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Ultra-High Energy Cosmic Rays in a Structured and Magnetized Cosmic Environment

Ultra-High Energy Cosmic Rays in a Structured and Magnetized Cosmic Environment. General facts and the experimental situation Acceleration (“bottom-up” scenario) Cosmic magnetic fields and their role in cosmic ray physics. G ü nter Sigl GReCO, Institut d’Astrophysique de Paris, CNRS

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Ultra-High Energy Cosmic Rays in a Structured and Magnetized Cosmic Environment

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  1. Ultra-High Energy Cosmic Rays in a Structured and Magnetized Cosmic Environment • General facts and the experimental situation • Acceleration (“bottom-up” scenario) • Cosmic magnetic fields and their role in cosmic ray physics Günter Sigl GReCO, Institut d’Astrophysique de Paris, CNRS http://www.iap.fr/users/sigl/homepage.html

  2. many Joules in one particle! The cosmic ray spectrum stretches over some 12 orders of magnitude in energy and some 30 orders of magnitude in differential flux:

  3. supernova remnants pulsars, galactic wind AGN, top-down ?? The structure of the spectrum and scenarios of its origin knee ankle toe ?

  4. electrons • -rays muons Atmospheric Showers and their Detection Fly’s Eye technique measures fluorescence emission The shower maximum is given by Xmax~ X0 + X1 log Ep where X0 depends on primary type for given energy Ep Ground array measures lateral distribution Primary energy proportional to density 600m from shower core

  5. ground arrays fluorescence detector ground array Current data at the highest energies

  6. but consistent with Haverah Park Discontinuity with AGASA, but better agreement with Akeno A Tension between the Newest Fluorescence Data (HiRes) and Ground Array Results? Or: Is there a Cut-Off after all?

  7. HiRes collaboration, astro-ph/0208301 Lowering the AGASA energy scale by about 20% brings it in accordance with HiRes up to the GZK cut-off, but not beyond. May need need an experiment combining ground array with fluorescence such as the Auger project to resolve this issue.

  8. But HiRes has also seen a >200 EeV event in stereo mode with only ~20% exposure of the mono-mode

  9. Next-Generation Ultra-High Energy Cosmic Ray Experiments compare to AGASA acceptance ~ 230 km2sr

  10. The southern Auger site is under construction.

  11. The Ultra-High Energy Cosmic Ray Mystery consists of (at least) Three Interrelated Challenges 1.) electromagnetically or strongly interacting particles above 1020 eV loose energy within less than about 50 Mpc. 2.) in most conventional scenarios exceptionally powerful acceleration sources within that distance are needed. 3.) The observed distribution seems to be very isotropic (except for a possible interesting small scale clustering)

  12. pair production energy loss -resonance pion production energy loss multi-pion production pion production rate The Greisen-Zatsepin-Kuzmin (GZK) effect Nucleons can produce pions on the cosmic microwave background  nucleon • sources must be in cosmological backyard Only Lorentz symmetry breaking at Г>1011 could avoid this conclusion.

  13. M.Boratav First Order Fermi Shock Acceleration This is the most widely accepted scenario of cosmic ray acceleration u1 u2 The fractional energy gain per shock crossing depends on the velocity jump at the shock. Together with loss processes this leads to a spectrum E-q with q > 2 typically. When the gyroradius becomes comparable to the shock size, the spectrum cuts off.

  14. A possible acceleration site associated with shocks in hot spots of active galaxies

  15. A possible acceleration site associated with shocks formed by colliding galaxies

  16. Or Can Plasma Waves in Relativistic Shocks Occuring in -Ray Bursts accelerate up to 1024 eV? Chen, Tajima, Takahashi, astro-ph/0205287

  17. galactic plane supergalactic plane Arrival Directions of Cosmic Rays above 4x1019 eV Akeno 20 km2, 17/02/1990 – 31/07/2001, zenith angle < 45o Red squares : events above 1020 eV, green circles : events of (4 – 10)x1019 eV Shaded circles = clustering within 2.5o. Chance probability of clustering from isotropic distribution is < 1%.

