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"Fundamental acceleration processes and CTA". From CTA observations to fundamental acceleration mechanisms... a difficult task:. many different acceleration mechanisms: Fermi 1, Fermi 2, shear, ... (Fermi acceleration at shock: most standard, nice powerlaw, few free parameters)
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"Fundamental acceleration processes and CTA" From CTA observations to fundamental acceleration mechanisms... a difficult task: • 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 Fermi at mildly relativistic internal shocks p shock Martin Lemoine - IAP
Fermi acceleration shock front rest frame Simple view of Fermi acceleration: Modern view of Fermi acceleration: Implications: unshocked upstream shocked downstream • test particle approximation: particles get accelerated as they bounce back and forth on magnetic inhomogeneities on both sides of the shock front vdown vsh • 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 • 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?
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 above 1GeV log10(J/J0) (>1 GeV) J0'10-7cm-2s-1sr-1 cluster, 16£ 16±, ±µ =0.4± filament, 16£ 16±, ±µ =0.4± cluster, 16£ 16±, ±µ =0.2± above 10 GeV log10(J/J0) (>10 GeV) J0'10-9cm-2s-1sr-1
Relativistic Fermi acceleration shocked unshocked shock front rest frame Limits: Consequences: (some) Open questions: • 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 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 )particles do not radiate via synchrotron, but via jitter radiation on small scale e.m. fluctuations • if the ambient magnetic field is too strong, accelerated particles cannot propagate far enough into the unshocked plasma (penetration length »rL / ¡sh3 !), 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... • 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...
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 • various interpretations, among which: • Wang et al. 09: inverse Compton, • E° as high as 70GeV implies • tacc'tLand offers a lower limit on unshocked magnetic field time • 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 energy Cosmic ray all-sky all-particle spectrum (x E3): Main questions: knee ankle very small flux at UHE: »1/km2/century at 1020eV sources: GRBs, blazars?? second knee Galactic supernovae remnants ...Sources of ultra-high energy cosmic rays are the most powerful accelerators known in Nature... • which source, which acceleration mechanism to reach E »1020 eV? • are secondaries (gamma-rays/ neutrinos) expected...?
Secondaries of ultra-high energy cosmic ray sources Assumptions: sources of UHE protons and nuclei embedded in magnetized clusters Other possibilities: 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...)
