A CLUSTER OF ULTRAHIGH ENERGY COSMIC RAYS Glennys R. Farrar Center for Cosmology and Particle Physics New York University Research supported by NSF, NASA and NYU

1/26/05G. F. Farrar, STSci Colloquium2 Results A pointlike cluster of 5 ultrahigh energy cosmic rays has been found, with energies eV –Data from 2 independent teams & techniques. –Analysis strongly suggests this is from a single source. –First source of UHECRs has probably been isolated. Vacant foreground explains absence of smearing Source candidates: –Merging rich cluster –Obscured AGN? –GRB?

1/26/05G. F. Farrar, STSci Colloquium3 Cosmic Rays Mostly composed of protons and nuclei. Rapidly falling energy spectrum.

1/26/05G. F. Farrar, STSci Colloquium4 Ultrahigh Energy (UHE) proton Energies above eV (= 10 Ecta-electron-Volts) 200 EeV Kinetic Energy of a golfball just off the tee, in a single elementary particle! Speed = c Relativistic time dilation: neutron lives ~ million years, instead of 15 minutes. Bending by magnetic fields decreases with energy => potential for UHECR astronomy. Very rare: ~ 1 per square kilometer, per century

1/26/05G. F. Farrar, STSci Colloquium5 Theories of acceleration How does nature do a trillion times better job than best human technology (Large Hadron Collider)? Cataclysmic event like gamma ray burst (GRB) -- collapse of massive star to black hole (?) Long, gradual acceleration in large scale magnetic shocks taking millions of years Jets in AGNs (accreting supermassive BH) Decay of invisible, super-heavy particle created in early moments of Big Bang

Example: AGN jet looks good

1/26/05G. F. Farrar, STSci Colloquium7 How are UHECRs detected? UHECR collides with air nucleus, creating ~ 1000 secondary particles, these collide …. => huge shower (beam) of charged particles Air fluoresces -- use telescope to see streak of light (High Resolution Fly’s Eye in Utah) Shower hits ground over many square kilometers -- see synchronous arrival (Akeno Giant Air Shower Array in Japan)

AGASA AUGER 2005

1/26/05G. F. Farrar, STSci Colloquium9 AGASA Photos

1/26/05G. F. Farrar, STSci Colloquium10 State of knowledge Data suggests UHECRs are protons –Nuclei or photons can’t be completely ruled out ~100 UHECR events above ~ 40 EeV detected in last decade Apparently no preferred arrival direction -- isotropic Uncertain whether predicted “Greisen Zatsepin Kuzmin” cutoff is violated or not (above ~ 100 EeV, scattering from relic Big Bang photons should drain away energy)

Energy Spectrum by AGASA (θ<45)

1/26/05G. F. Farrar, STSci Colloquium12 57 AGASA events above 40 EeV

1/26/05G. F. Farrar, STSci Colloquium HiRes Events above 10 EeV ApJ 610 (2004) L73, Abbasi et al

1/26/05G. F. Farrar, STSci Colloquium14 A cluster of UHE Cosmic Rays, consistent with a single source HiRes Collaboration + GRF astro-ph/ , Ap J in press –combined HiRes, AGASA data > 30, 40 EeV –4 events consistent with pointlike source –Simulation: 0.5% chance for some kind of strong clustering signal –Not a priori dataset, so rigorous chance probability cannot be given G. R. Farrar, astro-ph/ –More refined probability analysis on quad Chance probability to get quartet somewhere ~ 0.1 % Chance probability AGASA doublet -> quad = (7±1) 10^-4 –Include all published data > 10 EeV more events –Now 5 events in cluster (~ 10% probability by chance) –Magnetic field analysis constrains fields along trajectory –Quint also consistent with single point source

1/26/05G. F. Farrar, STSci Colloquium15 Big Picture: magnetic fields are crucial to interpreting UHECRs Hypothesis: magnetic field strengths are very inhomogeneous: strong in rich clusters (measured to be ~ micro Gauss), weak in voids. Most UHECR sources are obliterated due to strong magnetic deflections en route to detector. In a few lucky cases there may be only regions of weak fields in foreground of source. Present work: –Analyze the effect of large number of small deflections on apparent size of image (magnetic smearing) and compare to observations to infer properties of field and distance to source (GRF, astro-ph ) –Check density distribution in direction of source to see if picture above is consistent (A. Berlind, D. Hogg, GRF in preparation

1/26/05G. F. Farrar, STSci Colloquium16 Maximum Likelihood Analysis of Clustering including Magnetic Dispersion 4 highest energy events alone => E* = 29 o EeV 5 events (taking it to have average energy for its range, 15 EeV) => E* = 22.5 o EeV CONSISTENT! Therefore conclude there are likely to be FIVE EVENTS IN CLUSTER N.b., chance probability of quad becoming quint by chance is ~ 10%. Best constrained fit to galactic magnetic field has no GMF deflection; consistent w known constraints from observations, but not standard model of GMF

1/26/05G. F. Farrar, STSci Colloquium17 Implications Constraint on average extragalactic magnetic field strength, coherence length, and distance of source: 90 % cl limit on E* implies cosmic ray primaries are charged. First unambiguous evidence for multiple events from a single source excludes decaying superheavy particle as source.

1/26/05G. F. Farrar, STSci Colloquium18 What could the source be? Sloan Digital Sky Survey ( A. Berlind, D. Hogg + GRF) –Direction to UHECR source is almost empty out to 200 Mpc –Then: rich clusters of galaxies merging! –This may contain the source: Acceleration by intense shocks? Obscured AGN? No other especially remarkable objects identified in the field so far GRB still an open possibility

1/26/05G. F. Farrar, STSci Colloquium19 from SDSS: density in the source direction is exceptionally low out to 140 h -1 Mpc G. R. Farrar, A. A. Berlind, D. W. Hogg, forthcoming

1/26/05G. F. Farrar, STSci Colloquium20 Integrated Luminosity Density to SDSS Clusters GRF, A. A. Berlind, D. W. Hogg, forthcoming

1/26/05G. F. Farrar, STSci Colloquium21 Clusters of Galaxies in Direction of Source

1/26/05G. F. Farrar, STSci Colloquium22 Summary Evidence for a cluster of 5 UHECRs in the combined HiRes, AGASA data above 10, 40 EeV (entire published data). Chance probability to “promote” original AGASA pair ~ Definitive confirmation awaits further data. Source location constrained (SDSS) –Merging rich cluster? –Obscured AGN? GRB? Extragalactic magnetic fields probably have dramatic structures (e.g., walls and voids) like matter does. Rich opportunities ahead for UHECR astrophysics (HiRes continuing, Pierre Auger Observatory coming on-line)