The large-scale cosmic-ray anisotropy as observed with milagro

A. A. Abdo, B. T. Allen, T. Aune, D. Berley, S. Casanova, C. Chen, B. L. Dingus, R. W. Ellsworth, L. Fleysher, R. Fleysher, M. M. Gonzalez, J. A. Goodman, C. M. Hoffman, B. Hopper, P. H. Hüntemeyer, B. E. Kolterman, C. P. Lansdell, J. T. Linnemann, J. E. McEnery, A. I. MincerP. Nemethy, D. Noyes, J. Pretz, J. M. Ryan, P. M. Saz Parkinson, A. Shoup, G. Sinnis, A. J. Smith, G. W. Sullivan, V. Vasileiou, G. P. Walker, D. A. Williams, G. B. Yodh

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172 Scopus citations


Results are presented of a harmonic analysis of the large-scale cosmic-ray (CR) anisotropy as observed by the Milagro observatory. We show a two-dimensional display of the sidereal anisotropy projections in right ascension (R.A.) generated by the fitting of three harmonics to 18 separate declination bands. The Milagro observatory is a water Cherenkov detector located in the Jemez mountains near Los Alamos, New Mexico. With a high duty cycle and large field of view, Milagro is an excellent instrument for measuring this anisotropy with high sensitivity at TeV energies. The analysis is conducted using a seven-year data sample consisting of more than 95 billion events, the largest such data set in existence. We observe an anisotropy with a magnitude around 0.1% for CRs with a median energy of 6 TeV. The dominant feature is a deficit region of depth (2.49 ± 0.02 stat. ± 0.09 sys.) ×10-3 in the direction of the Galactic north pole centered at 189 deg R.A. We observe a steady increase in the magnitude of the signal over seven years.

Original languageEnglish (US)
Pages (from-to)2121-2130
Number of pages10
JournalAstrophysical Journal
Issue number2
StatePublished - Jun 2009

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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