottom left panel shows the corresponding degree of polarization in the radio band, as in the legend [ CR Acceleration and Propagation color scale is shown at the right. Only pixels where Fig. 4.— Fractional polarization PoS(ICRC2015)008 ]. ]). ]). erg) -ray 82 108 γ 112 , 100 yr 51 . IACTs − 53 10 –rays. 3 γ − & 50 –rays above 10 γ . Damiano Caprioli ]). ep K 120 . For the fiducial DSA , 2 ]), while the GeV flux is with time, the number of / ) 1 119 − 113 -rays produced via the decay max α γ ( E ν -ray telescopes is consistent with ∝ γ ) ν ( F ν 13 ]). On the other hand, SNRs exhibiting hard 114 ]). A natural caveat comes from the finite resolution of 118 , above a few PeV (PeVatron) should also emit , bremsstrahlung and pion decay return photon energy spectra (for bremsstrahlung) or a very steep electron spectra (for IC), , IC and pion decay provide a comparable flux in the TeV 3 5 α ep − − 117 K , E 10 120 yr old, but its line of sight is too close to the Galactic center for . 116 ∼ or steeper should be produced by pion decay, since leptonic processes , 2 ep − K 115 E ∝ mass, which can be considered the smoking gun for the hadronic nature of the ]. Yet, the question remains whether such CRs have been freshly accelerated, or ]. It is indeed possible that the Milky Way already hosts (or will host) a very 0 find that multi-PeV energies can only be achieved in powerful SNe ( , while IC returns a flatter spectrum of π α 122 111 − 2 and for max , 2 E ν = 121 ∝ ]. Historical SNRs such as Cas A, Tycho, and SN1006 are no longer expected to be α -ray measurements. Even with more optimistic scalings of ) γ ν 110 ( Pulsar wind nebulae do show acceleration of electrons and positrons up to PeV energies. The spectra of two middle-age SNRs (W44 and IC443) show the characteristic low-energy The overall spectral slope from GeV to TeV energies is by itself an indicator of the nature of Radio observations unequivocally attest to the presence of relativistic electrons in SNRs. The Nevertheless, such a lack of detection is not inconsistent with the SNR paradigm. For decades , F 5 ν -ray instruments, which often cannot resolve shock inhomogeneities or even the presence of MCs, 109 emission [ cutoff below the typically dominated by pion decay; bremsstrahlung may be important only if would require either a very large spectra (e.g., RX J1713.7–3946, RCWleptonic 86, scenario Vela (e.g., Jr., [ SN1006) are more naturally explained in the are diffuse CRs merely re-energized by theslope recent quite shock similar passage, to as that also of suggested diffuse but CRs. the spectral range, assuming standard ISM density and photon background (e.g., [ inconsistent with radio observations (e.g., [ the emission: spectra [ of neutral pions originated in nuclearFor collisions a between parent particle CRs spectrum and of theof thermal plasma (e.g., [ index of almost invaribaly measure photons produceddifficult by to particles disentangle close the to nature the ofTeV cutoff, the data emission which process. are makes In available it the for very origin few a cases of remnant, in the which emission instead, both by it GeV fitting and the may multi-wavelength be SNR possible emission from to radio assess to the TeV leptonic/hadronic γ which may allow for a much more complicated phenomenology (e.g., [ direct evidence of hadronic acceleration, instead, may be revealed by PeVatrons; G1.9+0.3 is only optimal young SNR that may be detectable in the next4.2 few years, especially Hadronic thanks or to leptonic? CTA. the maximum CR energy has beenjust thought to before be the achieved shock at the slows end down of because the of ejecta-dominated stage, the inertia of the swept-up material (e.g., [ CR Acceleration and Propagation accelerator able to produce hadrons a few hundred TeV. At theeven time if few of objects writing, show no no clear PeVatron evidence has of been a high-energy associated cutoff, with such as Galactic HESS SNRs, J1641–463 [ Now that the crucial role ofevolution the of NRH instability has beenexploding attested, in the most dense refined pre-SN models winds for and the that SNRs might act as PeVatrons only for young Galactic SNRs that fallno within detections the [ sensitivity of present PoS(ICRC2015)008 0 ]. ]; π for 3 at r . 123 125 2 ]. , , . − r 47 119 2 124 . , 2 , ≈ 123 114 , Damiano Caprioli α ], fig. 11). 47 47 10% and the level of ∼ ]. The velocity of such 15% [ 20 ([ ]. The non-linear balance cr 33 ∼ ∼ ξ 114 , 132 7 established by the CR relativistic 3 kpc; and its morphology is quite , ]. The NLDSA theory includes the ]) on the shock dynamics, predicting ' ). IC would likely dominate over ∼ 3 r 67 131 − , 127 -rays only if there is a large reservoir of γ , )[ 66 2.1 ]). A few possible solutions have been put 126 130 and induce an effective compression ratio ˜ 14 , at the highest energies. A simple way of thinking to 0 5 B . 1 / 129 − B , E δ ]. For typical SNR parameters, ∝ 128 A may be good indicators of hadronic emission, but are also ad v 133 2 or just be driven by the environment: the same SNR, with the − with the compression ratio CR acceleration in SNRs comes from the multi-wavelength emis- E cr ξ 2.1 local In principle, CRs do not feel the fluid compression ratio, but rather the and/or ep K ) is discussed in [ cr ξ concave CR spectra as flat as expected when CR acceleration is efficient [ cr ξ or steeper (e.g., IC443, W44, W28, W51, etc.) and are almost invariably associated with 5 . -ray spectra steeper than 2 Magnetic feedback. γ The best evidence for The number of Fermi detection of TeV-emitting SNRs is steadly increasing [ It is plausible that old SNRs can be detected in the − E for large this effect is considering Eq. forward to solve this apparent discrepancy between standard DSA and observations. compression ratio of the magneticfluctuations