A CLUSTER OFULTRAHIGH ENERGY

COSMIC RAYS

Glennys R. FarrarCenter for Cosmology and Particle Physics

New York University

Research supported by NSF, NASA and NYU

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Results

• A pointlike cluster of 5 ultrahigh energy cosmic rays has been found, with energies 1019-20 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?

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Cosmic Rays

• Mostly composed of protons and nuclei.

• Rapidly falling energy spectrum.

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Ultrahigh Energy (UHE) proton

• Energies above 1019 eV (= 10 Ecta-electron-Volts)• 200 EeV <=> Kinetic Energy of a golfball just off

the tee, in a single elementary particle!• Speed = 0.9999999999999999999999 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

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Theories of accelerationHow 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

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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

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AGASA Photos

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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)

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57 AGASA events above 40 EeV

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271 HiRes Events above 10 EeVApJ 610 (2004) L73, Abbasi et al

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A cluster of UHE Cosmic Rays, consistent with a single source

• HiRes Collaboration + GRF astro-ph/0412617, 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/0501388– 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 -- 214 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

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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-ph0501388)

– Check density distribution in direction of source to see if picture above is consistent (A. Berlind, D. Hogg, GRF in preparation

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Maximum Likelihood Analysis of Clustering including Magnetic Dispersion

• 4 highest energy events alone => E* = 29oEeV

• 5 events (taking it to have average energy for its range, 15 EeV) => E* = 22.5oEeV

• 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

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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.

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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

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fromSDSS: densityin the sourcedirection is exceptionally low out to 140 h-1 Mpc

G. R. Farrar, A. A. Berlind, D. W. Hogg, forthcoming

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Integrated Luminosity Density to SDSS Clusters

GRF, A. A. Berlind,D. W. Hogg, forthcoming

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Clusters of Galaxies in Direction of Source

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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 ~ 10-4

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)