1 / 54

Acceleration of Cosmic Rays

Acceleration of Cosmic Rays. E.G.Berezhko Yu.G.Shafer Institute of Cosmophysical Research and Aeronomy Yakutsk, Russia Introduction General properties of Cosmic Ray (CR) acceleration Diffusive shock acceleration

komala
Download Presentation

Acceleration of Cosmic Rays

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Acceleration of Cosmic Rays E.G.Berezhko Yu.G.Shafer Institute of Cosmophysical Research and Aeronomy Yakutsk, Russia • Introduction • General properties of Cosmic Ray (CR) acceleration • Diffusive shock acceleration • Acceleration of CRs in Supernova Remnants (SNRs) • Nonthermal emission of individual SNRs • SNRs as Galactic CR source • Some aspects of UHECR production in GRBs and extragalactic jets • Conclusions

  2. Cosmic Rays V.Hess (1912) I ≈ 1 particle/(cm2s) I ~ ε-γ γ≈ 2.7 Earth LCR ≈ 3×1041 erg/s Atmosphere CR origin problem: i) CR source (?) ii) Acceleration mechanism (?)

  3. General remarks • Cosmic Rays (CRs) = atomic nuclei = charged particles • Electric field is needed to generate (accelerate) CR population • High value large scale electric field is not expected • in space plasma • Electric field in space plasma is created • due to the movement of magnetized clouds • For efficient CR production (acceleration) the system, • which contains strong magnetic field and sufficient number • of rapidly moving clouds, is needed

  4. vi vf vi vf CR scattering on moving magnetized clouds CR vi Head-on collision: Δv = vf – vi >0 E = -[w B]/c w E vf scattering center × B w = 0 v >> w Δv = 0 E Overtaken collision: Δv = vf – vi <0 B w w = 0 w Elastic scattering: vf = vi vf > vi Elastic scattering: Larger rate of head-on then overtaken collisions efficient CR acceleration

  5. General remarks • CR acceleration, operated in the regions of powerful sources, • are the most meaningful • The main form of energy available in the space is • kinetic energy of large scale supersonic plasma motion • (stellar winds, expanding supernova remnants, jets) • Most relevant acceleration mechanisms are those, • which directly transform the energy of large scale motion • into the population of high energy particles • Intense formation of CR spectra are expected to take place • at the shocks and in shear flows

  6. Solar wind

  7. Diffusive shock acceleration of CRs Krymsky 1977 Bell 1978 log NCR p- Δp log p scattering centers  = ( + 2)/( -1) shock compression ratio

  8. Frictional Acceleration of Cosmic Rays Berezhko(1981) y Shear plasma flow w CR scattering center acceleration rate mean scattering time x Frictional CR acceleration is expected to be very efficient in relativistic/subrelativistic jets

  9. Energetic requirements to CR sources

  10. Requirements to the CR acceleration mechanism Jobs~ ε–γobs ~ Js/τesc observed CR spectrum γobs = 2.7 Τesc ~ ε-μ( μ = 0.5 - 0.7) CR residence time inside the Galaxy JS~ε-γS γS = γobs – μ = 2 – 2.2 source CR spectrum

  11. Supernova explosions Supernova explosions supply enough energy to replenish GCRs against their escape from the Galaxy If there is acceleration mechanism which convert ~10% of the explosion energy into CRS

  12. Cosmic Ray Flux Possible GCR sources: SNRs knee SNRs (?) Reacceleration (?) ankle Extragalactic (?) GZK cutoff (?)

  13. Cosmic Ray diffusive acceleration in Supernova Remnants ESN~ 1051erg Krymsky 1977 Bell 1978 shock compression ratio for strong shock

  14. Nonlinear kinetic (time-dependent) theory of CR acceleration in SNRs • Gas dynamic equations • CR transport equation • Suprathermal particle injection • Gas heating due to wave dissipation • Time-dependent (amplified) • magnetic field Applied to any individual SNR theory gives at any evolutionary phase t>0 :nuclear Np(p,r), NHe(p,r), … and electron Ne(p,r) momentum and spatial distributions, which in turn can be used for determination of the expected nonthermal emissions Fγ(εγ)

  15. Nonlinear kinetic model: basic equations Berezhko, Yelshin, Ksenofontov (1994) ρ(r, t) – gas density w(r, t) – gas velocity Pg(r, t) – gas pressure f (p, r, t) – CR distribution function Hydrodynamicequations CR transport equationsfor protons and electrons (Krymsky, 1964) CR pressure source term CR diffusion coefficient Synchrotron loss time u = Vs - w

  16. Particle spectrum in/near acceleration region injection rate (parameter) η > 10-5→ efficient CR production

  17. Nonlinear effects due to accelerated CRs • Modification of the shock structure due to CR pressure gradient • Non power law (concave) CR spectrum • Magnetic field amplification (Lucek & Bell, 2000) • Increase of maximum CR energy • Increase of π0-decay gamma-ray emission • over IC emission

  18. CR spectrum inside SNR p test particle limit maximum CR momentum due to geometrical factors (Berezhko 1996) ppmax~ RSVSB