fluctuations is they of are the coupledvelocity order with in the [ of shock the reference frame. Alfvéneffect may In velocity become the non-negligible presence and if of is magneticthe field typically CRs, amplification, much which instead, this in smaller turns than leadsbetween the to the flattening steeper induced fluid spectra by (Eq. efficientincreases CR acceleration with and the magnetic feedbackmagnetic (which also field amplification inferred in SNRs generally lead to spectral indexes odds with the prediction offlattening standard DSA for strong shocks, and even more inconsistent with the spherically symmetric and well resolved in theits radio morphology and and X-rays. multi-wavelength emission The by groups meansconcluded that of that have the investigated it most must refined be theoretical an models hadronic all accelerator, with an efficiency of 4.3 The origin of steep spectra backreaction of CRs (and amplified magnetic fields [ fluid with adiabatic index 4/3 (e.g., [ the beginning of the Sedov stage, exactly as required for explaining Tycho’s spectrum [ sion of single SNRs.remnant Tycho (SN1572) of is a arguably type-Ia the SN,light best so echo candidate that has for age, been such explosion measured, an energy, and analysis: which ejecta returns is mass a the are distance well of constrained; its targets, while the dominant emission mechanism inmedium, young which SNRs for may core-collapse depend SNe on is their aThe circumstellar complex cases mixture of of Tycho underdense and bubbles SN1006 andat are dense paradigmatic: the MCs. beginning they both of have the amuch Sedov type-Ia stage, more progenitor but and rarefied SN1006 are medium looks just (about more 0.05 leptonic vs because 1 it is protons expanding cm in a CR Acceleration and Propagation therefore, it is tempting∝ to work out aMCs, population while younger synthesis. remnants may show Older(e.g., either SNRs rather RX steep tend J1713.7–3946, (e.g., to Tycho RCW andtime show Cas 86, evolution spectra A) Vela of or Jr., flat spectra andsame SN1006). content of relativistic Such electrons and agas ions, trend reservoirs may look (e.g., may more warm either “hadronic” phase in reflectbackground ISM, the (e.g., the presence infra-red MCs) of and or dense optical more photons “leptonic” in star-forming in regions). the presence of an intense IC decay also in Tycho if the upstream density were smaller by a factor of PoS(ICRC2015)008 . If 1 − ) because km s 3 4 − ]. E 10 emission (e.g., Damiano Caprioli & 143 α neutral return flux sh ]. Another possible v ). Finally, it has been 141 ]) and often broad lines 3.1.4 ]. Such a neutral feedback , since for larger velocities 137 1 , − 140 emission is the only way of di- 136 as a result of the self-similar SNR 100) relevant for young SNRs has α 2 & − 0 E 3000 km s B ∝ / . B δ sh v 15 ]. The presence of escaping particles is also revealed by the -ray–bright SNRs are either associated with dense MCs (e.g., γ 132 ) than it would be in an ionized medium and accelerates particles r , ]. More recently it has been pointed out that such anomalous line 145 ] or by joining the Galactic pool when the SNR fades away. Trapped 138 It also possible that some of the assumptions of the DSA theory are vi- 144 Most of the ]. These scenarios assume that CR injection may happen also at quasi-perpendicular ]. Such a neutral return flux leads to the formation of a neutral-induced precursor, in 142 139 To connect the CR spectra inferred in SNRs and the flux of CRs measured at Earth it is crucial Neutral feedback. Other explanations. ]) or propagate into partially-ionized media,]). as revealed The by high their prominent resolution H of optical telescopes often allows to detect both a broad and a narrow -ray emission from nearby MCs, which in principle contains interesting information about CR 134 135 to understand how acceleratedescaping particles from are upstream released [ intoparticles the undergo ISM, strong which adiabatic losses, may soenergies. happen that The either only spectrum escaping by of particles escaping canSNR particles account emission, is for but in knee-like it principle may different still from be that a responsbile universal for power-law the reason for spectral steepening attively quasi-perpendicular sweep shocks CRs is through that the the shock,probable magnetic [ making their field return may to effec- the shock for further accelerationsuggested less that strong ion-neutral damping may steepen the CR spectrum (typically to 5. From accelerated particles to cosmic rays evolution in the Sedov stage [ γ shocks, which is not granted in the absence of energetic seeds, though (§ [ are anomalously-narrow, which initially suggestedin that CRs a rather large fraction than of in shock heat energy [ ended up which the incoming fluid ismuch slowed weaker (i.e., down with and a reduced significantlywith heated steeper up; spectra, hence, even the when shock the becomes CR backreaction is included [ of the partial evanescence of Alfvén waves above a critical energy of a few GeV [ [ Balmer line, whose widths arewhen determined ions by and the neutrals downstream and are upstream coupled plasma via temperature charge exchange. H widths may be produced also by the dynamical backreaction of the so-called olated, especially at quasi-perpendicular shocks and for large shock velocities CR Acceleration and Propagation Even if this mechanism seems towave work phase for velocities phenomenological in purposes, the the assumednot very scaling been non-linear of convincingly regime the proven, ( yet. rectly probing ion temperaturewith in respect SNRs to and the standard measurementsto predictions have be of revealed gaseous consistent interesting shocks: with deviations sometimes quasi-neutral narrow gas lines in are too the broad first place (e.g., [ induced by the population ofshock hot [ neutrals produced via charge exchange immediately behind the is expected to beionization important dominates at over charge SNR exchange and shocks the when neutral return flux vanishes. the magnetic field is not uniform,particles but across has the a average stochastic direction or of braided thescattering, structure, field resulting the may in transport anisotropic be and/or of more inhomogeneous complicated charged CR than distributions simple [ pitch-angle PoS(ICRC2015)008 ). 