an auger observatory view of centaurus a · 2009. 7. 20. · 17 17.3 17.5 17.8 18 18.3 18.5 18.8 19...
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Centaurus A June 2009
An Auger Observatory View of Centaurus A
Roger Clay, University of Adelaidebased on work particularly done with:
Bruce Dawson, AdelaideJose Bellido, AdelaideBen Whelan, Adelaide
and the Auger Collaboration
Centaurus A June 2009
An Auger Observatory View of Centaurus AThis talk will introduce the need to incorporate high energy
astrophysics into our understanding of mainstream astrophysics.
Some basic technique ideas will be discussed and then used to understand the Pierre Auger Observatory.
Some astrophysical ideas central to high energy astrophysics will be described.
Recent results from Auger will be presented.
Centaurus A June 2009
• Galactic Centres and AGN are of particular interest in high energy astrophysics, which concentrates on studies of the Universe through high energy radiations in many forms.
• Particles, including photons, at energies of GeV and above (TeV, PeV, EeV) are adding a new dimension to our understanding of familiar, and new, objects.
Centaurus A June 2009
• The Pierre Auger Observatory was conceived to attempt directional astronomy with charged particles.
• It is close to doing that and Centaurus A looks to be (should be) a likely source.
• The Observatory has already measured interactions with the CMB in the local Universe. It is now producing data which will teach us about magnetic fields in the same region.
• There are many remaining serious puzzles to be resolved before we will feel that we have a real understanding of the Universe out to 100 Mpc.
• First of all some context from lower energies……….
Centaurus A June 2009
As noted with H.E.S.S., our galaxy, the galactic centre, supernova remnants, pulsar wind nebulae, etc. are the types of object which are characteristically observed.
These are generally recognised as energetic/violent objects which are often observed through synchrotron and inverseCompton processes – clearly complementing radio astronomical studies.
Centaurus A June 2009
H.E.S.S. operates by observing specific sources for extended periods of integration time.
The alternative process involves imaging the whole of the accessible sky. Such a monitoring process is more traditional for ‘cosmic ray’ studies and is currently employed by MILAGRO, the Tibet Array, Pierre Auger etc…….
Centaurus A June 2009
Milagro – all skyAp.J. 595, 803, 2003
Centaurus A June 2009
• Synchrotron and inverseCompton processes producing observable photons require high energy particles (cosmic rays, energetic electrons, TeV etc.).
• Our goal is to directly observe such high energy particles and to determine their origins through directional measurements.
Centaurus A June 2009
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 powerlaw slope about –3.
It is rather featureless but steepens a little in the middle and flattens near the highest energies.
Radius of gyration 1 kpc in 1microGauss, 0.1 nT
Radius of gyration 100 kpc in 1microGauss, 0.1 nT
Centaurus A June 2009
Primary cosmic rays interact and a cascade develops which converts primary particle kinetic energy into
secondary particles.
Centaurus A June 2009
At energies of interest to Auger (above 1 EeV – 1018 eV), the flux is low and a large telescope is required.
In our case, with a 3000 square kilometre collecting area in Argentina. A northern observatory is planned with many times that area.
Centaurus A June 2009
Centaurus A June 2009
The Pierre Auger Observatory records the cascades using groundbased detectors and atmospheric fluorescence detectors.
Centaurus A June 2009
A large collecting area is required because of the low flux – 3000 square kilometres (1600 tanks).
Centaurus A June 2009
Observing the cascade with nitrogen fluorescence and particle detectors.
We deliberately avoid the forward directed Cerenkov light.
Centaurus A June 2009
We can occasionally see cascades from four fluorescence telescopes and the ground array. This gives us confidence in our calibrations and our understanding of the cascade physics (above accelerator energies).
Centaurus A June 2009
Comments on measurement uncertainties.
• Uncertainties in the primary energy are due to unknowns associated with the shower physics (above, but not greatly, accelerator energies), the atmospheric nitrogen fluorescence yield under various conditions, and many small effects at the few % level.
• The energy error budget adds to a couple of tens of percent. It is continually being improved and better understood.
Centaurus A June 2009
Comments on measurement uncertainties ….continued.
• The primary cosmic ray composition is unknown which means that the effect of any astrophysical magnetic fields is unknown. We tend to (erroneously) assume a simple proton composition.
• We have upper limits to photon and neutrino fluxes which are close to those expected from robust modelling (e.g. of GZK processes).
• It is possible that Auger events correlate best with AGN (or other) directions for the proton component and that other events re more highly charged (e.g. Hillas 2009).
Centaurus A June 2009
The cascade development gives us both energy and composition information.
Centaurus A June 2009
Centaurus A June 2009
Studies of the cascade development give us composition information.
Centaurus A June 2009
Auger directional data:
• The first data were published in 2007
• These were NOT ISOTROPIC. They are STILL NOT ISOTROPIC.
• The lack of isotropy seems to be correlated with directions of AGN within reasonable distances given the GZK process. But AGN are correlated with other things…………..
Centaurus A June 2009
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
Centaurus A June 2009
We expect to begin to see GZK photons in the nondistant future.
