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Open Problems with Very High Energy Cosmic Rays

This article discusses open issues and challenges related to the acceleration and observation of very high energy cosmic rays, including the spectrum, chemical composition, anisotropies, and the transition from galactic to extragalactic sources. It also explores the difficulties in particle acceleration, non-linear Fermi acceleration, and the importance of multifrequency observations.

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Open Problems with Very High Energy Cosmic Rays

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  1. Open Problems with Very High Energy Cosmic Rays Pasquale Blasi National Institute for Astrophysics Arcetri Astrophysical Observatory Firenze, Italy TeV Workshop, Madison, August 2006

  2. Outline • Some observational facts • Spectrum • Chemical Composition • Anisotropies • Where do CRs become extragalactic? • The dip • The Ankle • Mixed Composition • Open issues with acceleration of CRs • Non linear Fermi Acceleration • Relativistic shocks • Help of multifrequency observations

  3. Observations I: Spectrum Observations – I: Spectrum Knee 2nd Knee Dip GZK?

  4. The actual discrepancy is at 1-2 standard deviations

  5. De Marco, PB and Olinto 2006

  6. Chemical Composition

  7. Helium knee Proton knee ? Iron

  8. Knee Region TRANSITION ? ? From A. Watson (2006) DIP ANKLE

  9. Anisotropies

  10. Observations – III: Anisotropy HiRes-I data with E>1019.5 eV with mono (52 events versus 47 of AGASA) The banana-shaped error boxes are [4.9-6.1]x[0.4-1.5] degrees HiRes stereo data (271 events with E>1019 eV versus about 900 of AGASA). The error boxes are about 0.6 degrees wide. Only 27 events above 4 1019 eV

  11. Small Scale Anisotropies PB & De Marco 2003 10-5 Mpc-3

  12. Statistical Volatility of the SSA signal De Marco, PB & Olinto 2006 THE SSA AND THE SPECTRUM OF AGASA DO NOT APPEAR COMPATIBLE WITH EACH OTHER (5 sigma)

  13. De Marco, PB and Olinto 2006 >4 1019 eV 105 km2 sr yr >1020eV

  14. De Marco, PB and Olinto 2006 15 years of Operation of Auger South

  15. Transition from Galactic to Extragalactic Cosmic Rays • The Ankle scenario • The Dip scenario • The mixed Composition scenario

  16. Transition at the Ankle • A steep GAL spectrum • encounters a flat EX-GAL • spectrum • The GAL spectrum should • extend to >1019 eV and is • expected to be Fe • dominated • The chemical composition • of the EX-GAL part can • have heavy nuclei Aloisio, Berezinsky, PB, Gazizov, Grigorieva, Hnatyk 2006

  17. Transition at the Dip A DIP IS GENERATED DUE ONLY TO PAIR PRODUCTION. ITS POSITION IS FIXED BY PARTICLE PHYSICS INTERACTIONS • STILL TRUE that a STEEP GAL • spectrum meets a FLAT EX-GAL • spectrum • 2. The Low energy flattening due to • either pair prod. or most likely • magnetic horizon • The GAL spectrum is expected to • END at <1018 eV (mainly Fe) • The EX-GAL spectrum is • predicted to be mainly protons • at E>1018 eV (No more that 15% • He allowed) • 5. Steep injection spectrum unless • complex injection Aloisio, Berezinsky, PB, Gazizov, Grigorieva, Hnatyk 2006

  18. Kachelriess and Semikoz 2005 Aloisio, Berezinsky, PB, Grigorieva and Gazizov 2006

  19. Transition due to Mixed Composition Allard et a. 2006 • Particles at injection have an arbitrary chemical composition • Relatively flat spectra at injection are required • The inferred galactic spectrum has a cutoff at energies about the same • as in the DIP scenario • 4.At 1019 eV there is still an appreciable fraction of heavy elements • 5. At 1020 eV, as in all other models, there are mainly protons

  20. Some Open Problems with Acceleration of Cosmic Rays The lesson we can learn from galactic cosmic rays…

  21. A CRUCIAL ISSUE: the maximum energy of accelerated particles 1. The maximum energy is determined in general by the balance between the Acceleration time and the shortest between the lifetime of the shock and the loss time of particles 2. For the ISM, the diffusion coefficient derived from propagation is roughly For a typical SNR the maximum energy comes out as FRACTIONS OF GeV !!! Similar numbers would be obtained for galactic sources of similar age and in similar conditions. PARTICLE ACCELERATION AT ASTROPHYSICAL SHOCKS IS EFFICIENT ONLY IF PARTICLES GENERATE THEIR OWN SCATTERING CENTERS!!! NON-LINEAR PARTICLE ACCELERATION