4.3 ]. Such CR- ). 2 156 Damiano Caprioli 3 is also found to , / 1 . Such scalings are 2 155 / ≈ ; iii) for intermediate 1 , A δ E v 154 ∝ ≈ , can account for the low level v D ], consistent with the inferred 200GeV (see § δ , is about seven orders of mag- 153 G ∼ µ 133 3 is also consistent with the steep / 1 100 B 200GV CRs diffuse in the pre-existing ≈ / & δ ; ii) below a few GV, CRs efficiently drive 3 GeV / E 1 E 20 ]) and about CR propagation in weakly-ionized ∝ 10 -ray observations of young SNRs (see § 16 γ 147 D ' , ]. Finally, ) E 41 146 ( , B ] for some attempts), especially because the saturation of D 134 150 , 65 and make the spectrum observed at Earth merely the result of a 149 . 3) inferred from . 2 2 ≈ ≈ δ δ ] and references therein). + − ]. In addition, only relatively small values of α 148 5 65 . ]. By assuming resonant streaming instability and non-linear Landau damping, 2 5 for the energy scaling of the Galactic confinement time. . 5 the intrinsic fluctuations in the distribution of SN explosions may significantly ≈ . 0 152 0 , α . & δ 151 δ On larger scales, the CR self-confinement is expected to be important also for determining their The local excess of energetic particles immediately outside the sources should generate gra- The coupling of low-energy CRs with the ISM is hence predicted to be strong, which im- When such a dual modality of particle release is accounted for, the total CR spectrum should 1%) of CR anisotropy below the knee [ . remarkably consistent with the “spectral hardening” observed at driven winds may play a pivotal rolegalactic in halos galaxy in formation, heavy suppressing elements. star formation and enriching nitude smaller than the Galacticproperly one at parameterize GeV the energies, transport and of noneproblem CRs of and around these the diffusion their large coefficients scales sources. can cally involved make or The it numerically strong very (see, non-linearity hard e.g., of to [ the address such a problem either analyti- waves with a Kolmogorov power spectrum and Alfénic modes and areenergies advected CRs with diffuse in the the self-generated self-generated waves Alfvénic at turbulence and transport in the Milky Way, and in particularof the anisotropy inferred in Galactic diffusion their coefficient arrival and directions.CRs the level and By the considering spectrum the of non-linearsolution the resonant for ISM coupling the magnetic between CR turbulence equilibrium itGalaxy distribution is [ and possible for to the work self-generated out diffusion a coefficient self-consistent in the the NRH instability in this context has not been assessed self-consistently, yet. 5.2 CR self-confinement in the Galaxy three regimes of CRs diffusion can be individuated: i) for dients in the CR distribution,to which self-confine are escaping expected particles. to drive Suchby plasma a instabilities one “sphere that of source eventually influence” dominates tend (the overbecause region the of where the the Galactic contribution steepness CR of sea)diffusion the becomes coefficient diffuse inferred increasingly spectrum in and large SNRs, can at be high as energies large as a kpc. The self-generated plies that CRs can ablate ionizedcharge-exchange gas is from rapid the Galactic enough) disk while (and they possibly escape quasi-neutral the material if Galaxy [ values of provide a more universal connection betweenthat the for injected and the diffuseloosen CR the spectra, constraint in the sense ( CR spectra ( 5.1 CR self-confinement around sources be a bit steeper than that at the beginning of the Sedov stage [ random realization [ CR Acceleration and Propagation diffusion close to their sources (e.g.,environments [ (e.g., [ PoS(ICRC2015)008 ]. 157 ), which 3.1.1 Damiano Caprioli -ray astronomy and kinetic γ ). Nevertheless, there are still 3.1.4 ) but covering the multi-dimensional pa- ]) or might as well be a signature that a 3.2 158 ) and reproducing DSA ab initio (§ 17 4 ), the CR-driven instabilities and damping mechanisms needed 5 Understanding how SNRs affect their circumstellar medium is of primary importance for as- PIC simulations have just started unraveling injection, acceleration, and thermalization of elec- The NRH instability is very likely the main channel of magnetic field amplification in SNRs Despite recent progresses (§ After many decades, the SNR paradigm still represents the most plausible explanation for The chemical composition of Galactic CRs is significantly much heavier than solar, especially sessing the mechanisms that regulateCRs star may formation and, play in a turnand crucial galaxy, possibly formation role also and in evolution. in transportingceleration the and at intracluster depositing non-relativistic medium energy shocks in and hasfor galaxy finally momentum embedding clusters. entered in theoretically its the Now and quantitative that ISM, observationallygalactic age, simulations, the motivated the whose sub-grid theory resolution time models of is may rapidly in CR be approaching cosmological ac- the ripe and scalesAcknowledgments of individual SNRs. I would like to thankotal the topic, ICRC the 2015 University organizers of Delaware forthe and their Shakti the kind IUPAP P. invitation (Commission Duggal to C4) Award for talklino, 2015, the about J. honor