The Pierre Auger Observatory & Ultra High Energy Cosmic Rays Nick Cowan UW Astronomy January 2006

Outline What are UHECRs? How are they produced? Why you should you be pumped about Auger?

UHECRs: Primer Cosmic rays are high energy charged particles which are produced out there and rain down on our planet. (they’re not really rays) 85% protons, 12% He nuclei, 2% electrons, 1% heavier nuclei. Gamma-rays are high energy photons but are often associated with Cosmic Rays. Ultra-High Energy Cosmic Rays are those which have energies in excess of eV (1 EeV).

Anatomy of a Power Law Cosmic Ray fluxes follow a power law with slope -2.7 The “Knee” is a relative steepening of the power law around E = 3x10 15 eV The “Ankle” is a flattening of the curve around E = 3x10 18 eV

The Usual Suspects The Sun and other stars account for the very lowest energy CRs. Galactic Supernova Remnants work well for E < They would result in elemental signatures in CRs. Since Galactic SN are irregular both in time and space, need a mechanism to smooth out the CR spectrum…

Giant Galactic Halo The Milky Way has a 100 kpc halo of ionized gas around it. There is a shock front where this Galactic wind hits the inter-galactic medium. CRs produced in the Galaxy could bounce off this shock front, becoming more homogenous in space and time.

Fermi’s Supernova Pump Cosmic Rays gain energy by “bouncing” off magnetic fields they encounter in space. This works great for lower energy Cosmic Rays but not so well for UHECRs.

Extragalactic CR Sources? Hypernovae (really big star blowing itself up) Pulsars (rapidly spinning neutron star spewing stuff in our direction) Active Galactic Nuclei (stuff falling into a supermassive black hole) Topological defects in space time (remnants from the era of GUT)

GZK Cutoff Since UHECRs are travelling so fast, CMB photons are blue-shifted to energies of 130-odd MeV, the rest-mass energy of pions. In the rest frame of the cosmic ray, the photon is converted into a pion. In the laboratory frame, however, the rest-mass energy of the pion comes from the cosmic ray itself. Cosmic rays consisting of heavy nuclei move less slowly (for a given E) and hence are less sucseptible to this effect.

Interaction with IR background Same story as GZK cutoff, but this time with electrons rather than pions. Photons need only be blueshifted to 1 MeV or so. (and they are higer energy to begin with) Even the slower moving Fe nuclei can do this.

Bottom line: Nobody can make it! A proton of energy 2x10 20 eV lasts about sec before interacting with the CMB, which corresponds to a distance of ~10 Mpc. An Fe nucleus with the same energy lasts even less long before photodisintingration… A gamma-ray of this energy tends to produce electron-positron pairs and so doesn’t last sec, either. What is there within 10 Mpc of us?

Nothing!!!

Single Source? The CR spectrum exhibits structure, which seems hard to explain with sources dispersed at various redshifts. Maybe a single nearby source is responsible for all of the UHECRs? If that were the case, we should see anisotropy in CRs, which we don’t. Maybe CRs are being deflected to give the appearance of isotropy?

Larmor Radius Charged particles get deflected by B-fields Galactic B-fields are too weak to bend UHECRs very much. We should be able to trace them back to their source. We have yet to find an astonomical object associated with UHECRs.

Pierre Auger ( ) First person to observe an extended air shower. “In 1953, [he] declared that from that date on, particle accelerators took the place of cosmic rays, which more or less faded into the background. And yet, even today accelerators have not caught up with cosmic rays.”

Extended Air Showers Cosmic Rays hitting the Earth’s atmosphere lose kinetic energy, resulting in the creation of all sorts of exotic particles and gamma rays. By counting up high energy particles that have fallen on a CR’s footprint, we can estimate its energy.

Pierre Auger Observatory This observatory consists of two hybrid detectors, one being built in the Pampas in Argentina and one to be built in southeastern Colorado. The southern array is made of 1,600 water tanks plus a bunch of UV detectors.

Cherenkov Detectors 3000 gallons of water The speed of light is less in water than in air, so speeding particles traveling close to the speed of light in air find themselves travelling faster than the speed of light in water, resulting in the emission of photons to slow them down. Photomultiplier tubes then do their thing.

Fluorescence Detectors CRs also interact with nitrogen in the atmosphere, resulting in fluorescence. This UV light can be seen on dark, moonless nights. Using an array of fluorescence detectors, one can determine the direction and energy of the shower.

Auger: Preliminary Results Cherenkov detectors give large quantities of showers, while the occasional fluorescence detection gives high quality data. Even though the observatory is only 60% built and has only been operational for 1 year, it has already matched the best UHECR data from previous experiments. Up to now, the data is consistent with previous observations.

Summary: Why Auger is a big deal We’re not sure how particles could be accelerated to eV in the first place. Even we did know how to make UHECRs, we don’t know how to get them here unmolested. But we’ve detected them! Is there a mechanism right here in the MW which could produce UHECRs? Is there some wrinkle in Special Relativity which allows UHECRs to reach us from more distance sources? As the results from Auger arrive, we may be able to answer these questions.

References Yamamoto et al Duffy & Blundell 2005 Erlykin & Wolfendale 2005 Stecker 1998 Cronin, Gaisser & Swordy