Neutrino signal from extended Galactic sources in IceCube

Context. The Galactic plane is the brightest source of gamma rays in the sky. It should also be one of the brightest very-high-energy neutrino sources, if a neutrino flux comparable to the gamma-ray flux is produced by the cosmic ray interactions in the interstellar medium. Aims. We explore the detectability of the neutrino flux from the entire Galactic plane or from a part of it with the IceCube neutrino detector. Methods. We calculated the normalization and the spectral index of the neutrino power-law spectrum from different regions of the Galactic plane, based on the observed spectral characteristics of the pion decay gamma-ray diffuse emission observed by the Fermi/LAT telescope in the energy band above 100 GeV. We compared the neutrino flux calculated in this way with the sensitivity of IceCube for the detection of extended sources. Results. Assuming a binned extended source analysis method, we find that the only possible evidence of neutrino emission for sources located in the northern hemisphere after 20 years of exposure is from the Cygnus region. For other parts of the Galactic plane even a 20 year exposure with IceCube is not sufficient for the detection. Taking into account marginal significance of the detectable source in the Cygnus region, we find a precise position and size of the source region that optimizes the signal-to-noise ratio for neutrinos. We also calculated the low-energy threshold above which the neutrino signal can be detected with the highest signal-to-noise ratio. This calculation of precise source position, size, and energy range, based on the gamma-ray data, can be used to remove the so-called trial factor in the analysis of the real neutrino data of IceCube. We notice that the diffuse neutrino emission from the inner Galactic plane in the southern hemisphere is much brighter. A neutrino detector with characteristics equivalent to IceCube, but placed at the northern hemisphere (such as KM3NeT), would detect several isolated neutrino sources in the Galactic plane with just a five-year exposure at the 5 sigma level. Their discovery would identify the cosmic ray sources in the Galaxy that have been active during the last 10(4) years. Conclusions. The detection of the diffuse neutrino emission from cosmic ray interactions in the Galactic disk is challenging, but marginally feasible with a 20 year exposure by IceCube. Dramatically shorter (2-5 yr) exposure times would be required for the Galactic plane signal detection with an IceCube-like neutrino detector in the northern hemisphere.