and such of Park, a all receiving piv- T. myresearch Jones, collaborators, was H. partially among Kang, supported which: by M.Max-Planck/Princeton E. NASA Center Vietri, Amato, (grant for and Plasma NNX14AQ34G G. Physics. in to Mor- particular D.C.) and P. Blasi facilitated by and the A. Spitkovsky. This rameter space relevant for SNR shocksratios will and require 2D/3D much setups. more Yet, effort, the especiallytron prospects dynamics for for from realistic studying first mass the principles physical and processes extrapolate crucial them to to the astrophysical elec- scales are strong. trons at non-relativistic shocks from first principles (§ to describe CR transportExamples around of sources effects that and may play ininduced a the either pivotal by role Milky the are large-scale Way ion-neutral Galactic have damping magnetic and field not anisotropic or been by transport, the singled anisotropic wave out, damping [ yet. significant fraction of the Galactic CRswere is the produced case, in a high-metallicity star-forming crucial regions. questionphenomena is If qualitatively whether this different collective from effects (e.g., the multiple linear SN superposition explosions) of lead “ordinary” to SNRs. and has the potential to foster thedescription acceleration of of its PeV non-linear regime protons has in not verythe been young put field sources, forward, configuration but yet. a at In coherent its addition, saturation iteither is is because conducive not to of clear CR whether the spectra finite steeperconfinement velocity than of of the energetic the DSA particles effective prediction, in scattering its centers filamentary or structures. because of an incomplete some tiles that need to be placed in the mosaic of understanding CR acceleration and transport. the origin of Galactic CRs. Thanks to the recent developments of in the knee region asan a enrichment consequence may of be due the to slightly dust flatter sputtering spectra (e.g., of [ species other than H. Such CR Acceleration and Propagation 6. Conclusions simulations, observers and theoristsevidence have of unraveled hadronic few acceleration long-standingalso in issues, allowed SNRs to finding build (§ the a self-consistent direct model for ion injection (§ PoS(ICRC2015)008 , , ]. -Ray γ Braz. J. (Sept., , ]. ]. (Jan., 2015) (1984) 425–444. Damiano Caprioli (Apr., 2013) 22 811 (July, 2005) ]. 87 (June, 2015) ApJ ArXiv e-prints , , arXiv:1108.4838 ]. ]. (May, 1978) 415–419. ]. 65 arXiv:1105.4521 Phys. Rev. D , arXiv:1502.0160 ]. ArXiv e-prints Ann. Rev. of A&A A&A , , astro-ph/0508014 , , arXiv:1303.3565 ]. (Nov., 2011) 14, [ Ankle-like feature in the energy spectrum of Bulletin of the Russian Academy of Science, Phys. 742 ]. , ]. Observation of a knee in the p+he energy spectrum (Jan., 2012) 10, [ arXiv:1103.4055 arXiv:1007.1925 Escape model for Galactic cosmic rays and an early Low-Energy Break in the Spectrum of Galactic Energy Spectra of Primary and Secondary 1 arXiv:1112.5541 ApJ 18 , PAMELA Measurements of Cosmic-Ray Proton and Precision measurement of the proton flux in primary Energy Spectra of Cosmic-ray Nuclei at High Energies Discrepant hardening observed in cosmic-ray elemental Energy spectra of abundant nuclei of primary cosmic rays (Apr., 2015) 083009, [ arXiv:1107.5576 JCAP , Cosmic ray energy spectrum from measurements of air 91 Measurements of cosmic-ray proton and helium spectra from Non-linear diffusive acceleration of heavy nuclei in supernova (Apr, 2015) 171103. arXiv:0911.1889 ]. (Dec., 2013) 748–758, [ (July, 2013) 150–153. , 2015. 114 8 ]. (Apr., 2011) 69–, [ ]. arXiv:1310.6133 341 arXiv:1101.3246 332 ]. (Jan., 2011) 447–456, [ Phys. Rev. D (Feb., 2012) 051105, [ (Apr., 2012) 63, [ , ]. Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. 34 ICRC #366 108 749 Science Voyager 1 Observes Low-Energy Galactic Cosmic Rays in a Region Depleted of (2010), no. 1 L89. th , [AMS-02 Collaboration], Blazars as Ultra-high-energy Cosmic-ray Sources: Implications for TeV Science APh Very local interstellar spectra for galactic electrons, protons and helium A review of experimental results at the knee , , ApJ Phys. Rev. Lett. Possible physics scenarios behind cosmic-ray ”anomalies” PRL The Origin of Ultra-High-Energy Cosmic Rays 714 [BESS Collaboration], , , , “Espresso” Acceleration of Ultra-High-Energy Cosmic Rays On the sources of ultra-high energy cosmic rays et al. arXiv:1304.7114 Frontiers of Physics (2014) 581–588, [ ApJ (Dec., 2009) 593–603, [ , et al. , arXiv:1509.0423 arXiv:1505.0673 44 707 (June, 2009) 564–567, [ Proceedings 34 arXiv:1506.0126 astro-ph/0508014 Observations L38, [ 2015) [ Cosmic-Ray Nuclei Measured with TRACER extragalactic transition remnant shocks cosmic rays from rigidity 1 gvspace to station 1.8 tv with the alpha magnetic spectrometer on the international ApJ spectra the BESS-Polar long-duration balloon flights over[ Antarctica 73 from the data of ATIC-2 experiment: Final results I: spectrum and chemical composition Cosmic Rays [ Helium Spectra below 1 pev by measuring particlein densities very close to the eas core with the argo-ybj experiment showers light elements of cosmic rays observed with KASCADE-Grande Heliospheric Ions Phys. 081101, [ [6] D. Caprioli, P. Blasi, and E. Amato, [9] M. Aguilar [5] P. Blasi and E. Amato, [3] A. Neronov, D. V. Semikoz, and A. M. Taylor, [4] J. R. Hörandel, [7] T. K. Gaisser, T. Stanev, and S. Tilav, [8] O. Adriani et al. [PAMELA Collaboration], [1] E. C. Stone et al., [2] M. S. Potgieter, [21] K. Murase et al., [22] D. Caprioli, [20] G. Cavallo, [10] H. S. Ahn et al. [CREAM Collaboration], [12] A. Obermeier et al. [TRACER Collaboration], [15] P. D. Serpico, [16] G. Giacinti, M. Kachelrieß, and D. V. Semikoz, [19] A. M. Hillas, [14] A. D. Panov et al. [ATIC Collaboration], [11] H. S. Ahn et al. [CREAM Collaboration], [13] K. Abe [17] A. D’Amone, I. De Mitri, L. Perrone, and S. A., [18] W. D. Apel et al. [KASCADE-Grande Collaboration], CR Acceleration and Propagation References PoS(ICRC2015)008 , , , 22 ArXiv ]. (Aug., (July, 509 , 453 75 ICRC (Apr., 790 ]. ApJ , A&AR ApJ ]. , in , , ]. 