Ultra-High Energy Cosmic Ray Research with the Pierre Auger Observatory Methods, Results, What We Learn, and expansion to Colorado Bill Robinson
Mysteries of Ultra-High Energy Cosmic Rays • What are they made of through the range of energies? • What accelerates them? • How energetic can they get? • Why do we detect UHECRs that are too energetic to be allowed by current theory? • Where do they come from? And why do different detectors get different results?
(S. Swordy, AUGER design report) power law E-2.8 Above knee: supernova remnant shocks Below knee; where from? Upper limit; GZK cutoff at ankle, most energetic yet 3 E 20 eV or 300 EeV [1 EeV = 10 E18, Exaelectron Volt]
Simulated Longitudinal development of 50 1 EeV Cascades M. Ambrosio et. al., Astroparticle Physics 24, 355-371 (2005).
Interaction Depth of shower maximum (PAO ~ 850 g/cm^2; sea level = 1000 = interstellar space 10^8 LY)
1 EeV Anisotropy from Akeno Ratio of # of observed events to expected ones in equatorial coordinate. Solid line = Galactic Plane, G.C. is galactic center. Amplitude: ~ 4%
Galactic region of excess (Akeno) Ion gyroradius [cgs]; m = mass in proton units Z = ionization E = energy in eV Galactic B = 3 microgauss R for 1 EeV proton = ~300 pc
Anisotropies in HE Arrival Direction From the Japanese Akeno observatory, above 40 EeV, 1990—2002 But there’s a lot of controversy about this…..
Problems • Shaded circles show clustering within 2.5 degrees; chance probability of 0.9% for just one triple event • BUT this is not corroborated by other observatories; when they show anisotropies they are in other directions • No sources in arrival directions away from galaxy* • Neutrons of 10 EeV have gamma ~ 10 E9, so a range of about 10 kpc; galactic center within range. Immune to field deflections. No anisotropies below 10 E17.9 • Impossible to detect directly if neutrons are primaries • Only Akeno shows galactic center and Cygnus clustering • Extremely low flux and contradictory results are good arguments for more observatories using multiple detection methods in different regions • Have to separate gamma ray showers from nuclei
PAO fails to find excess… 10 E17.9 < E < 10 E 18.5, 5 degree windows, GC at cross, line is galactic plane; 2.3 years of Auger data, no abnormally over-dense regions, cannot resolve probable source at GC (must use gamma rays to investigate)
GZK Cutoff? (2003, pre-PAO) arXiv:hep-ph/o206217 v5 27 Feb 2003, “Has the GZK suppression been discovered?”
UHECR Energy loss and calibration; Energy loss in the shower; with Nem = particle density along shower direction x, Nph = photons reaching fluorescence detector, R(x) = distance from shower point x and FD, T(x) = atmospheric transmission (<1) Energy loss in the shower; Energy loss in the shower; Night atmosphere assumed horizontally Invariant; only steered through zenith angle, probes to 30 km tracing losses to molecular and aerosol scattering. Requires clear moonless nights; 14% duty cycle. Calibrationwith Lidar
Auger fluorescence telescope Diameter: 2.2 meters Aperture: 3.8 sq. m. 440 photomultiplier tubes Schematic shows positions of diffusers for optical calibrations
Central Laser Facility (PAO South) • UV laser (355 nm) fires 7 ns, 7 mJ pulse every 15 minutes to calibrate FDs; scattered luminosity approx. same as strong shower • Located equidistant from 3 of the 4 FD eyes • Polarization randomized (better than circular) • Can fire in any direction • Weather checked by instruments every 5 minutes
PAO proposed initial Colorado site (Middle of Nowhere)
Conclusion • More detectors needed! • Colorado site ideal; funding on order of $100 million; room for huge expansion • Primary questions remain unresolved • Existing PAO works both physically and politically as a model of international cooperation
References Websites: History of the Air Fluorescence Technique, www.cosmic-ray.org/reading/fluor.html Pierre Auger Observatory; www.auger.org and www.augernorth.org AGASA; www-akeno.icrr.u-tokyo.ac.jp/AGASA/ Interaction Depth www.lbl.gov/abc/cosmic/SKliewer/Cosmic_Rays/Interaction.htm Journals: M. Ambrosio et. al., Astroparticle Physics 24, 355-371 (2005). Pierre Auger Collaboration, arXiv:astro-ph 3, 0607382 (2006). N. Hayashitda et al., Astroparticle Physics 10, 310 (1999). J. Bahcall and Eli Waxman, arXiv:hep-ph 5, 0206217 (27 Feb 2003). A. Filipcic et. al., Astroparticle Physics 18, 502 (2003). Optical Relative Calibration for Auger Fluorescence Detector, and Performance of the PAO Surface Array, Pune (2005) 00, 101-106 The Central Laser Facility at the PAO, Subm. To Nucl. Inst. Meth. ‘06