COSMIC RAYS

At the Earth’ Surface

We see cascades from CR primaries interacting with the atmosphere. Need to correct for that to understand their astronomical nature

What We Detect

Composition

• 98% nucleons (protons + heavier nuclei)• 2% electrons and positrons

At low energies (108 – 1010 eV), we have87% hydrogen12% helium 1% heavier nuclei

Three Anomalies1. Relative overabundance in light elements Li, Be, B

2. Slight underabundance of H (by number)

3. A striking deficiency of electrons

WHY?

4. These are spallation products (heavier nuclei break up in collisions in the ISM

5. Supernova ejecta (accelerators of CRs) are relatively rich in heavy elements, less so in H.

Cosmic Ray Energies

Relativistic?

Take γ > 1.05 to be ‘relativistic’ (that is, K.E. is large compared to rest-mass energy)

Some cosmic rays have energies corresponding to γ ~ 1011 - something we can’t do on Earth!

The Spectrum

Linear in a log-log plot: a power law

J (E) = K E –Γ

(consider the units!)

Γ is the spectral index;has a value of ~2.7 - 3

Meaning?

Low-energy CRs are abundant, high-energy ones are very rare (but staggering!)

Not a Single Power Law

Some structure: a knee, an ankle

What does this tell us?

How About Electrons?

We see the same sort of law, but the low-energy ones are strongly modulated by the solar wind

After correction for that,

Je(E) = 412 E -3.44

(between 3 GeV and 2 TeV)

Note that this is STEEPER than for nucleons. Why?

Electrons Lose Energy

1. Through synchrotron radiation; and

2. Because of inverse Compton scattering

Synchrotron Radiation

(We come back to this in Chapter 8.)

In Astronomy(need electrons moving at relativistic speeds)

In the general galactic magnetic field; in pulsars

Compton Scattering?

In Compton scattering, slow-moving electrons are invested with some energy by collisions with photons (that lose some energy)

Inverse Compton

Here, relativistic electrons lose energy to radiation with which they interact

e.g. relativistic electrons interact with the sea of low-energy photons in the cosmic microwave background

The Origin of Cosmic Rays

Very high energies! (~1020 eV)

(LHC can reach only ~1013 per proton, a factor of ten million less!)

Can we identify the sources?

Problems

CRs are deflected by magnetic fields (all but the most energetic) so their arrival direction does not point back to the source.

Low energy CRs are from the Sun (the ‘solar wind’; flux depends on solar cycle and activity: the ‘solar modulation’)

SupernovaeHigher energy CRs are from outside the Solar System, probably from supernovae. They would be accelerated by the Fermi acceleration mechanism, involving reflections from magnetic fields in shock waves etc.

Sometimes this yields gamma rays that are not deflected by interstellar magnetic field, could point back to the source.

Follow this link:

Proof that supernovae are the source of Cosmic Rays

The Highest Energies

Above the knee, CRs have enough energy to escape the galaxy. Those we receive could be from other galaxies.

But there is a limit to how far they can travel: the protons interact with CMBR photons and degrade. The GZK energy marks this important distinguishing point.