Astrophysics of the Highest Energy Cosmic Rays Paul Sommers Cracow, Poland January 10, 2004

Overview A bit of history The GZK expectation Explanations for super-GZK cosmic rays What about charged particle astronomy? Observations to come

John Linsley at Volcano Ranch (circa 1960)

Extremely High Energy Cosmic Ray Experiments Volcano Ranch ( ) (?) Haverah Park ( ) Sydney SUGAR Array ( ) Yakutsk ( ) Fly’s Eye ( )  1985 ICRC (La Jolla) contention AGASA ( ) HiRes (1998- )  1999 ICRC (Salt Lake City) concordance  2001 ICRC (Hamburg) divergence Auger (2004- ) Telescope Array (Starting)

HiRes & AGASA Spectra HiRes Collab ‘02 What is the spectrum? [Olinto]

The GZK Expectation  Based on special relativity and laboratory physics (pion photoproduction): High energy cosmic ray sees CMB as a beam of gamma rays (E>140 MeV) in its restframe.  Characteristic energy attenuation time is about 10 8 years for cosmic rays above the GZK threshold.  Other cosmic rays (below the GZK threshold) have been accumulating for approximately years.  Expect suppression by about 1/100 above the GZK threshold (relative to what the spectrum would be without the GZK effect).

Explanations for Possibly Absent GZK Suppression * All high energy cosmic rays are young: age < 10 8 yrs. (e.g. galactic halo sources, Cen A single source, galactic iron,... ) * All high energy cosmic rays are old: age > 10 9 yrs. (e.g. uhecrons, neutrinos,... ) * Super-GZK particles are younger than sub-GZK ones, but hard new spectrum hides GZK feature (decays of supermassive relic particles, topological defect annihilations,...) * Lorentz invariance is violated.

Scenarios in which “all” UHE cosmic rays are young Galactic iron nuclei (isotropized by halo B-field) Dominance by sources in local (supercluster) overdensity (No. Not enough overdensity. Still strong GZK effect.) Single nearby source: (isotropized by strong B-fields) Galactic center, Cen A, M87, M82,... OR: All UHE particles are attenuated in less than 10 8 years by unknown interactions, possibly with dark matter or dark energy.

Scenarios with all “old” particles Neutrinos Hadronic-size cross section at UHE energy Z-burst particle production on relic neutrinos UHEcrons (supersymmetric or other particles that are immune to interactions with the CMB) From distant BL Lac sources (Tinyakov & Tkatchev) From radio loud AGNs (Farrar & Biermann)

High Energy Top-Down Scenario Source spectra Propagated spectra E E 3 dN/dE E

High Energy Cosmic Ray Puzzles * How does Nature produce particles with E>50 J ? Step 1: Where and what are the sources ? * Why is there not an obvious GZK suppression ? * Why do super-GZK arrival directions not point to obvious powerful astrophysical sources ?

Maximum Particle Energy E EeV <  (v/c)   G R kpc

Multiple theories to test if GZK cutoff is absent. Lots of source models if GZK feature is there. Most UHE models expect protons. Spectrum and composition are unlikely to provide clear understanding of cosmic ray origins. A celestial (anisotropy) fingerprint is essential ! (Discrete sources or large-scale pattern)

Larmor radius: R kpc = E EeV / (Z B  G ) R Mpc = E EeV / (Z B nG ) Charged Particle Astronomy above GZK Threshold (~50 EeV)

5-year Auger Full-Sky Simulation ( E > eV and  < 60 o ) arrival directions Relative exposure as function of sin(declination) Auger North + Auger South

Summary The GZK spectral suppression is a simple consequence of special relativity unless all the observed UHE cosmic rays are all somehow young, all old, or there is a new hard source spectrum at the highest energies. Present experimental results are inconclusive. Composition and anisotropy measurements are needed as well as a definitive energy spectrum.

100 Myrs Proton energy loss due to pion photoproduction on CMB [Cronin]