  19. Main nonthermal emission produced by Cosmic Rays(how one can “see” CR sources) • Synchrotron radiation radio B X-ray e • Inverse Compton scattering  gamma-rays  e  • Nuclear collisions  N gamma-rays p

  20. Nonthermal emission of SNRs • Test for CR acceleration theory • Determination of SNR physical parameters: • - CR acceleration efficiency • - Interior magnetic field B

  21. Relevant SNR parameters SNR aget known for historical SNRs ISM density NH influences SNR dynamics and gamma-ray production; deduced from thermal X-rays influences CR acceleration & synchrotron losses; deduced from fit of observed synchrotron spectrum; expected to be strongly amplified B >> BISM magnetic fieldB influences accelerated CR number, shock modification, CR spectral shape; deduced from observed shape of radio emission injection rate η (fraction of gas particles, involved in acceleration)

  22. CR spectrum inside SNR p e test particle limit radio X-ray due to synchrotron losses Steep radio-synchrotron spectrum Sν ~ν-α (>0.5,  >2) is indirect evidence of i) efficient proton acceleration and ii) high magnetic field B>>10G α = (γ – 1)/2

  23. Cassiopeia A Type Ib Distance 3.4 kpc Age 345 yr Radius 2 pc Circumstellar medium: free WR wind + swept up RSG wind + free RSG wind Tuffs(1986), VLA

  24. Circumstellar medium MS → RSG → BSG → SN Borkowski et al. (1996) Ng, cm-3 CSM number density current SN shock position 10 shell 1 RSG wind BSG wind 1 r, pc 2 d = 3.4 kpc Mej = 2 MSun ESN = 0.4×1051 erg

  25. Berezhko et al. (2003)

  26. Synchrotron Emission from Cassiopeia A Experiment: radio (Baars et al. 1977), 1.2 mm data (Mezger et al. 1986), 6 m data (Tuffs et al. 1997), X-ray data (Allen et al. 1997) Proton injection rate η = 3×10--3 Interior magnetic field Bd ≈ 0.5 mG Strong SN shock modification Steep concave spectrum at ν < 1012 Hz Smooth connection with X-ray region (ν > 1018 Hz) α≈ 0.8

  27. Magnetic field inside SNRs Emission (X-ray, γ-ray) due to high energy electrons Rs J Low field L ρ ρ Line of sight J High (amplified) field ρ Unique possibility of magnetic field determination! -Rs Rs 0

  28. Chandra SN 1006 Chandra Cassiopeia A • Filamentary structure of X-ray emission • of young SNRs • consequence of strongly amplified magnetic field, • leading to strong synchrotron losses

  29. Projected X-ray brightness of Cassiopeia A direct evidence for magnetic field amplification Theory:Berezhko & Völk (2004) Experiment (Vink & Laming 2003) confirms high internal magnetic field extracted from the fit of volume Integrated synchrotron flux (Berezhko, Pühlhofer & Völk 2003) Bd = 10 μG Bd = 500 μG For strong losses L emissivity scale brightness scale angular distance

  30. Integral gamma-ray energy spectrum of Cas A Components: Hadronic (π0) Inverse Compton (IC) Nonthermal bremsstrahlung (NB) Confirmation of HEGRA measurement is very much needed Already done by Magic (ICRC, Merida 2007)!

  31. SNR RX J1713.7-3946 X-rays (nonthermal) ROSAT(Pfeffermann & Aschenbach 1996) ASCA (Koyama et al. 1997; Slane et al. 1999) XMM (Cassam-Chenai et al. 2004; Hiraga et al. 2005) Gamma-ray image (HESS) Aharonian et al. (2005) Radio-emission ATCA (Lazendic et al. 2004) VHE gamma-rays CANGAROO (Muraishi et al. 2000) CANGAROO II (Enomoto et al. 2002) HESS (Aharonian et al. 2005)

  32. Spatially integrated spectral energy distribution of RX J1713.7-3946 Experiment: Aharonian et al. (2006) Theory: Berezhko & Völk (2006) required interior magnetic field Bd = 126 μG

  33. Magnetic field amplification ρISM Results of modeling (Lucek & Bell, 2000) + Spectral properties of SNR synchrotron emission + Fine structure of nonthermal X-ray emission Beff VS BISM SNR magnetic field is considerably amplified L Beff2/8π ≈ 10-2ρISMVS2 Bd = Beff >> BISM

  34. SNR magnetic field • Influences synchrotron emission • Determines CR diffusion mobility: Κ ~ p/(ZBd) CR diffusion coefficient (Bohm limit) • Influences CR maximum momentum pmax: pmax~ Z e Bd RS VS nuclear charge number

  35. Energy spectrum of CRs, produced in SNRs Berezhko & Völk (2007) Amplified magnetic field Bd2/(8π) ≈ 10-2ρ0VS2 Bd >> BISM