221 ]. PRL ApJ , , Damiano Caprioli ApJ (May, 2005) 95–+. , ]. 31 (Jan., 1978) 147–156. Extremely fast acceleration astro-ph/0603723 182 (Apr., 1949) 1169–1174. ]. 75 astro-ph/0409453 arXiv:0807.4730 arXiv:1210.4546 arXiv:1407.1657 ]. MNRAS , Proceedings of the National Academy of , arXiv:1203.0570 ]. Magnetic field amplification in Tycho and other (Oct., 2007) 576–578. Common Solution to the Cosmic Ray Anisotropy Cosmic ray nuclei, antiprotons and gamma rays ]. Physical Review (June, 1977) 1306–1308. , , pp. 132–137, 1978. (Oct., 2008) 18, [ 449 (July, 2006) 387–395, [ (Mar., 2013) 36, [ (Aug., 2014) 93, [ 19 ]. 234 10 3 Measurement of Boron and Carbon Fluxes in Cosmic astro-ph/9912240 ]. 453 ]. (Apr., 2005) 229–240, [ 791 ]. arXiv:1105.4529 Ultra-High-Energy Cosmic Rays from Young Neutron Star Nature The acceleration of cosmic rays by shock waves Journal of Physics G Nuclear Physics , JCAP JCAP Newly Born Pulsars as Sources of Ultrahigh Energy Cosmic , ]. 433 A&A ApJ , Propagation of Cosmic-Ray Nucleons in the Galaxy , , , Particle acceleration by astrophysical shocks (May, 2012) 211102, [ arXiv:1201.5197 ]. A&A , 108 1105.6342 Strong evidence for hadron acceleration in Tycho’s supernova remnant arXiv:1011.0037 Cosmic Rays from Super-novae PRL astro-ph/ (Jan., 2012) 11, [ , Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. arXiv:1509.0087 Cosmic ray electrons, positrons and the synchrotron emission of the Galaxy: 1 Magnetic Field Amplification in the Thin X-Ray Rims of SN1006 astro-ph/9506081 astro-ph/9505082 (Apr., 2000) L123–L126, [ Observational constraints on energetic particle diffusion in young SNRs: amplified A regular mechanism for the acceleration of charged particles on the front of a Constraints on Cosmic-ray Propagation Models from A Global Bayesian Analysis JCAP (May, 2012) 118, [ Nonthermal particles and photons in starburst regions and superbubbles Energy Dependence of Synchrotron X-Ray Rims in Tycho’s Supernova Remnant TOPICAL REVIEW: Can diffusive shock acceleration in supernova remnants account , Cosmological Gamma-Ray Bursts and the Highest Energy Cosmic Rays Akademiia Nauk SSSR Doklady 533 , The acceleration of cosmic rays in shock fronts. I (1934) 259–263. arXiv:1406.3630 750 The Acceleration of Ultra–High-Energy Cosmic Rays in Gamma-Ray Bursts International Cosmic Ray Conference On the Origin of the Cosmic Radiation (Feb., 2012) A81, [ (Mar., 2011) 106, [ (Sept., 2015) [ 20 ApJ , ApJ 538 , 729 e-prints and Gradient Problems consistent analysis and implications (Nov., 1995) 883, [ shell-type supernova remnants 2014) 85, [ for high-energy galactic cosmic rays? II: anisotropy Science in the galaxy: a new diffusion model magnetic field and maximum energy 1995) 386–389, [ Winds (Nov., 2014) 77. shock wave (Dec., 1998) 212–228, [ ApJ of cosmic rays in a supernova remnant A&A Rays vol. 11 of 1978) L29–L32. Rays with the PAMELA Experiment [39] C. Evoli, D. Gaggero, D. Grasso, and L. Maccione, [40] G. Di Bernardo et al., [38] C. Evoli, D. Gaggero, D. Grasso, and L. Maccione, [45] A. Tran et al., [36] A. W. Strong and I. V. Moskalenko, [37] R. Trotta et al., [41] P. Blasi and E. Amato, [42] H. J. Völk, E. G. Berezhko, and L. T.[43] Ksenofontov, E. Parizot et al., [44] S. M. Ressler et al., [47] G. Morlino and D. Caprioli, [24] M. Vietri, [25] P. Blasi, R. I. Epstein, and A. V. Olinto, [46] Y. Uchiyama, F. A. Aharonian, T. Tanaka, T. Takahashi, and Y. Maeda, [30] E. Fermi, [26] K. Fang, K. Kotera, and A. V. Olinto, [27] W. Baade and F. Zwicky, [31] G. F. Krymskii, [32] W. I. Axford, E. Leer, and G. Skadron, [23] E. Waxman, [35] O. Adriani et al. [PAMELA Collaboration], [34] R. D. Blandford and J. P. Ostriker, [33] A. R. Bell, [28] A. M. Hillas, [29] A. M. Bykov, CR Acceleration and Propagation PoS(ICRC2015)008 663 58 809 , Space , , , ApJ ApJ , , ]. (2005) (Sept., 1975a) 20 Damiano Caprioli 172 ]. ]. ]. MNRAS Space Science Reviews , , ]. arXiv:1407.1971 ]. (Feb., 2015) 085003, ]. Modern Physics Letters A , ]. arXiv:1401.7679 114 ]. Magnetohydrodynamic-particle-in-cell Method physics/0611174 PRL arXiv:1310.2943 , Spatial structure of X-ray filaments in SN 1006 (Apr., 2015) 115, [ 20 (June, 2014) 062308. arXiv:1407.2261 802 Turbulence and Magnetic Field Amplification in 21 arXiv:0901.0486 ]. Simultaneous Acceleration of Protons and Electrons at 1005.2127 (Sept., 1997) 19789–19804. (Oct., 2014) 46, [ ApJ arXiv:0912.2972 , ]. 102 794 arXiv:1304.4956 Nonlinear theory of diffusive acceleration of particles by shock (Mar., 2014) 91, [ The maximum energy of cosmic rays accelerated by supernova Cosmic-ray shock acceleration in the presence of self-excited Simulations of Ion Acceleration at Non-relativistic Shocks. III. Simulations of Ion Acceleration at Non-relativistic Shocks: I. Simulations of Ion Acceleration at Non-relativistic Shocks: II. ApJ JGR Magnetic Field Amplification by Shocks in Turbulent Fluids The plasma physics of shock acceleration , 783 , (Apr., 2001) 429–481. Microphysics of Quasi-parallel Shocks in Collisionless Plasmas (Mar., 2007) 419–425, [ (Oct., 2014) 47, [ 64 ApJ 176 794 ]. , Physics of Plasmas The link between shocks, turbulence, and magnetic reconnection in , Injection and acceleration of thermal protons at quasi-parallel shocks: A hybrid ApJ (Apr., 2009) 825–833, [ arXiv:1412.1087 Comparison of different methods for non-linear diffusive shock acceleration (Sept., 1983b) 249–257. , dHybrid: A massively parallel code for hybrid simulations of space plasmas (Feb., 1983a) 223–228. (June, 2013) 84, [ 695 Turbulent Amplification