  18. HiRes sees no significant anisotropy above 1018 eV

  19. Cosmic Magnetic Fields and their Role in Cosmic Ray Physics 1.) Cosmic rays up to ~1018 eV are confined in the Galaxy Energy densities in cosmic rays, in the galactic magnetic field, in the turbulent flow, and gravitational energy are of comparable magnitude. The galactic cosmic ray luminosity LCR required to maintain its observed density uCR in the galactic volume Vgal for a confinement time tCR~107 y, LCR ~ uCRVgal / tCR, is ~10% of the kinetic energy rate of galactic supernovae. 2.) Cosmic rays above ~1019 eV are probably extragalactic and may be deflected mostly by extragalactic fields BXG rather than by galactic fields. However, very little is known about about BXG: It could be as small as 10-20 G (primordial seeds, Biermann battery) or up to fractions of micro Gauss if concentrated in the local Supercluster (equipartition with plasma). 3.) Magnetic fields are main players in cosmic ray acceleration.

  20. Example: Magnetic field of 10-10 Gauss, coherence scale 1 Mpc burst source at 50 Mpc distance To get an impression on the numbers involved: time delay cuts through the energy-time distribution: differential spectrum Lemoine, Sigl

  21. Transition rectilinear-diffusive regime Neglect energy losses for simplicity. Time delay over distance d in a field Brms of coherence length λc for small deflection: This becomes comparable to distance d at energy Ec: In the rectilinear regime for total differential power Q(E) injected inside d, the differential flux reads

  22. In the diffusive regime characterized by a diffusion constant D(E), particles are confined during a time scale which leads to the flux For a given power spectrum B(k) of the magnetic field an often used (very approximate) estimate of the diffusion coefficient is where Brms2=∫0∞dkk2<B2(k)>, and the gyroradius is

  23. IF E<<Ec and IF energy losses can be approximated as continuous, dE/dt=-b(E) (this is not the case for pion production), the local cosmic ray density n(E,r) obeys the diffusion equation Where now q(E,r) is the differential injection rate per volume, Q(E)=∫d3rq(E,r). Analytical solutions exist (Syrovatskii), but the necessary assumptions are in general too restrictive for ultra-high energy cosmic rays. Monte Carlo codes are therefore in general indispensable.

  24. Medina Tanco, Lect.Not.Phys 542, p.155 Strong fields in our Supergalactic Neighbourhood ?

  25. Principle of deflection code sphere around observer source A particle is registered every time a trajectory crosses the sphere around the observer. This version to be applied for individual source/magnetic field realizations and inhomogeneous structures. sphere around source A particle is registered every time a trajectory crosses the sphere around the source. This version to be applied for homogeneous structures and if only interested in average distributions. source

  26. Effects of a single source: Numerical simulations A source at 3.4 Mpc distance injecting protons with spectrum E-2.4 up to 1022 eV A uniform Kolmogorov magnetic field of strength 0.3 micro Gauss and largest turbulent eddy size of 1 Mpc. 105 trajectories, 251 images between 20 and 300 EeV, 2.5o angular resolution Isola, Lemoine, Sigl Conclusions: 1.) Isotropy is inconsistent with only one source. 2.) Strong fields produce interesting lensing (clustering) effects.

