Flare Duty Cycle of Gamma-Ray Blazars and Implications for High-Energy Neutrino Emission

Abstract

Gamma-ray flares of blazars may be accompanied by high-energy neutrinos due to interactions of high-energy cosmic rays in the jet with photons, as suggested by the detection of the high-energy neutrino IceCube-170922A during a major gamma-ray flare from blazar TXS 0506+056 at the $\sim3\sigma$ significance level. In this work, we present a statistical study of gamma-ray emission from blazars to constrain the contribution of gamma-ray flares to their neutrino output. We construct weekly binned light curves for 145 gamma-ray bright blazars in the {\it Fermi} Large Area Telescope (LAT) Monitored Source List adding TXS 0506+056. We derive the fraction of time spent in the flaring state (flare duty cycle) and the fraction of energy released during each flare from the light curves with a Bayesian blocks algorithm. We find that blazars with lower flare duty cycles and energy fractions are more numerous among our sample. We identify a significant difference in flare duty cycles between blazar sub-classes at a significance level of 5~\%. Then using a general scaling relation for the neutrino and gamma-ray luminosities, $L_{\nu} \propto (L_{\gamma})^{\gamma}$ with a weighting exponent of ${\gamma} = 1.0 - 2.0$, normalized to the quiescent gamma-ray or X-ray flux of each blazar, we evaluate the neutrino energy flux of each gamma-ray flare. The gamma-ray flare distribution indicates that blazar neutrino emission may be dominated by flares for $\gamma\gtrsim1.5$. The neutrino energy fluxes for one-week and 10-year bins are compared with the declination-dependent IceCube sensitivity to constrain the standard neutrino emission models for gamma-ray flares. Finally, we present the upper-limit contribution of blazar gamma-ray flares to the isotropic diffuse neutrino flux.