Analysing the gamma-ray activity of neutrino emitter candidates: comparing TXS 0506+056 with other blazars

On 2017 September 22, the IceCube collaboration sent an alert for an EHE (Extreme High Energy) event, corresponding to the reconstruction of a muonic neutrino (IC-170922A) with energy similar to 290 TeV. A multiwavelength follow-up campaign associated this neutrino event with a gamma-ray flaring state of the BL Lac TXS 0506+056 located at z = 0.3365. From the same position of the sky, a muonic neutrino excess is observed in a time window of 110 d around 2014 December 13. These observations together suggest TXS 0506+056 as a possible neutrino emitter. We report here a long-term gamma-ray monitoring of this source and we compare it with other blazars spatially correlated with astrophysical muonic neutrino events observed by IceCube. We characterize the most significant gamma-ray flares of the blazars in the sample and introduce the blazar duty cycle as an important parameter to be considered when assessing a possible neutrino counterpart. For the selected blazars, we show the expected neutrino flux variability with different time binning assuming the gamma-rays observed by Fermi-LAT as a product of a leptohadronic emission scenario. The neutrino expectations from the blazar sample are then compared with the IceCube discovery flux searching for the optimal time bin in a multimessenger context. This analysis indicates that the detection of a single flare represents a challenge for a single cubic kilometre detector, underlining the importance of increasing the size of current neutrino telescopes and obtaining a good monitoring of the entire sky through a global neutrino network.

Automated summary

The IceCube collaboration detected an Extreme High Energy (EHE) event on September 22, 2017, and linked it to a gamma-ray flaring state of the BL Lac TXS 0506+056. Observations also showed a muonic neutrino excess around the same area of the sky on December 13, 2014, suggesting TXS 0506+056 as a potential neutrino emitter. Through long-term gamma-ray monitoring and comparison with other blazars, researchers highlighted the importance of considering the blazar duty cycle when assessing a possible neutrino counterpart.