Centaurus A June 2009
Simple idea of a spectrum depending on the source distance.
Source Differentialspectral index.
(Surprisingly noncritical.)
These are TIMES.
This assumes a power law source spectrum
Centaurus A June 2009
Suppose that sources are randomly distributed in space – how will the spectrum depend on intergalactic diffusion properties – i.e. magnetic field strength and turbulence scales.
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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.
10100 nG possible depending on the turbulence parameters.
Centaurus A June 2009
The spectrum is now also sensitive to the LOCAL (real) distribution of sources.
Another way of looking at this is to recognise that, above the ‘GZK cutoff’, the flux will have useful directional
properties because we can only see locally.Fractional Anisotropy
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log(E)
log(
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100 nG
10 nG
1 nG
Centaurus A June 2009
• So………………
• We expect to see an anisotropy for sources at distances less than about 100 Mpc because more distant sources have strongly attenuated fluxes.
• This is seen.
• This means that the dominant source distribution is anisotropic but it does not tell us how to interpret that.
• But we still need to identify some sources. Are there a few very strong ones or many rather similar ones?
Centaurus A June 2009
Hillas, arXiv 2009
A uniform exposure plot of (2007 data) Auger events (circles) is NOT isotropic.
Centaurus A June 2009
Hillas, arXiv 2009
The Auger events show evidence of smallscale clustering.
Centaurus A June 2009
Hillas, arXiv 2009
There is some evidence that cosmic rays (circles) are correlated with AGN directions.
This broad brush correlation may not seem great, there is clustering near Cen A.
Centaurus A June 2009
Hillas, arXiv 2009
On the other hand, AGN and the highest energy cosmic rays do seem to have a correlated component.
How can this be?
Centaurus A June 2009
Hillas, arXiv 2009
It also works for a subset (extended radio sources) of sources.
Centaurus A June 2009
• Physics/reality check:
• Radius of gyration of a 1 EeV (1018 eV) proton in a 1 microgauss regular field is 1 kpc.
• Of course, this scales linearly so as we saw, 100 EeV has 100 kpc.
• More highly charged nuclei (iron – 26 times) have smaller radii of gyration.
• Cen A is many times this distance away so retention of direction requires submicrogauss fields or small turbulence scales.
Centaurus A June 2009
Cartoon of possible scattering field sites.
Source cluster. This fills with particles which are emitted from its surface. Subtends a small angle. May be large propagation delays.
B
Possible intergalactic or halo field.
May cause loss of flux or systematic deviations – ‘magnetic spectrometer’.
Possible local group field. Scattering here results in large deviations. Could be large delays.
Milky Way and Halo
Centaurus A June 2009
Galactic Deflections of protons 20 EeV and above (regular field).(Stanev 1996)
Centaurus A June 2009
• Time for the official sky map (circa 2007).
Centaurus A June 2009
Recent Auger data
Crosses – AGN
Circles Auger events
AGN and Auger directions correlate.
Centaurus A June 2009
Auger Directions on this image
Auger point spread
Galactic Field Deviation
Centaurus A June 2009
2009 ICRC result
Centaurus A June 2009
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Galactic Longitude
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Cen A
These events make up about one third of all PAO events at the highest energies – even though this is not in the area of greatest sky exposure. They arguably contribute strongly to the 2007 published excess of correlations with AGN because they are within the supergalactic plane.
Centaurus A June 2009
To incorporate the other surrounding events, we must invoke intergalactic scattering in turbulent fields, a rather natural requirement.
e.g. a 10 kpc turbulence scale plus a 0.05 nT field out to Cen A would give a spread of around 300.
or 10 kpc turbulence for 0.1 nT in a galactic halo to 100 kpc
=> around 200.
Neither too unnatural and not too bad a fit.
And all this takes extra time – say an extra 108 years to get from Virgo – an interesting number.
Centaurus A June 2009
Conclusions.• Auger has measured a GZK ‘cutoff’.• The cosmos out to 100 Mpc is not isotropic so we expect
Auger data to be anisotropic. They are.• There is evidence for small angle correlation with AGN
directions. This is still somewhat perplexing to me.• However, there is a clear correlation with the
supergalactic plane and an apparent largescale correlation centred on Cen A.
• The data are still sparse, but it is not clear that the more energetic particles are more concentrated on Cen A.
• There are basic theory issues. How does Cen A, or even our Milky Way accelerate particle to EeV energies.
Centaurus A June 2009
Nonconclusions.
• So, there are loose ends and, of course, more data will be required to resolve them.
• Why is there no clear change in mean event energy with angular distance from the Cen A direction?
• Do regular intergalactic/halo fields play a role?• Is it a coincidence that the higher energy events are to
the north of Cen A?• Why is there clustering around Cen A but nothing
towards Virgo?• What role is composition playing?
Centaurus A June 2009
With charged particles, there is an extra degree of freedom – the choice of magnetic field structure. What if it had a regular component, an extended galactic halo perhaps……Maybe we get Virgo?
Centaurus A June 2009
Thanks
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