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What can we conclude from UHECR observations?

What can we conclude from UHECR observations?. Glennys R. Farrar Center for Cosmology and Particle Physics New York University. Key (secure) Ingredients. Spectrum Composition Clusters of UHE events Correlations with Candidate Sources.

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What can we conclude from UHECR observations?

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  1. What can we conclude from UHECR observations? Glennys R. Farrar Center for Cosmology and Particle Physics New York University

  2. Key (secure) Ingredients • Spectrum • Composition • Clusters of UHE events • Correlations with Candidate Sources > What can be inferred alread > Near-term expected improvements G.Farrar, ISMD

  3. Spectrum • Absolute energy scale still uncertain • Air Fluorescence-Surface discrepancy ~ 30%? • Awaiting definitive air fluorescence yields (Sept 07?) • Air shower simulations: systematic problems • Berezinsky et al: normalizing E’s by position of dip gives consistent spectra • Spectrum drops off at highest energies • No need to worry about GZK violation • But premature to claim that GZK cutoff has been confirmed (absolute E scale, max. acceleration?) G.Farrar, ISMD

  4. Energy losses produce characteristic dip in spectrumBerezinsky et al; ICRC 07 • Nucleon + CMB photon => e+e- production => energy loss • As long as only protons (< ~10% nuclei) spectrum develops a dip at a definite energy. • Use dip to normalize energy of different experiments. G.Farrar, ISMD

  5. Rescaling energies to dip=> consistency! • Left: published spectra x E3 from different expts • Right: spectra after shifting energies by factors AG = 0.9; HR = 1.2; Yk = 0.75; Auger = 1.2 • Shift => consistent normalizations of all spectra, for Auger = 1.4. (HR = 1.2 + difference in AF yield assumptions => Auger ~ 1.35) G.Farrar, ISMD

  6. Composition • Strong limit on photons (Auger ICRC07) • < 10% at 20 EeV • Excludes non-contrived “top down” models • Very confusing situation on nucleon vs. nucleus question! G.Farrar, ISMD

  7. “Elongation Rate” nowwell measured (Auger) • Xmax increases with energy. • For fixed total energy, Xmax is lower for nuclei. • Predicted Xmax vs E depends on UHE event generator. • Data suggests heavy light during galactic-extragalactic transition • Then, light  heavy at still higher energies! • This is VERY unexpected! G.Farrar, ISMD

  8. There is a problem with present simulations! • If the energy of a hybrid event is fixed according to the AF method, the predicted number of muons in the shower is too low by ~ 40%; other similar effects. • EPOS increases the mu/e ratio but gets other features wrong. • Some aspect(s) of the UHE hadron interaction are not yet understood, possibly • inclusive nuclear modeling? • Leading particle effects? (MINOS mu+/mu- is wrong) • ??? • May be responsible for energy normalization discrepancies, seemingly puzzling elongation rates. G.Farrar, ISMD

  9. Clusters of UHE events • Most UHECRs are isolated on ~ few o scale • AGASA 96 (57 events > 40 EeV): 1 triplet, 5 pairs • AGASA + HiRes 04,05 • AGASA triplet “promoted” to quadruplet with “HE” HiRes data (37 events) • … to quintuplet w all published HiRes data (+234 events) • Promotion probability 2 10-3 • Auger sees no statistically significant small scale clustering signal (ICRC07), but • Toward galactic center • Much more extragalactic structure More severe magnetic deflection? G.Farrar, ISMD

  10. Extragalactic Magnetic Fieldsmay be low in some directions, large in othersDolag et alastro-ph/0310902, 0410419; Sigl et al astro-ph/0302388,0401084 Deflection map of CR’s above 4 1019 eV, within 100 Mpc. EGMF deflections may be small except in isolated directions G.Farrar, ISMD

  11. Possible Acceleration Mechanisms • Cataclysmic event, e.g., gamma ray burst (GRB), massive star collapse to black hole, magnetar birth • Jets in AGNs (accreting supermassive BH), e.g., BL Lacs, powerful radio galaxies, … • Gradual acceleration in large scale magnetic shocks taking millions of years • New Physics -- e.g., decay of invisible, super-heavy particle created in early moments of Big Bang (mostly excluded now by photon limits…).

  12. Inferring Source(s) from “bias” • Simulated Large Scale Structure(Millenium Run, Springel et al 2004) • Clustering of UHECR (if from galaxies) (Berlind, Farrar 2007) • Powerful because insensitive to magnetic deflection • Can only discriminate characteristic mass of source, not specific source type (e.g., GRB, AGN have same bias) 1000 UHECRs

  13. BLLac-UHECR correlationGorbunov et al, astro-ph/0406654; Abbassi et al, astro-ph/0507120; R. Jansson & GRF, ICRC07 ~130 BLLacs in HiRes domain 15 correlate with 1 UHECR (7 if random) 2 correlate with 2 UHECRs 10 excess correlations, probability < 10-4

  14. Bursting vs Continuous Source??? • Spectrum discriminates! • Bursting source • At any given time, ~ factor-2 spread in energies. • But beware… G.Farrar, ISMD

  15. GZK energy loss DISTORTS A CONTINUOUS SOURCE! Observed Spectrum of Continuous Source with GZK (dN/dEsrc ~ Esrc-p) Green: no GZK losses Red: with GZK losses G.Farrar, ISMD

  16. For Each Cluster: • Estimate source distance from energies (use GZK) • Spectrum of individual source: • Include GZK distortions • Compare observed energies with prediction of continuous and bursting sources. • Fit angular distribution: • Different for bursting and continuous cases • Can infer for each cluster: • Flux (if continuous) or total energy of burst • < B2 x magnetic coherence length> • If bursting, delay time since burst G.Farrar, ISMD

  17. The Ursa Major UHECR Cluster5 events from AGASA & HiRes + Galaxies from SDSS R. Abassi et al (GRF and HiRes), ApJ 623, 264 (2005); GRF astro-ph/0501388,GRF, A. Berlind, D. Hogg astro-ph/0507657, ApJ 642, L89(2005) G.Farrar, ISMD

  18. Probability Distribution of Source Distancefor Ursa Major Cluster • Figure: • Assuming mean GZK energy loss • Solid: unrenormalized energies, • Dashed: BGG renormalized energies • Source distance ~100-150 Mpc well compatible with matter distribution as determined from SDSS (GRF, Berlind, Hogg 05) G.Farrar, ISMD

  19. UHECR multiplets • Cluster candidate in published AGASA-HiRes data: • 5 events, chance probability 2 10-3 • Energies (15), 38, 53, 55, 78 EeV (1 EeV = 1018 eV) • (b, l) =(55o,145o)(“Ursa Major Cluster”) • Projecting from AGASA-HiRes suggests with a dataset like Auger • 1 multiplet with 8-10 UHECRs  1 multiplet with ~ 6 UHECRs • 5 multiplets with ~ 4 UHECRs • Need good analysis tools to avoid using false multiplets. Maximum Likelihood method has been developed (GRF) • If(when!)such multiplets are found, they will give a powerful constraint on GMF and sources. G.Farrar, ISMD

  20. Conclusions • UHECRs are an astrophysical phenomenon. • Large statistics => many powerful tools can be used. • Near-term: Infer sources from “bias”, correlations, and cluster properties • Medium term: • UHE/nuclear modeling, accurate event energies, > statistics • Composition, GZK • Longer term: • Astrophysics, UHE particle physics • UHECRS will soon become an astrophysical & UHE particle physics TOOL!

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