  27. Same scenario, averaged over many magnetic field realisations

  28. That the flux produced by CenA is too anisotropic can also be seen from the realization averaged spectra visible by detectors in different locations southern hemisphere AGASA, northern hemisphere solid angle averaged Isola, Lemoine, Sigl, Phys.Rev.D 65 (2002) 023004

  29. Summary of spectral effects in rectilinear regime in diffusive regime in rectilinear regime in diffusive regime Continuous source distribution following the Gaussian profile. B=3x10-7 G, d=10 Mpc

  30. More detailed scenarios of large scale magnetic fields use large scale structure simulations with magnetic fields followed passively and normalized to a few micro Gauss in galaxy clusters. We use a (75 Mpc)3 box, repeated by periodic boundary conditions, to take into account sources at cosmological distances. We then consider different observer and source positions for structured and unstructured distributions with and without magnetization. We analyze these scenarios and compare them with data based on large scale multi-poles, auto-correlations, and clustering. Sigl, Miniati, Ensslin, astro-ph/0309695

  31. Observer immersed in fields of order 0.1 micro Gauss Observer immersed in fields of order 10-11 Gauss

  32. Observer immersed in fields of order 10-11 Gauss: Cut thru local magnetic field strength Filling factors of magnetic fields from the large scale structure simulation.

  33. Result: Magnetized, structured sources are marginally favored if the observer is immersed in negligible fields. Strong field observer: ruled out by isotropy around 1019 eV. Weak field observer: allowed. However, even if fields around observer are negligible, deflection in magnetized structures surrounding the sources lead to off-sets of arrival direction from source direction up to >10 degrees up to 1020 eV in our simulations. This is contrast to Dolag et al., astro-ph/0310902. => Particle astronomy not necessarily possible !

  34. Unmagnetized, Unstructured Sources Source density=2.4x10-4 Mpc-3 Source density=2.4x10-6 Mpc-3 Autocorrelation function sensitive to source density in this case Comparison with AGASA data => The required source density is ~ 10-5 Mpc-3. Similar numbers were found in several independent studies, e.g. Yoshiguchi et al. ApJ. 586 (2003) 1211, Blasi and de Marco, astro-ph/0307067

  35. Magnetized, Structured Sources Comparing predicted autocorrelations for source density = 2.4x10-4 Mpc-3 (upper set) and 2.4x10-5 Mpc-3 (lower set) for an Auger-type exposure. Deflection in magnetic fields makes autocorrelation and power spectrum much less dependent on source density and distribution !

  36. The spectrum in the magnetized source scenario shows a pronounced GZK cut-off. Deflection can be substantial even up to 1020 eV.

  37. Comparing predicted autocorrelations for source density = 2.4x10-5 Mpc-3 with (lower set) and without (upper set) magnetization for an Auger-type exposure. In the future, a suppressed auto-correlation function will be a signature of magnetized sources.

  38. Generalization to heavy nuclei All secondary nuclei are followed and registered upon crossing a sphere around the source. B=10-12 G, E>1019 eV Example: If source injects heavy nuclei, diffusion can enhance the heavy component relative to the weak-field case. B=2x10-8 G, E>1019 eV Here we assume E-2 iron injection up to 1022 eV. Bertone, Isola, Lemoine, Sigl, astro-ph/0209192

  39. B=2x10-8 G, d=7.1 Mpc Composition as function of energy. However, the injection spectrum necessary to reproduce observed spectrum is ~E-1.6 and thus rather hard. B=2x10-8 G, d=3.2 Mpc

  40. 2nd example: Helium primaries do not survive beyond ~20 Mpc at the highest energies B=10-12 G, E>1020 eV Bertone, Isola, Lemoine, Sigl, astro-ph/0209192

  41. Conclusions 1.) The origin of very high energy cosmic rays is one of the fundamental unsolved questions of astroparticle physics. This is especially true at the highest energies, but even the origin of Galactic cosmic rays is not resolved beyond doubt. 2.) Acceleration and sky distribution of cosmic rays are strongly linked to the in part poorly known strength and distribution of cosmic magnetic fields. 3.) Already current cosmic ray data (isotropy) favor an observer immersed in fields < 10-11 G. Future data (auto-correlation) will test source magnetization. 4.) The coming 3-5 years promise an about 100-fold increase of ultra-high energy cosmic ray data due to experiments that are under either construction or in the proposal stage.

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