Extending the Search for Neutrino Point Sources with IceCube above the Horizon

Point source searches with the IceCube neutrino telescope have been restricted to one hemisphere, due to the exclusive selection of upward going events as a way of rejecting the atmospheric muon background. We show that the region above the horizon can be included by suppressing the background through energy-sensitive cuts. This improves the sensitivity above PeV energies, previously not accessible for declinations of more than a few degrees below the horizon due to the absorption of neutrinos in Earth. We present results based on data collected with 22 strings of IceCube, extending its field of view and energy reach for point source searches. No significant excess above the atmospheric background is observed in a sky scan and in tests of source candidates. Upper limits are reported, which for the first time cover point sources in the southern sky up to EeV energies.

Figure 1

${\nu }_{\mu }+{\overline{\nu }}_{\mu }$ effective areas for this analysis with 22 strings. At positive declinations, highest energies are suppressed through absorption. The line for $-50°$ to $-20°$ is lowered due to strong cuts above the horizon.

Figure 2

Sensitivity to point sources with $E^{-2}$ ${\nu }_{\mu }+{\overline{\nu }}_{\mu }$ spectra vs declination, comparing this analysis with [6] and ANTARES [13]. Differences in the energy coverage at southern declinations are discussed in the text and Fig. 3.

Figure 3

Left: Sky map of the directions of the 1877 events in equatorial coordinates. The legend indicates the negative logarithm of the $p$ value for compatibility with background in each angular search bin before trial corrections. Right: Energies for a simulated $E^{-2}$ neutrino spectrum as a function of declination. Indicated is the fraction of events per angle and energy bin at final cut level. The solid lines describe the region of 90% signal containment in each declination band.

Figure 4

Flux limits from this work for 2 sources: Centaurus A limit (blue dashed) with models CH08 [2] and KOT08 [18]; 3C279 flare search (red solid) with model Reimer09 [21].