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Fits of the HiRes Spectra to Astrophysical Models

Fits of the HiRes Spectra to Astrophysical Models. Douglas Bergman Rutgers University ICRC 2003, Tsukuba, Japan 6 August 2003. HiRes Mono Spectra. Ankle is evident Hint of GZK Want to find the significance of these features by fitting. Continuation above GZK?.

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Fits of the HiRes Spectra to Astrophysical Models

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  1. Fits of the HiRes Spectra to Astrophysical Models Douglas Bergman Rutgers University ICRC 2003, Tsukuba, Japan 6 August 2003

  2. HiRes Mono Spectra • Ankle is evident • Hint of GZK • Want to find the significance of these features by fitting

  3. Continuation above GZK? • Does the spectrum continue at the same spectral index above the GZK? • Expect 29.8 see 11 • Poisson probability of 7.3 x 10-5

  4. Fit to Event Distribution • All fits done using binned maximum likelihood method on numbers of events

  5. Two Component Model • Use latest HiRes composition measurements to constrain new fits • Assume the iron (heavy) component comes from galactic sources • Proton (light) component comes from extragalactic sources • 33% protons at 1017 eV • 80% protons at 1018 eV • 100% protons at 1020 eV

  6. Energy Loss in XG UHECR’s • Model the GZK effect in XG protons using the formalism of Berezinsky, Gazizov and Grigorieva (hep-ph/0904357) • Pion production causes high energy protons to loose energy quickly • Lower energy protons still loose energy due to electron pair production

  7. Uniform Source Model • Integrate over uniform source density out to z = 4 • Can allow density of sources to evolve as (1+z)m • m = 0, c2 = 78.6/39 • m = 3, c2 = 64.7/39 • Source density is not static

  8. Galactic Survey Model • There is a local over-density of galaxies, might be true of UHECR sources too • Use actual distribution of galaxies • This figure taken from Blanton et al. astro-ph/0009466

  9. Galactic Survey Model • Integrate over observed galactic density out to z = 4 • c2 improves by 2-6 from introduction of galactic source density • m = 0, c2 = 72.8/39 • m = 3, c2 = 63.0/39

  10. Best Evolution Parameter • Allow m to vary between 0 and 4 • Best fit for m = 2.5 with an uncertainty of 0.5 • Stellar Formation Rate evolves with m = 3

  11. HiRes Spectra: Galactic Survey Fit • Best fit for m = 2.5 c2 = 60.4/39 • Some of c2 comes from difference of energy scales between HiRes-I and HiRes-II

  12. HiRes Mono & Fly’s Eye Stereo • Add Fly’s Eye Stereo measurement into fit • Energy scaled down by 7% • Helps constrain fit a lower energies • c2 = 87.7/63 with m = 2.4 • Softening of spectrum between second knee and ankle caused by e+e- pair production turning on

  13. Conclusion • The HiRes spectra are not consistent with a continuation above the GZK at the same spectral slope seen above the “ankle” • The observed fluorescence spectra are consistent with a two component source model with protons from extragalactic sources subject to the GZK mechanism • Fits to the fluorescence spectra favor source densities which evolve with z in a similar way as the Stellar Formation Rate • 2nd Knee and Ankle caused by e+e- pair production

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