Time-dependent neutrino emission from Mrk 421 during flares and predictions for IceCube

TL;DR

This study models neutrino emission from blazar Mrk 421 during a 2010 flare, correlates it with gamma-ray data, and predicts IceCube detection prospects, highlighting the importance of flare frequency and flux correlation for neutrino observability.

Contribution

It provides the first detailed modeling of neutrino flux during a multi-wavelength flare of Mrk 421 and assesses IceCube detection likelihood based on flux correlations and long-term data.

Findings

Neutrino flux correlates with photon flux during flares.

Expected IceCube event rate exceeds detection thresholds.

Non-detection would constrain neutrino-gamma flux correlation.

Abstract

Blazars are prime candidate sources for the high energy neutrinos recently detected by IceCube. Being one of the brightest sources in the extragalactic X-ray and $\gamma$-ray sky as well as one of the nearest blazars to Earth, Mrk 421 is an excellent source for testing the scenario of the blazar-neutrino connection. Here, we model the spectral energy distribution of Mrk 421 during a 13-day flare in 2010 with unprecedented multi-wavelength coverage, and calculate the respective neutrino flux. We find a correlation between the $>1$ PeV neutrino and photon fluxes, in all energy bands. Using typical IceCube through-going muon event samples with good angular resolution and high statistics, we derive the mean event rate above 100 TeV ($\sim0.57$ evt/yr) and show that it is comparable to that expected from a four-month quiescent period in 2009. Due to the short duration of the flare, an accumulation of similar flares over several years would be necessary to produce a meaningful signal for IceCube. To better assess this, we apply the correlation between the neutrino and $\gamma$-ray fluxes to the 6.9 yr Fermi-LAT light curve of Mrk 421. We find that the mean event count above 1 PeV for the full IceCube detector livetime is $3.59\pm0.60$ ($2.73\pm0.38$) $\nu_\mu+\bar{\nu}_\mu$ with (without) major flares included in our analysis. This estimate exceeds, within the uncertainties, the $95\%$ ($90\%$) threshold value for the detection of one or more muon (anti-)neutrinos. Meanwhile, the most conservative scenario, where no correlation of $\gamma$-rays and neutrinos is assumed, predicts $1.60\pm0.16$ $\nu_\mu+\bar{\nu}_\mu$ events. We conclude that a non-detection of high-energy neutrinos by IceCube would probe the neutrino/$\gamma$-ray flux correlation during major flares or/and the hadronic contribution to the blazar emission.