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 (1958-1968) (?)

Haverah Park (1968-1987)

Sydney SUGAR Array (1968-1979)

Yakutsk (1974-1995)

Fly’s Eye (1981-1991)

1985 ICRC (La Jolla) contention

AGASA (1992-2004)

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 108

years for cosmic rays above the GZK threshold.

Other cosmic rays (below the GZK threshold) have been accumulating for approximately 1010 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 < 108 yrs.

(e.g. galactic halo sources, Cen A single source, galactic iron, . . . )

* All high energy cosmic rays are old: age > 109 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 108 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

E3 dN/dE

E3 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

EEeV < (v/c) G Rkpc

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:

Rkpc = EEeV / (Z BG)

RMpc = EEeV / (Z BnG)

Charged Particle Astronomy

above GZK Threshold (~50 EeV)

5-year Auger Full-Sky Simulation

( E > 1019 eV and < 60o )

36000 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]