of a Magnetic Field Driven by the Dynamo Effect at Rippled 125 (Oct., 2013) 513–533. astro-ph/0511235 (Sept., 2010) 1773–1783, [ (June, 2010) L21–L25, [ 118 770 Particle Acceleration and Wave Excitation in Quasi-parallel High-Mach-number Cosmic ray streaming. I - Effect of Alfven waves on particles ApJ 178 On the Origin of Very High Energy Cosmic Rays 407 405 , ApJ Rep. Prog. Phys. A&A A&A , , , , arXiv:1412.0672 557–566. MNRAS [ Particle Diffusion Nonrelativistic Quasiparallel Collisionless Shocks Comp. Phys. Commun. (1991) 259–346. waves Collisionless Shocks: PIC Simulation shocks Acceleration Efficiency for Coupling Cosmic Rays with a Thermal Plasma: Application to Non-relativistic Shocks (July, 2007) L41–L44. Supernova Remnants: Interactions Between a StrongMedium Shock Wave and Multiphase Interstellar waves (Aug., 2015) 55, [ Shocks astrophysical and laboratory plasmas. Berlin; New York: Springer, Scientific computation, 2002. collisionless plasmas Magnetic Field Amplification MNRAS 3055–3076, [ simulation parameter survey Sci. Rev. [69] J. Skilling, [68] D. Caprioli et al., [64] T. N. Kato, [67] M. A. Malkov and L. O’C. Drury, [61] D. Caprioli and A. Spitkovsky, [62] X.-N. Bai, D. Caprioli, L. Sironi, and A. Spitkovsky, [63] J. Park, D. Caprioli, and A. Spitkovsky, [65] L. Gargaté et al., [66] F. C. Jones and D. C. Ellison, [52] P. Blasi, [54] P. O. Lagage and C. J. Cesarsky, [55] D. Caprioli and A. Spitkovsky, [50] T. Inoue, R. Yamazaki, and S.-i. Inutsuka, [49] J. Giacalone and J. R. Jokipii, [51] F. Fraschetti, [53] P. O. Lagage and C. J. Cesarsky, [56] A. S. Lipatov, The hybrid multiscale simulation technology: an[57] introduction withH. application Karimabadi to et al., [48] G. Morlino, E. Amato, P. Blasi, and D. Caprioli, [59] D. Burgess and M. Scholer, [60] D. Caprioli and A. Spitkovsky, [58] J. Giacalone et al., CR Acceleration and Propagation PoS(ICRC2015)008 , , (Jan., (Oct., 419 ]. 17 Damiano Caprioli ]. ]. ]. MNRAS , GeoRL ]. , (Feb., 2015) 974–978. ]. ]. ]. 347 ]. (Mar., 2002) 1037. Microphysics of Cosmic Ray arXiv:1301.3173 astro-ph/ (June, 2006) 6104. 107 Science ]. , arXiv:1304.7081 arXiv:1305.4656 111 astro-ph/0612424 Stochastic electron acceleration during (Aug., 1983) 973–1027. (Sept., 2006) 1251–1258, (Mar., 2004) 187–195. (Aug., 2012) 8107. The Origin of Radially Aligned Magnetic Fields in arXiv:1409.8291 46 ]. arXiv:1002.1701 371 arXiv:0806.1223 604 117 21 (May, 2013) [ Numerical Simulations of Local Shock Reformation and J. Geophys. Res. arXiv:0810.4565 , ApJ ]. , JGR (May, 2007) 190–202, [ Simulations and Theory of Ion Injection at Non-relativistic , (Apr., 2013) 2873–2884, [ MNRAS (Aug., 2013) L20, [ , arXiv:1211.6765 Electron Injection by Whistler Waves in Non-relativistic Shocks Nonlinear Study of Bell’s Cosmic Ray Current-Driven The maximum momentum of particles accelerated at cosmic ray 661 430 772 ApJ Cosmic-Ray-induced Filamentation Instability in Collisionless Non-linear theory of cosmic ray shocks including self-generated The dynamics and upstream distributions of ions reflected at the (Jan., 2015) L28, [ On microinstabilities in the foot of high Mach number perpendicular , (Mar., 2007) 1471–1478, [ ApJ Space Sci. Rev. Formation of reflected electron bursts by the nonstationarity and (Sept., 1984) 7407–7422. Electron Injection at High Mach Number Quasi-perpendicular Shocks: , , 798 arXiv:1009.3319 Universal behaviour of shock precursors in the presence of efficient A filamentation instability for streaming cosmic rays MNRAS 375 89 ]. (Dec., 1982) 191–200. , Non-linear particle acceleration at non-relativistic shock waves in the A kinetic approach to cosmic-ray-induced streaming instability at supernova ApJ (Feb., 2009) 1591–1600, [ (Mar., 2010) L127–L132, [ , (Mar., 2009) 626–642, [ 116 JGR arXiv:1109.5690 , Reports of Progress in Physics (Mar., 2013) L20, [ The Nonlinear Saturation of the Non-resonant Kinetically Driven Streaming MNRAS , 392 711 694 , An introduction to the theory of diffusive shock acceleration of energetic particles in A&A 765 , (Sept., 2004) 550–558. Diffuse ions at a quasi-parallel collisionless shock - Simulations Particle acceleration and generation of diffuse superthermal ions at a quasi-parallel ApJ Turbulent amplification of magnetic field and diffusive shock acceleration of cosmic rays ApJ , , (May, 2011) 63, [ ApJ 353 MNRAS Journal of Geophysical Research (Space Physics) , , , 733 astro-ph/0606592 collisionless shock: Hybrid simulations Surfing and Drift Acceleration modified shocks Collisionless Shocks Ion Acceleration in Supernova Remnants 1990) 1821–1824. earth’s bow shock presence of self-generated turbulence Shocks [ tenuous plasmas spontaneous turbulent reconnection in a strong shock wave shocks cosmic ray acceleration Alfven waves ApJ Instability Young Supernova Remnants Instability nonuniformity of a collisionless shock front shocks MNRAS 2012) 2433–2440, [ Driven Plasma Instabilities [85] Y. Su et al., [86] M. Scholer, [88] T. Amano and M. Hoshino, [90] Y. Matsumoto, T. Amano, T. N. Kato, and M. Hoshino, [87] D. Burgess and S. J. Schwartz, [79] E. Amato and P. Blasi, [81] L. O’C. Drury, [82] P. Blasi, E. Amato, and D. Caprioli, [83] D. Caprioli, A. Pop, and A. Spitkovsky, [84] R. E. Lee, S. C. Chapman, and R. O. Dendy, [89] M. A. Riquelme and A. Spitkovsky, [80] B. Reville and A. R. Bell, [72] D. Caprioli and A. Spitkovsky, [78] E. Amato and P. Blasi, [73] A. M. Bykov, A. Brandenburg, M. A. Malkov, and S.[74] M. Osipov, T. Inoue, J. Shimoda, Y. Ohira, and R. Yamazaki, [75] M. A. Riquelme and A. Spitkovsky, [76] L. Gargaté et al., [77] J. F. McKenzie and H. J. Völk, [91] B. Lembège and P. Savoini, [92] S. Matsukiyo and M. Scholer, [70] A. R. Bell, [71] B. Reville and A. R. Bell, CR Acceleration and Propagation PoS(ICRC2015)008 , , ]. ]. ]. , in 435 , ]. , in ]. , in (Oct., 2014) MNRAS ]. , 794 Low-frequency Damiano Caprioli ]. The Youngest ]. , in ApJ (Apr., 2013) 104, , arXiv:0803.1487 arXiv:1209.2196 ]. arXiv:1505.0278 145 (June, 2015) A32, AJ , arXiv:1401.7519 578 arXiv:1503.0300 ]. A&A ]. astro-ph/0702674 , arXiv:1101.1454 astro-ph/9305037 (Oct., 2012) A124, [ arXiv:1302.2150 (June, 2008) L41–L44, [ 546 High-energy gamma-ray and neutrino On the Radio Polarization Signature of Efficient ]. Electron and proton acceleration efficiency by (Aug., 2015) 2198–2211, [ 680 (Mar., 2014) 30007, [ (Sept., 2015) 1–10, [ 22 The gamma-ray visibility of supernova remnants. A test A&A 23 451 , 69 ApJ , arXiv:1201.1502 (Feb., 2011) L28+, [ arXiv:1409.7393 APh (June, 2007) 879–891, [ On the cosmic ray spectrum from type II supernovae expanding , Non-thermal Electron Acceleration in Low Mach Number Non-thermal Electron Acceleration in Low Mach Number (Oct., 2004) 121–131. 728 MNRAS , 661 (July, 1994) 959–971, [ , 2015. 425 Cosmic ray acceleration in young supernova remnants ApJl , 2015. , 2015. , 2015. Cosmic Rays in Galaxy Clusters and Their Nonthermal Emission , ]. ApJ 287 , arXiv:1307.6575 (Oct., 2013) 2748–2760, [ A&A (A. Keiling, D.-H. Lee, and V. Nakariakov, eds.), Geophys. Monogr. Ser. Cosmic particle acceleration after a decade of vhe gamma-ray observations , The ”toothbrush-relic”: evidence for a coherent linear 2-Mpc scale shock ]. ]. A&A 434 (Dec., 2014) 47, [ Geometry of the non-thermal emission in SN 1006. Azimuthal variations of Evidence for Particle Acceleration to the Knee of the Cosmic Ray Spectrum in ICRC #012 XMM-Newton observations of the merging galaxy cluster CIZA J2242.8+5301 , Particle Acceleration in Supernova Remnants and the Production of Thermal Acceleration of cosmic rays and gamma-ray emission from supernova remnants in th Low-frequency waves at and upstream of collisionless shocks ICRC #017 ICRC #004 ICRC #834 797 th th th Cosmic particle acceleration after a decade of vhe gamma-ray observations (Mar., 2013) 2617–2633, [ Hess j1641-463, a very hard spectrum tev gamma-ray source in the galactic plane MNRAS ApJ Cosmic particle acceleration after a decade of vhe gamma-ray observations , , 429 arXiv:1406.5190 Proceedings 34 arXiv:1410.8697 arXiv:1302.4678 and Nonthermal Radiation [ in Proceedings 34 Proceedings 34 Proceedings 34 (Oct., 2013) 1174–1185, [ and Inefficient Particle Acceleration in Supernova Remnant SN 1006 wave in a massive merging galaxy cluster? of cosmic ray origin American Geophysical Union, Washington, D.C., 2015. accepted July 7, 2015. the Galaxy MNRAS Waves in Space Plasmas Galactic Supernova Remnant: G1.9+0.3 backgrounds from clusters of galaxies and radio constraints merger shocks in galaxy clusters International Journal of Modern Physics D in their red giant presupernova wind Tycho’s Supernova Remnant Collisionless Shocks. II. Firehose-mediated Fermi Acceleration andConditions its Dependence on Pre-shock cosmic-ray acceleration Collisionless Shocks. I. Particle Energy Spectra153, and [ Acceleration Mechanism [ [95] E. M. Reynoso, J. P. Hughes, and D. A. Moffett, [96] S. P. Reynolds, K. J. Borkowski, D. A. Green, U. Hwang, I. Harrus, and R. Petre, [94] X. Guo, L. Sironi, and R. Narayan, [97] R. Rothenflug et al., [98] K. A. Eriksen et al., [99] F. Zandanel, I. Tamborra, S. Gabici, and S. Ando, [93] X. Guo, L. Sironi, and R. Narayan, [113] D. C. Ellison et al., [106] R. Buehler, [107] F. Aharonian, [108] I. Oya et al., [109] K. M. Schure and A. R. Bell, [102] R. J. van Weeren et al., [103] G. A. Ogrean et al., [104] L. B. Wilson, III, [105] M. Lemoine–Goumard, [111] P. Cristofari et al., [112] L. O’C. Drury, F. Aharonian, and H. J. Völk, [110] M. Cardillo, E. Amato, and P. Blasi, [100] F. Vazza, D. Eckert, M. Brüggen, and B. Huber, [101] G. Brunetti and T. W. Jones, CR Acceleration and Propagation PoS(ICRC2015)008 , ]. 7 (May, ApJ Ap. J. (Nov., , 5 , ]. ]. ]. (Aug., JCAP IAU 445 , JCAP 580 , ]. Damiano Caprioli ]. MNRAS A&A , , arXiv:0803.2521 ]. , in Origin of Cosmic Rays: , p. 363, 1981. ]. ]. arXiv:0804.2884 ]. ]. arXiv:1002.2738 arXiv:0807.4261 ]. ’enic drift on the shape of cosmic ray Spatial Structure and Collisionless IAU Symp. . \ arXiv:0810.0094 arXiv:0909.2285 ]. (Dec., 2008) 1089, [ Efficient Cosmic Ray Acceleration, (J. van Leeuwen, ed.), vol. 291 of 689 The Nature of Gamma-Ray Emission of Tycho’s arXiv:0807.2754 The First Fermi-LAT SNR Catalog SNR and Cosmic arXiv:1001.1932 ApJ arXiv:1211.5398 Dynamical Effects of Self-Generated Magnetic Fields Dynamical feedback of self-generated magnetic fields , arXiv:1507.0363 arXiv:0912.2964 23 (July, 2010) 372–378, [ arXiv:1001.5150 (May, 2009) 895–906, [ A high-energy catalogue of Galactic supernova (Oct., 1978) L27–L30. (June, 2008) L139–L142, [ 717 ]. Nonthermal Radiation of Young Supernova Remnants: The 395 ]. arXiv:1210.5264 225 IAU Symposium 679 (July, 2008) [ (Jan., 2009) 240–250, [ The influence of the Alfv Optical emission from a fast shock wave - The remnants of The contribution of supernova remnants to the galactic cosmic arXiv:1302.3307 Gamma-ray emission from SNR RX J1713.7-3946 and the origin ]. ApJ (Jan., 2010) 965–980, [ ]. , , in A Simple Model of Nonlinear Diffusive Shock Acceleration ApJ ApJ (July, 2015) [ 392 , , Hadronic gamma-rays from RX J1713.7-3946? 708 MNRAS Hydromagnetic shock structure in the presence of cosmic rays Shock structure including CR acceleration (Jan., 2013) 14, [ (Mar., 2014) 33. , ApJ 763 783 , MNRAS (Mar., 2010) 287–293, [ Fermi Large Area Telescope Observations of Supernova Remnants , (Apr., 2010) 160–168, [ Detection of the Characteristic Pion-Decay Signature in Supernova Remnants ApJ ApJ , , 712 33 ArXiv e-prints arXiv:1406.2322 (Feb., 2010) L151–L155, [ astro-ph/0807.2754 arXiv:1206.1360 , Direct Evidence for Hadronic Cosmic-Ray Acceleration in the Supernova Remnant , Study of TeV shell supernova remnants at gamma-ray energies ApJ A CR-hydro-NEI Model of the Structure and Broadband Emission from Tycho’s APh , arXiv:1103.2624 710 , arXiv:1506.0230 (Feb., 2013) 807–811, [ Cosmic-ray acceleration in supernova remnants: non-linear theory revised Understanding hadronic gamma-ray emission from supernova remnants , pp. 483–485, Mar., 2013. 