Multi-messenger astronomy can help to investigate the sources of the
high-energy neutrinos observed by the high-energy neutrino telescope IceCube.
We consider the hypothesis that the highest energy neutrinos are produced by BL
Lacs, arguing that this is not contradicted severely by any known fact. We
check the BL Lac hypothesis by searching for correlations between the
through-going muon events of IceCube and the BL Lacs of the second catalog of
Fermi-LAT (2FHL). We expect 10.2 ± 2.4 correlated events but we find that
just 1 event has a BL Lac as counterpart. We also assess the probability of
observing one multiplet from the same source, finding that the present null
result is not yet of critical significance. We conclude that the hypothesis
that the BL Lacs are the main emitters of the highest-energy neutrinos observed
by IceCube is disfavored at 3.7{\sigma}. We discuss implications and possible
ways out; for example, this could work if the angular resolution was 4{\deg},
which is much more than expected.Comment: 10 pages, 6 figures. Accepted for publication in Astronomy &
Astrophysic
