Searches for signals from cosmic point-like sources of high energy neutrinos in 5 years of AMANDA-II data

by

AMANDA-II is a neutrino telescope located in the glacial ice at the South Pole. It is optimized to detect neutrino induced muon tracks with energies larger than 100 GeV by their ?erenkov light emission. We analyzed the data collected in 1001 effective days of detector operation between the years 2000 and 2004 for a signal from point-like sources of neutrinos. Such a signal is expected from cosmic objects that accelerate hadrons to very high energies, which subsequently interact with ambient protons or photons. The dominant event class recorded in AMANDA-II are muons produced in the interactions of cosmic rays in the atmosphere. Due to their energy loss, the muons cannot penetrate the Earth and have down-going directions. The main signature to identify a neutrino induced event is therefore its up-going direction. A sample of 4282 up-going events is extracted from the 10 billion events triggered in the period selected for this analysis. The search for point sources is accomplished on this data sample by looking for a localized excess over the isotropic background of atmospheric neutrinos. The procedure is applied for the directions of candidate sources, like Active Galactic Nuclei, Supernova remnants and X-ray binaries. In a second step, the full northern sky is scanned for unknown sources. Further, we investigate methods to enhance the detection chance for sources which are suspected to be highly variable neutrino emitters. We search for an excess of neutrino events in periods of high activity of a source, defined by the intensity of its electromagnetic emission in certain frequency bands. An additional test based on a sliding time window is applied to the dataset to find flares of neutrinos, which would not be visible in the time-integrated search. Neither a localized excess nor a neutrino flare has been found in the analyzed dataset. Therefore, we calculate upper limits on the neutrino fluxes which are compatible with this observation. The average upper limit achieved for a combined ?µ+?? flux with a spectrum of d?/dE ? E?2 is E2 d?/dE = 1.0 · 10?7 GeV cm?2 s?1 assuming a flavor ratio of 1 : 1. It represents the most stringent upper limit on neutrino fluxes from point-like sources reported so far.