Are blazars above the blazar sequence a significant source of IceCube neutrinos?

Abstract

ABSTRACT The recent association of two flaring blazars (TXS 0506+056 and 3HSP J095507.9+355101) with IceCube neutrinos strongly suggests that blazars are at least one source of high-energy neutrinos. A peculiar characteristic of these two blazars is that they both appear to lie above the well-known blazar sequence, and they move further above the blazar sequence during the neutrino emission. Although the reason for this peculiarity is unclear, it may indicate a link between blazar SED properties and neutrino emission. Here, we test whether blazars lying above the blazar sequence are overrepresented in IceCube neutrino error regions. We spatially cross-match IceCube error regions with a sample of γ-ray blazars from the Fermi-LAT 4LAC catalogue, as a function of the blazars’ distance in synchrotron peak frequency above the blazar sequence. We do not find a statistically significant excess of counterparts, and thus conclude that the current data cannot confirm that the sources above the blazar sequence are a significant source of IceCube neutrinos. However, we show that the inclusion of more recently detected IceCube events increases the likelihood of an excess of blazars above the blazar sequence, and suggest that this excess may reach statistical significance in the future as the sample of IceCube neutrino detections increases.