Connecting blazars with ultra high energy cosmic rays and astrophysical neutrinos
We present a strong hint of a connection between high energy $\gamma$-ray emitting blazars, very high energy neutrinos, and ultra high energy cosmic rays. We first identify potential hadronic sources by filtering $\gamma$-ray emitters %from existing catalogs that are in spatial coincidence with the high energy neutrinos detected by IceCube. The neutrino filtered $\gamma$-ray emitters are then correlated with the ultra high energy cosmic rays from the Pierre Auger Observatory and the Telescope Array by scanning in $\gamma$-ray flux ($F_{\gamma}$) and angular separation ($\theta$) between sources and cosmic rays. A maximal excess of 80 cosmic rays (42.5 expected) is found at $\theta\leq10^{\circ}$ from the neutrino filtered $\gamma$-ray emitters selected from the second hard {\it Fermi}-LAT catalogue (2FHL) and for $F_\gamma\left(>50\:\mathrm{GeV}\right)\geq1.8\times10^{-11}\:\mathrm{ph}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$. The probability for this to happen is $2.4 \times 10^{-5}$, which translates to $\sim 2.4 \times 10^{-3}$ after compensation for all the considered trials. No excess of cosmic rays is instead observed for the complement sample of $\gamma$-ray emitters (i.e. not in spatial connection with IceCube neutrinos). A likelihood ratio test comparing the connection between the neutrino filtered and the complement source samples with the cosmic rays favours a connection between neutrino filtered emitters and cosmic rays with a probability of $\sim1.8\times10^{-3}$ ($2.9\sigma)$ after compensation for all the considered trials. The neutrino filtered $\gamma$-ray sources that make up the cosmic rays excess are blazars of the high synchrotron peak type. More statistics is needed to further investigate these sources as candidate cosmic ray and neutrino emitters.