What is cosmic radiation and where does it come from?

Frederik Rühr,

Kirchhoff-Institut für Physik, Universität Heidelberg

IRTG Seminar, 26. Oktober 2007

“The origin of cosmic rays is one of the major unsolved astrophysical problems”

(Claus Grupen, Astroparticle Physics, 2005)

What are cosmic rays?

Why cosmic rays?

What are cosmic rays?

• Charged particles (above a given magnetic rigidity)

– Protons: 86%– α-particles: 11%– Electrons: 2%– Nuclei, up to uranium: 1%– (Positrons and antiprotons?)

• Neutral particles– γ-rays– Neutrinos and antineutrinos

(numbers from Perkins, Particle Astrophysics, 2003)

What are cosmic rays?

• Charged particles (above a given magnetic rigidity)

– Protons: 86%– α-particles: 11%– Electrons: 2%– Nuclei, up to uranium: 1%– (Positrons and antiprotons?)

• Neutral particles– γ-rays– Neutrinos and antineutrinos

(numbers from Perkins, Particle Astrophysics, 2003)

I will focus on this

Where do they come from?

• Bottom up approach– Charged particles of low energy are

accelerated to the observed high energies

• Top-down approach– Decay of cosmic strings, topological defects,

…

Where do they come from?

• Time dependant magnetic fields create electric fields that can accelerate charged particles– Cyclotron mechanism

• E.g. particles going round in circles around sunspots could be accelerated up to 100 GeV, gaining ~1 GeV per full orbit

• Orbits are only stable in the presence of guiding forces

– Acceleration by sunspot pairs• Moving pairs of sunspots -> moving magnetic dipole ->

electric field• More plausible, but resulting in lower maximum energies

Acceleration of cosmic rays

• Charged particles traversing shock fronts (e.g. supernova shells) can gain energy

• If they transverse the shock front multiple times, due to e.g. being mirrored by magnetic clouds, energies up to 1014Z eV could be reached

• Multiple reflection between two shock fronts is also an option

• The estimated energy spectrum resulting from this mechanism is expected to run with E-2.1, compared to the -2.7 measured (but the difference can possibly be accounted for by an energy dependence of the escape chance of particles)

Supernova shock fronts (blue)

Energies above 1014 eV?

• No mechanisms are known• Can we look for the origin of highly energetic charged

primary cosmic rays?

The short answer is: NO!

Charged cosmic rays up to 1018 eV appear to be isotropic, due to losing all their directional information in cosmic magnetic fields

The knee, possible explanations

• Galactic confinement– Particles down to the knee

can be contained in our galaxy due to its magnetic field

– below the knee, particles start to leak out of our galaxy

• Maximum energy supplied by supernovae

The ankle, a theory

• Extragalactical component?• The spectrum could be the

combination of a hard extragalactic spectrum and a steeper galactic spectrum

Toes? The GZK cut-off

• It is expected that the CMB becomes opaque for protons above 1019 eV

• Thus the protons have a limited range

(Greisen, Zatsepin and Kuzmin)

James Cronin, 2007

The problem• Fly`s Eye measured an Oh-My-God

particle in 1991, with an energy of 3.2x1020 eV

• The assumption is that the ankle has an extragalactic origin

• Particles of that energy must originate very close to our galaxy

?

A closer look at the toes

• According to AGASA measurements there is no energy cut-off visible at all

-> No Toes

What's so great about these energies?

• According to the GZK cut-off they have to originate very close to us

• In addition we could actually pinpoint their sources!

James Cronin, 2007

James Cronin, 2007

Pierre Auger Data

James Cronin, 2007

Pierre Auger Data

Summary• We know do not really have any top-down models to explain the

primary cosmic ray flux• We know a few bottom up approaches, but they party don’t make

much sense, and are very limited in energy• We do have several explanations ready for the knee, the ankle and

supposed toes, but the big picture does not make sense at the moment

Michael Turner, 2007