Cosmic neutrinos from temporarily gamma-suppressed blazars

TL;DR

This paper proposes that gamma-ray suppression during blazar activity can enhance neutrino production detection, explaining past neutrino-blazar associations and suggesting improved search strategies for astrophysical neutrino sources.

Contribution

It introduces the concept of temporary gamma-ray suppression in blazars as a mechanism for efficient neutrino production, supported by observational evidence from multiple sources.

Findings

Gamma-ray flux was at a local minimum during neutrino detection.

Radio emission was high during neutrino events, indicating active outflows.

Similar gamma suppression observed in multiple blazars at neutrino times.

Abstract

Despite the uncovered association of a high-energy neutrino with the apparent flaring state of blazar TXS 0506+056 in 2017, the mechanisms leading to astrophysical particle acceleration and neutrino production are still uncertain. Recent studies found that when transparent to $\gamma$-rays, $\gamma$-flaring blazars do not have the opacity for protons to produce neutrinos. Here we present observational evidence for an alternative explanation, in which $\gamma$-ray emission is suppressed during efficient neutrino production. A large proton and target photon density help produce neutrinos while temporarily suppress the observable $\gamma$-emission due to a large $\gamma \gamma$ opacity. We show that the Fermi-LAT $\gamma$-flux of blazar PKS 1502+106 was at a local minimum when IceCube recorded the coincident high-energy neutrino IC-190730A. Using data from the OVRO 40-meter Telescope, we find that radio emission from PKS 1502+106 at the time period of the coincident neutrino IC-190730A was in a high state, in contrast to earlier time periods when radio and $\gamma$ fluxes are correlated for both low and high states. This points to an active outflow that is $\gamma$-suppressed at the time of neutrino production. We find similar local $\gamma$-suppression in other blazars, including in MAGIC's TeV flux of TXS\,0506+056 and Fermi-LAT's flux of blazar PKS B1424-418 at the time of coincident IceCube neutrino detections. Using temporary $\gamma$-suppression, neutrino-blazar coincidence searches could be substantially more sensitive than previously assumed, enabling the identification of the origin of IceCube's diffuse neutrino flux possibly with already existing data.