  22. Pitch angle scattering and Spatial Diffusion The Alfven waves can be imagined as small perturbations on top of a background B-field The equation of motion of a particle in this field is In the reference frame of the waves, the momentum of the particle remains unchanged in module but changes in direction due to the perturbation: The Diffusion coeff reduces To the Bohm Diffusion for F(p)~1

  23. Maximum Energy a la Lagage-Cesarsky In the LC approach the lowest diffusion coefficient, namely the highest energy, can be achieved when F(p)~1 and the diffusion coefficient is Bohm-like. For a life-time of the source of the order of 1000 yr, we easily get Emax ~ 104-5 GeV Wave growthHERE IS THE CRUCIAL PART! Bell 1978 Wave damping

  24. Maximum Level of Turbulent Self- Generated Field UPSTREAM Stationarity B SHOCK B Integrating Breaking of Linear Theory… SHOCK For typical parameters of a SNR one has δB/B~20.

  25. Possible Observational Evidence for Amplified Magnetic Fields Volk, Berezhko & Ksenofontov (2005) Rim 1 Rim 2 Lower Fields 240 µG 360 G • Add to all this, the enourmous • Predictions of Non-linear Diffusive • Shock Acceleration!!! • Curved spectra • Heating suppression downstream • B-field amplification

  26. Hybrid Simulations (PIC+MHD) used to follow the field Amplification when the linear theory starts to fail Bell 2004 For typical parameters of a SNR we getδB/B~300 The Max energy for a SNR becomes ~1017 eV for protons and 3 1018 eV for Iron nuclei (for Bohm diffusion)

  27. General lesson and concerns • The max energy as estimated through Hillas-like plots may well be incorrect by orders of magnitude if the ambient field is used instead of the self-generated field • δB/B>>1 is predicted by a quasi-linear approach, though confirmed by PIC • PIC carried out in very simple approach. No nonlinear effects taken into account in these PIC simulations… • THERE ARE NO such calculations in the case of RELATIVISTIC SHOCKS

  28. Particle Acceleration At Relativistic Shocks

  29. Peculiar Aspects of Particle Acceleration at relativistic shocks we find that p2 = (1/3) 2 and 2 = 1/3 Taub’s relations No equipartition ultrarelativistic pressureless For a homogeneous magnetic field downstream NO PARTICLE is expected to return back to the shock. The turbulent structure of the B-field is crucial!

  30. Relativistic mirror vs Fermi acceleration Reflection at the relativistic mirror works ONLY at the first interaction. After that E ~42 E

  31. Universal or non-universal… Most calculations of particle acceleration at relativistic shocks lead to the so-called UNIVERSAL SPECTRUM This important result is obtained by solving the same equation: ASSUMPTIONS: the scattering is diffusive and in the so-called Small Pitch Angle Scattering (SPAS) regime (Dµµ is constant ONLY for the isotropic case)

  32. The transport equation in the most general case is Vietri 2003 PB & Vietri 2005 Arbitrary Scattering Function For instance, in the case of Large Angle Scattering: Universal Blasi & Vietri 2005

  33. More non-universality… Lemoine et al. 2006 Anisotropic Turbulence Isotropic turbulence Again, one ends up again in a situation of quasi-coherent perpendicular Field downstream: PARTICLES DO NOT RETURN → SPECTRUM STEEPENS Caveat: what if the field upstream does not have a coherence length? (e.g. Scale invariant spectrum 1/k)

  34. The role of multifrequency observations • For Galactic CRs the role of Multi-ν observations is obvious (e.g. gamma, X, radio from SNR,…) • A similar role would be desirable for UHECRs • Interactions inside the source during the acceleration • Interactions during the propagation from source to observer • The sources of UHECRs might be identified through the SSA of the arrival directions, but this depends on Extra-Gal B-field, chemical composition, Gal B-field, etc... • Correlations with Large Scale Structures (Longo et al. 2006) would also be helpful.

  35. A simulated Auger South Sky De Marco

  36. CONCLUSIONS • A lot of indirect evidence is accumulating in favor of SNRs being sources of CRs in the Galaxy, up to <1018 eV…but no iron solid proof • The transition between Gal to Extra-Gal CRs takes place either at the second knee or at the ankle (very important for the origin of UHECRs; crucial the composition) • Acceleration at shock fronts presents us with exciting new developments and challenges to reach high Emax even of relevance for UHECRs • The spectrum of UHECRs is being investigated by Auger…just some patience is needed. It would be precious to gather information about the composition in the transition region • UHECRs are still orphans…may be something even bigger than Auger South will be needed to “see” the sources (Auger North, Super Auger, EUSO-like?)

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