• 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:

Emin , Emax [Ã timescale tacc(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 tacc(E), ®(E)- data interpretation: most effort on source modelling ( tacc » tL , ® » 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 relativisticinternal shocks

Fermi accelerationSimple view of Fermi acceleration:

unshockedupstream

shockeddownstream

vdown vsh

shock frontrest 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: vsh » 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 magnetisedIGM shock waves: non-relativistic, unmagnetized ?GRB: moderately to ultra-relativistic, weakly magnetised?PWNe: ultra-relativistic, strongly magnetised?

Acceleration at IGM shock waves and magnetic fieldsIGM 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

log10(J/J0) (>10 GeV)

J0' 10-9 cm-2 s-1 sr-1

above 1GeV

above 10 GeVcluster, 16£ 16±, ±µ =0.2±

filament, 16£ 16±, ±µ =0.4±

cluster, 16£ 16±, ±µ =0.4± log10(J/J0) (>1 GeV)

J0' 10-7 cm-2 s-1 sr-1

Relativistic Fermi accelerationLimits:• the ambient magnetic field inhibits Fermi acceleration: B?down » ¡ shB?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 cB

) 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 » rL / ¡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 frontrest frame

Relativistic Fermi acceleration: an exampleObservations of GRB 080916C:

• Fermi LAT detection of high energy emission >1 GeV, delayed by several seconds with respect to lower energy

ener

gy

time

• various interpretations, among which:

o Wang et al. 09: inverse Compton, E° as high as 70GeV impliestacc ' tL 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 & 1020 eV, but requires huge magnetic energy content

Acceleration mechanism vs energyCosmic ray all-sky all-particle spectrum (x E3):

very small flux at UHE:» 1/km2/century at 1020eV

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 » 1020 eV?• are secondaries (gamma-rays/ neutrinos) expected...?

Secondaries of ultra-high energy cosmic ray sourcesAssumptions: 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 1018eV 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: