1. many different acceleration mechanisms: Fermi 1, Fermi 2, shear,... (Fermi acceleration at shock: most standard, nice powerlaw, few free parameters) main signatures to be determined: E min, E max [ Ã timescale t acc (E) ], spectral slope h ® i, running d ® / d ln E only secondary photon spectra are observed, reconstruction process is difficult and source physics dependent... different ways of addressing this problem: - acceleration physics: idealized source configurations ) calculate t acc (E), ® (E) - data interpretation: most effort on source modelling ( t acc » t L, ® » best fit "Fundamental acceleration processes and CTA" From CTA observations to fundamental acceleration mechanisms... a difficult task: Martin Lemoine - IAP p shock Fermi at mildly relativistic internal shocks

2. Fermi acceleration Simple view of Fermi acceleration: unshocked upstream shocked downstream v down v sh shock front rest frame test particle approximation: particles get accelerated as they bounce back and forth on magnetic inhomogeneities on both sides of the shock front Modern view of Fermi acceleration: relativistic regime: v sh » c, how well does Fermi acceleration operate? test particle approximation is not a good approximation: cosmic ray energy density/pressure represents a sizeable contribution... ) modification of the shock jump conditions, non-linear Fermi acceleration theory and observations suggest that the coupling between accelerated particles and e.m. waves is of fundamental importance, for both non-relativistic and relativistic shocks Implications: there exists an intimate link between the physics of (relativistic or not) collisionless shock waves, accelerations mechanisms, source physics, hence observational data at VHE a new numerical tool to probe acceleration physics: Particle-In-Cell (PIC) simulations... astrophysical objects probe different physical conditions... SNR: non-relativistic, weakly magnetised IGM shock waves: non-relativistic, unmagnetized ? GRB: moderately to ultra-relativistic, weakly magnetised? PWNe: ultra-relativistic, strongly magnetised?

3. Acceleration at IGM shock waves and magnetic fields IGM shock waves: acceleration can proceed if the unshocked medium is magnetized: gamma-ray observations would allow to measure this unshocked (primeval?) magnetic field and/or constrain the amplication mechanisms... Keshet et al. 03 log 10 (J/J 0 ) (>10 GeV) J 0 ' 10 -9 cm -2 s -1 sr -1 above 1GeV above 10 GeV cluster, 16 £ 16 ±, ±µ =0.2 ± filament, 16 £ 16 ±, ±µ =0.4 ± cluster, 16 £ 16 ±, ±µ =0.4 ± log 10 (J/J 0 ) (>1 GeV) J 0 ' 10 -7 cm -2 s -1 sr -1

4. Relativistic Fermi acceleration Limits: the ambient magnetic field inhibits Fermi acceleration: B ? down » ¡ sh B ? up, therefore B is mostly perpendicular, particle is trapped on B line and advected away from the shock far in the shocked region unshockedshocked c/3 c B ) Fermi acceleration requires energy transfer between shock and magnetic field...... accelerated particles are the likely agent of transfer via e.m. beam-plasma instabilities Consequences: if the ambient magnetic field is too strong, accelerated particles cannot propagate far enough into the unshocked plasma (penetration length » r L / ¡ sh 3 !), hence instabilities cannot grow, hence Fermi acceleration is inhibited: ) Fermi acceleration should not operate at strongly magnetized PWNe terminal shocks, in magnetized GRB external shocks (?)... much to be learned from VHE observations... (some) Open questions: spectral slope, running and maximal energy still unknown... Fermi acceleration at moderately relativistic shock waves (ex. GRB internal shocks)... time dependence of the shock structure and Fermi acceleration... ) particles do not radiate via synchrotron, but via jitter radiation on small scale e.m. fluctuations shock front rest frame

5. Relativistic Fermi acceleration: an example Observations of GRB 080916C: Fermi LAT detection of high energy emission >1 GeV, delayed by several seconds with respect to lower energy energy time various interpretations, among which: o Wang et al. 09: inverse Compton, E ° as high as 70GeV implies t acc ' t L and offers a lower limit on unshocked magnetic field o Razzaque et al. 09: VHE emission is proton synchrotron radiation, delay » proton cooling time; implies acceleration of p to & 10 20 eV, but requires huge magnetic energy content

6. Acceleration mechanism vs energy Cosmic ray all-sky all-particle spectrum (x E 3 ): very small flux at UHE: » 1/km 2 /century at 10 20 eV sources: GRBs, blazars?? knee second knee ankle Galactic supernovae remnants...Sources of ultra-high energy cosmic rays are the most powerful accelerators known in Nature... Main questions: which source, which acceleration mechanism to reach E » 10 20 eV? are secondaries (gamma-rays/ neutrinos) expected...?

7. Secondaries of ultra-high energy cosmic ray sources Assumptions: sources of UHE protons and nuclei embedded in magnetized clusters Kotera et al. 09 ) detection of gamma-rays from UHE sources in galaxy clusters in unlikely even for CTA, even with optimistic assumptions Gabici & Aharonian 05 suggest to detect the >GeV synchrotron light of 10 18 eV e + e - pairs produced by UHE protons interacting with the CMB: unlikely for 'modern' source luminosities... secondaries emitted in the source itself: also unlikely for reasons of temporal coincidences between arrival of UHE protons and VHE gamma-rays (magnetic fields...) Other possibilities: