Lowering IceCube’s energy threshold for point source searches in the southern sky

Observation of a point sourceof astrophysicalneutrinos would be a "smoking gun" signature of a cosmic-ray accelerator. While IceCube has recently discovered a diffuse flux of astrophysicalneutrinos, no localized point sourcehas been observed. Previous IceCube searches for point sourcesin the southern skywere restricted by either an energy threshold above a few hundred TeV or poor neutrino angular resolution. Here we present a search for southern sky point sourceswith greatly improved sensitivities to neutrinos with energies below 100 TeV. By selecting charged-currentν μ interacting inside the detector, we reduce the atmospheric background while retaining efficiency for astrophysicalneutrino-induced events reconstructedwith sub-degree angular resolution. The new event sample covers three years of detector data and leads to a factor of 10 improvement in sensitivity to point sourcesemitting below 100 TeV in the southern sky. No statistically significant evidence of point sourceswas found, and upper limits are set on neutrino emission from individual sources. A posteriori analysis of the highest-energy (∼100 TeV) starting event in the sample found that this event alone represents a 2.8σ deviation from the hypothesis that the data consists only of atmospheric background.