339 ApJl , (Aug., 1981) 344. (Nov., 1999) 385–399. 248 Shock structure including cosmic ray acceleration, vol. 94 of Electron Heating in Balmer-dominated Shocks ray spectrum Tycho’s supernova and SN 1006 remnants and pulsar wind nebulae Symposium in Cosmic-Ray-modified Shocks in cosmic ray modified shocks Interacting with Molecular Clouds Case of RX J1713.7-3946 Ray Implications Supernova Remnant Supernova Remnant 2014) L70–L73, [ Science 526 spectra in SNRs (July, 2012) 38, [ of galactic cosmic rays IC 443 2011) 26–+, [ 2015) A74, [ Hydrodynamics, and Self-Consistent Thermal X-Ray EmissionJ1713.7-3946 Applied to Supernova Remnant RX [129] L. O’C. Drury and H. J. Völk, [136] M. van Adelsberg, K. Heng, R. McCray, and J. C. Raymond, [126] D. Caprioli, P. Blasi, E. Amato, and M. Vietri, [128] L. O’C. Drury and H. J. Völk, [132] D. Caprioli, E. Amato, and P. Blasi, [135] R. A. Chevalier and J. C. Raymond, [134] D. Castro and P. Slane, [116] V. N. Zirakashvili and F. A. Aharonian, [123] P. Slane et al., [124] Brandt et al. [for the Fermi LAT Collaboration], [125] S. Safi-Harb, G. Ferrand, and H. Matheson, [127] D. Caprioli, P. Blasi, E. Amato, and M. Vietri, [130] E. G. Berezhko and D. C. Ellison, [133] D. Caprioli, [131] V. N. Zirakashvili and V. S. Ptuskin, [119] E. G. Berezhko, L. T. Ksenofontov, and H. J. Völk, [121] M. Ackermann et al., CR Acceleration and Propagation [114] D. Caprioli, [115] G. Morlino, E. Amato, and P. Blasi, [117] D. C. Ellison, D. J. Patnaude, P. Slane, and J. Raymond, [118] F. Acero et al., [120] S. Gabici and F. A. Aharonian, [122] M. Tavani et al., PoS(ICRC2015)008 , , , , 418 ]. (Sept., 755 ApJ , (Sept., 1997) ]. MNRAS , Damiano Caprioli 487 (July, 2009) ]. ]. ArXiv e-prints ]. 396 , ApJ , arXiv:1007.4869 ]. MNRAS ]. , Transport of relativistic nucleons (May, 2013) 148, (Feb., 1975) 107–120. astro-ph/9604056 Collisionless Shocks in a Partially Collisionless Shocks in a Partially ]. 768 arXiv:1207.3706 arXiv:1301.7264 196 ]. ApJ arXiv:1206.1384 ]. , Publications of the Astronomical Society of , Cosmic-ray acceleration and escape from ApJ (May, 1997) 434–443. , On the mechanism for breaks in the cosmic ray Galactic winds. I - Cosmic ray and wave-driven ]. 321 arXiv:1507.0059 arXiv:1503.0146 (Jan., 2011) 1577–1582, [ 24 Broad-band non-thermal emission from molecular A&A ]. Cosmic ray acceleration at oblique shocks 410 Galactic Cosmic Rays from Supernova Remnants. I. A , -rays from molecular clouds illuminated by cosmic rays γ Stochastic particle acceleration at shocks in the presence of arXiv:0807.4259 Spectral Breaks as a Signature of Cosmic Ray Induced , 2015. Cosmic ray penetration in diffuse clouds Cosmic-ray transport in the presence of a cr-drive galactic On the spectrum of high-energy cosmic rays produced by On the escape of particles from cosmic ray modified shocks (Aug., 2012) 061101, [ (Oct., 1996) 1010–1016, [ ]. Non-linear cosmic ray Galactic transport in the light of AMS-02 (May, 2013) 415–429, [ (May, 1991) 79–98. (July, 2015) [ (Aug., 2012) 082901, [ astro-ph/0408025 arXiv:0906.4553 MNRAS ]. , 109 314 19 431 245 ]. Toward a theory of interstellar turbulence. 2: Strong alfvenic PRL arXiv:1108.0582 ICRC #552 A&A , A&A th , ]. (Sept., 2015) 091505, [ , MNRAS (Jan., 1995) 763–775. , 22 Measuring the Cosmic-Ray Acceleration Efficiency of a Supernova Remnant arXiv:1202.3080 ArXiv e-prints , 438 Cosmic ray confinement and transport models for probing their putative sources Galactic winds driven by cosmic rays arXiv:0901.4549 (July, 2009) 2065–2073, [ (2010) 23–44. (Aug., 2009) 719–, [ ApJ , Physics of Plasmas (Jan., 2005) 755–765, [ 27 Balmer-dominated shocks: A concise review , Proceedings 34 396 astro-ph/9704267 325 arXiv:1509.0512 429 , in arXiv:1211.6148 2015) [ escaping from interacting SNRs Turbulence in the Galaxy and Voyager data (Aug., 2012) 121, [ [ supernova remnants Physics of Plasmas winds from the Galaxy in a galactic wind driven by cosmic rays. 182–+, [ braided magnetic fields. clouds illuminated by cosmic rays from nearby supernova remnants wind turbulence Cosmic-Ray Composition Controlled by Volatility and Mass-to-Charge Ratio Ionized Medium. I. Neutral Return Flux and its Effects on Acceleration of Test Particles Ionized Medium. III. Efficient Cosmic Ray Acceleration spectrum Australia Science (Dec., 2011) 1208–1216, [ MNRAS supernova remnants in the presence ofA&A strong cosmic-ray streaming instability and wave dissipation 1629–1639, [ [148] G. Morlino, S. Gabici, and J. Krause, [149] A. R. Bell, K. M. Schure, B. Reville, and[150] G. Giacinti, M. A. Malkov, [151] P. Blasi, E. Amato, and P. D. Serpico, [152] R. Aloisio, P. Blasi, and P. Serpico, [153] F. M. Ipavich, [154] D. Breitschwerdt, J. F. McKenzie, and H. J. Völk, [155] V. S. Ptuskin, H. J. Völk, V. N. Zirakashvili, and D. Breitschwerdt, [147] Y. Ohira, K. Murase, and R. Yamazaki, [156] S. Recchia, P. Blasi, and G. Morlino, [157] P. Goldreich and S. Sridhar, [141] J. G. Kirk, P. Duffy, and Y. A. Gallant, [158] J. Meyer, L. O. Drury, and D. C. Ellison, [139] P. Blasi, G. Morlino, R. Bandiera, E. Amato, and D. Caprioli, [140] G. Morlino, P. Blasi, R. Bandiera, E. Amato, and D. Caprioli, CR Acceleration and Propagation [137] K. Heng, [138] E. A. Helder et al., [142] A. R. Bell, K. M. Schure, and B. Reville, [143] M. A. Malkov, P. H. Diamond, and R. Z.[144] Sagdeev, D. Caprioli, P. Blasi, and E. Amato, [145] V. S. Ptuskin and V. N. Zirakashvili, [146] S. Gabici, F. A. Aharonian, and S. Casanova,