Constraints on Astrophysical Magnetic Fields from UHE Cosmic Rays

Magnetic Field Workshop November 2007

Constraints on Astrophysical Magnetic Fields from UHE Cosmic Rays

Roger Clay, University of Adelaidebased on work done with:

Roland Crocker, Adelaide and MonashBruce Dawson, AdelaideBen Whelan, Adelaideand the Auger Collaboration


Cosmic Rays and Magnetic Fields

Cosmic rays are charged particles so their propagation is affected by magnetic fields.

1. By changing their direction.

If we know a point source and its corresponding particles, we should be able to deduce the characteristics of the intervening fields.

2. By increasing their path length to reach us.

If we know the source spectrum, the observed spectrum and any attenuation process, we can deduce the intervening fields also.

We can now attempt these tasks. The latter first – the spectrum.


The Cosmic Ray Spectrum

Things to note are:It covers a huge range of energies so the physics is likely to change from one part to another.

It is steep, with a power-law slope about –3.

It is rather featureless but steepens a little in the middle and flattens near the highest energies.


Primary cosmic rays interact and a cascade develops which converts primary particle kinetic energy into

secondary particles.


The Pierre Auger Observatory records the cascades using ground-based detectors and atmospheric fluorescence detectors.


A large collecting area is required because of the low flux – 3000 square kilometres (almost complete – 1500 tanks).

(current exposure~ 7000 km2 sr yr)


At energies above a few times 1019 eV, interactions with the CMB result in a ‘small’ interaction mean free path for protons.

THIS IS THE ‘GZK’ EFFECT (but notice that the energy loss per interaction is not huge – the energy

attenuation length is typically ~90 Mpc)

Mean free path below 10 Mpc

For energies above 60 EeV


Simple idea of a spectrum depending on the source distance.

Source Differentialspectral index.

(Surprisingly non-critical.)


But spectra are difficult to measure!

~ Auger


Additional information will come from photon observations – Auger currently just has upper limits to photon fluxes.

This relative photon flux is probably somewhat optimistic – but should not be too far in error.

It is realistic to expect Auger to reach this physics.


Real Auger spectra to 2007 showing the cut-off.


Suppose that sources are randomly distributed in space – how will the spectrum depend on the diffusion properties – i.e. field strength and

turbulence scales.

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17 17.3 17.5 17.8 18 18.3 18.5 18.8 19 19.3 19.5 19.8 20 20.3 20.5

MW 0.2 at2

10kpc 100nG

10kpc 10nG

10kpc 1 nG

1Mpc 10 nG

1Mpc 1nG

1 Mpc 100nG

Log(E3*flux)

Log(E eV)

Milky Way Extragalactic

1 nG too low.

10-100 nG possible depending on the turbulence parameters.


Building a spectrum to fit observed data leads one to realise that the spectrum is sensitive to parameters of the intergalactic magnetic field.

Galactic and Extragalactic Fluxes

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log(E)

log(

flux*

E**3

)

MW 0.2 at2

10kpc 100nG

10kpc 10nG

10kpc 1 nG

1Mpc 10 nG

Turbulence scale

Field strength

To get a steep cut-off we need the lower turbulence scale or a non-random source distribution


Another way of looking at this is to recognise that, above the ‘GZK cut-off’, the flux will have useful directional

properties.

Fractional Anisotropy

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

17 18 19 20

log(E)

log

(fra

ctio

nal

an

iso

tro

py)

100 nG

10 nG

1 nG


AGASA highest energies

Supergalactic plane


Data from the Sydney University SUGAR array

We will use these directions later


Developing a ‘Prescription’

• Data prior to May 2006 searched.• Directions compared to the Veron-Cetty and Veron

catalog of AGN.• Minimum probability of random for angular uncertainty

for 3.1o, zmax = 0.018 (75 Mpc), Eth = 56EeV

• These all make sense! Use these for later data until chance of isotropy is below 1%.

• Reject isotropy 31 August 2007 8 events compared to 2.7 expected – actual chance is 0.0017.

• Then look at the properties of the whole dataset.


AGASA REGIONSC2

C3

C1

SUGAR

Centaurus Wall?

VIRGO


Auger Directions on this image

Auger point spread

Galactic Field Deviation


Summary so far:

• The shape of the GZK cut-off is now being measured and depends on the properties of the intergalactic (and more local) magnetic fields.

• At the highest energies, the sources must be rather local.

• There is apparent correlation with known astrophysical structures (3o uncertainty) such as the supergalactic plane (or a component of its contents – AGN?).

• There are some unexpected directions (in the voids).


Correlating with the supergalactic plane (?)

• AGASA, SUGAR and Auger all suggest this.

• To the extent that it is true, the angular deviation above 60 EeV (GZK) must be << 30o (very conservative upper limit).

• This is amazing: a. The intergalactic fields must be quite small (ball park 10 nG).

• b. It makes us astronomers.


Centaurus A (Speculation)

• Centaurus A stands out in the Auger data as a source close to a preferred sky direction.

• Cen is a little below 4 Mpc distant – and is huge on the sky.

• The spread of directions around Cen A is much below 10 degrees. This confirms that the intervening magnetic field strength is below 10nG or the turbulence scale is much below 1 Mpc. 10nG + 10kpc scale fits both spectral and directional data.

• Note that Cen A should be resolved – the sources extend in the supergalactic plane either side but may suggest a southern lobe source.


Where now?

We are just learning to play this game.

• WHAT HAS BEEN PRESENTED IS EQUIVALENT TO ONE YEAR OF DATA AT OUR PRESENT RATE.

• Each further year will add that much more.

We can already find multiplets of events. These may or may not be real but the next step is to search for consistent field models for groups of events. Presently, we can fit a Bi-Symmetric Spiral galaxy model (plus halo) –only- to our multiplet data. It fits rather well for Cen A.


• Tomography of the intervening fields should now become possible.

• Then we can ask whether all the possible sources are known – of course, we are looking back in time at a different rate to photon astronomers so the sources may not correspond.

• We can now put real galaxy directions and estimated activity into our models.

Then, where do the particles below the GZK come from????


THANKS

Documents

Constraints on Astrophysical Magnetic Fields from UHE Cosmic Rays