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Small-Scale Anisotropy in the Cosmic Radiation: Results from Milagro

This study presents the results from the Milagro Gamma Ray Observatory, focusing on small-scale anisotropies in the cosmic radiation. The observatory has collected a large amount of cosmic ray events and has discovered anomalous energy spectra and regions of clumpy hit distributions, indicating the presence of non-gamma-ray sources. The findings suggest the need for further investigation into the nature of these effects and their possible explanations.

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Small-Scale Anisotropy in the Cosmic Radiation: Results from Milagro

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  1. Small-Scale Anisotropy in the Cosmic Radiation: Results from Milagro John Pretz Los Alamos National Laboratory

  2. Milagro Gamma Ray Observatory A. Abdo, B. Allen, D. Berley, T. DeYoung,B.L. Dingus, R.W. Ellsworth, M.M. Gonzalez, J.A. Goodman, C.M. Hoffman,P. Huntemeyer, B. Kolterman, C.P. Lansdell, J.T. Linnemann, J.E. McEnery, A.I. Mincer, P. Nemethy, J. Pretz, J.M. Ryan, P.M. Saz Parkinson, A. Shoup, G. Sinnis, A.J. Smith, G.W. Sullivan, D.A. Williams, V. Vasileiou, G.B. Yodh

  3. e m g 8 meters 50 meters 80 meters Milagro • 2600m asl • 898 detectors • 450(t)/273(b) in pond • 175 water tanks • 2-35 TeV median energy • 1700 Hz trigger rate • ~220 billion events • Angular resolution between 1.2 and 0.4 degrees by fitting the shower plane.

  4. Proton MC Proton MC g MC g MC Data Data Background Rejection in Milagro • Hadronic showers contain penetrating component: ’s & hadrons • Cosmic-ray showers lead to clumpier bottom layer hit distributions • Gamma-ray showers give smooth hit distributions

  5. Milagro as a Gamma Ray Observatory • Six high-significance sources in the Galactic Plane (ApJL 700. L127-L131. ApJL 664. L91-L94) • Eight additional marginal-significance associations with bright Fermi sources (ApJL 700. L127-L131) • Diffuse emission in the Galactic Plane at ~20 TeV in excess of GALPROP predictions. Particularly high in the Cygnus region. (ApJ 688. 1078-1083. ApJL 658. L33-L36) • Measurement of the energy spectrum from the Crab (arXiv:1110.0409) Milagro map of the Galactic Plane Energy spectrum of the Crab Nebula Diffuse Emission from Cygnus Region

  6. Background Rejection (Cont’d) Background Rejection Parameter Gamma MC Data mxPE: maximum # PEs in bottom layer PMT Nb2: Number of PMTs in bottom with >2 PEs Proton MC The compactness distribution for gamma rays is dependent upon the energy spectrum of the source. Softer spectra have more proton-like C distributions

  7. Background Estimation and Subtraction • Assume that the cosmic ray arrival directions can be separated into a spatial piece and a temporal piece. • For one integration duration (2 hours), measure all-time spatial distribution and all-sky time variation. • Convolve to get the background estimate. • A high-pass spatial frequency filter arXiv:1110.0409

  8. Intermediate-Scale Anisotropy Fractional Excess x103 B A

  9. Not a Systematic Effect • Background (black line) developed using only data for which detector was live for 24 hours without interruption. • Is the same as the previously reported large-scale anisotropy (using different method). • Red line shows data in declination band 10o-20o. Not a Systematic in bkgrnd est. From large-scale anisotropy

  10. Energy Spectrum of Excess • Region A has a hard spectrum relative to the CR background • ~E-1.45±0.45 with cutoff between 4 and 20 TeV.

  11. The Excess is Due to Cosmic Rays Region A Proton hypothesis • Compactness distribution consistent with soft-spectrum gammas or protons • ln(fout) distribution inconsistent with soft spectrum source • Fit excess to dN/dE = Eg e-(E/Ec) • Region A gamma hypothesis c2=124/16 proton c2=10.3/16 • Region B gamma hypothesis c2=84.8/16 proton c2=19/16

  12. High Altitude Water Cherenkov Detector • Re-deploy Milagro PMTs and front-end electronics at a high-altitude site near Sierra Negra, Mexico • Large (7.3m x 4.5m) close-packed water tanks. • Higher altitude and larger triggering area gives ~10x increase in area at 100 GeV • Superior gamma / hadron separation. • Comparable effective area to Milagro at 10 TeV • Potential improvements to cosmic ray anisotropy measurement. • Extend to lower energies. • Better measure the energy spectrum. • Measure the composition? • Operations begin 2013 / Completed detector 2014

  13. Conclusions • Milagro has collected over 200 billion cosmic-ray events. • Search for very small anisotropies with high statistical power • Discovered ~10o anisotropies in CR arrival directions • Not due to gamma rays • One of the regions has an anomalous energy spectrum • The explanation of these affects is still unclear. Hard to explain with typical cosmic ray diffusion. • Larmor radius of 10 TeV proton in 1 mG is 0.01 pc • Decay length of 10 TeV neutron is 0.1 pc • Could a nearby source be responsible along with non-standard diffusion?

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