  36. Cosmic Ray Flux CR sources: Supernova remnants knee 1 Supernova remnants knee 2 Extragalactic (?) GZK cutoff (?)

  37. Energy spectrum of CRs Dip p + γ→ p + e+ + e- Dip scenario GZK cutoff p + γ→ N + π Experiment: Akeno-AGASA (Takeda et al. 2003) HiRes (Abbasi et al. 2005) Yakutsk (Egorova et al. 2004) CR spectrum, produced in SNRs CR spectrum from JEG~ε-2.7 extragalactic sources (Berezinsky et al.2006)

  38. Energy spectrum of CRs Ankle scenario Extragalactic (AGNs, GRBs…) JEG~ε-2 Berezinsky et al.(2006) SNRs SNRs + reacceleration

  39. Mean logarithm of CR atomic number Ankle scenario Experiment: KASKADE (Hörandel 2005) Yakutsk (Ivanov et al.2003) HiRes (Hörandel 2003) Dip scenario Precise measurements of CR composition is needed to discriminate two scenarios

  40. Fireball model of Gamma-ray bursts Rees & Meszaros (1992) ForwardShock R Energy release (supernova ?) E ISM dΩ ~ 10-2 π Fireball Γ ≈ 100 Lorentz factor E ≈ 1051 erg(?) ESS≈ 3×1053 erg spherically symmetric analog Γ ~ (ESS/NISM)1/2 R-3/2 R ~ t1/4

  41. CR acceleration in GRBs relativistic shock (Γ >> 1) Achterberg et al. (2001) assumption: isotropic CR diffusion in downstream region εmax≈ e BuΓ R c maximum proton energy unamplified magnetic field Bu = BISM = 10 μG εmax≈ 5 × 107mpc2 amplified magnetic field Bu2/8π = 0.1Γ2ρISMc2 εmax≈ 5 × 1013mpc2 NCR(ε)~ ε-γ γ≈ 2.2 GRBs are powerful extragalactic sources of CRs (?)

  42. Problem Bd~ Γ2 Bu >> Bu strongly anisotropic CR diffusion low chance for CRs to recross shock from downstream to upstream inefficient CR production Bdll >> Bd┴ & downstream upstream (e.g. Ostrowski & Niemiec, 2006) VS Bd Bu shock

  43. CR acceleration at late evolutionary stage (nonrelativistic shock) εmax≈ e Bu R c R(Γ = 1) =(ESS/3ρISMc2)1/3 ρISM = NISMmp InterStellar Medium density Bu2/8π = 0.1ρISMc2 amplified magnetic field For ESS= 3× 1053 erg, NISM = 1 cm-3 εmax = 3 × 1010 mpc2 However assumption Lγ = Qe, Pe~ Γ2ρISM c2 seems to be unrealistic Realistic numbers: Pp ~ Γ2ρISM c2 Pe = 10-2Pp then ESS≈1055 erg εmax ≈ 1011 mpc2

  44. Active Galactic Nuclei Jets Powerful source of nonthermal emission Powerful source of Cosmic Rays Γ ≈ 10Lorentz factor Shear flow Effective frictional acceleration (e.g. Ostrowski, 2004) Shock Diffusive shock acceleration

  45. Conclusions • CR acceleration in SNRs is able to provide the observed Galactic CR • spectrum up to the energy ε≈ 1017 eV • Two possibility for Galactic CR spectrum formation: • - Dip scenario ( CRs from Galactic SNRs at ε < 1017 eV + • Extragalactic CRs at ε > 1018 eV ) • - Ankle scenario ( CRs from Galactic SNRs at ε < 1017 eV + • Reaccelerated CRs at 1017 < ε < 1018 eV + • Extragalactic CRs at ε >019eV) • Precise measurements of CR spectrum and composition at ε > 1017 eV • are needed to discriminate the above two possibilities • Acceleration by subrelativistic/nonrelativistic shocks in GRBs • (or AGN jets) and frictional acceleration in AGN jets are • potential sources of Ultra High Energy CRs

  46. Supernovae = star explosions lg( Luminosity) 0 H lines SN I SN II H lines -4 -8 300 0 100 200 t, day

  47. SN Ia ( 15 % ) MCO<1.4MSun No central objects SNR in uniform ISM thermonuclear explosion SN II/Ib ( 85 %) MCO>1.4MSun pulsar / black hole SNR in CSM, modified by progenitor star wind core collapse ν detected from SN1987 A

  48. Cosmic Ray Flux CR sources: Supernova remnants knee 1 Supernova remnants (?) Reacceleration (?) knee 2 Extragalactic (?) GZK cutoff (?)

  49. Structure of the shock modified due to CR backreaction Flow speed u downstream upstream precursor CR pressure classical (unmodified) shock σ = u0/u2 = σS = u1/u2 =4 subshock modified shock σ > 4, σS < 4 x shock front p < mpc γ > 2 Acceleration sites p >> mpc γ < 2

  50. Cutoff of CR spectrum due to CR interaction with CMB Zatcepin, Kuzmin (1966) Greisen (1966) Cosmic microwave background (CMB) radiation Galaxy CR source π± π0 E = 1030 eV E=6×1019 eV

More Related