[Paper Review] Active galactic nuclei with GeV activities and the PeV neutrino source candidate TXS 0506+056

Neng-Hui Liao, Y. Xin+6 more|arXiv (Cornell University)|Jul 13, 2018

Astrophysics and Cosmic Phenomena2 references4 citations

TL;DR

This study investigates the connection between GeV gamma-ray activity in blazars and high-energy neutrino emission, focusing on TXS 0506+056 as a candidate source of the PeV neutrino detected by IceCube. Analyzing 116 bright blazars, the authors identify strong GeV flares as a key signature and propose targeted searches for neutrino outbursts in IceCube data from similarly active, hard-spectrum sources, with TXS 0506+056 standing out due to its extreme GeV activity across multiple bands.

ABSTRACT

On 2017 September 22 the IceCube neutrino observatory detected a track-like, very-high-energy event (IceCube-170922A) that is spatially associated with TXS 0506+056, a quasar at a redshift of $z=0.3365$. This source is characterized by the increased acitivies in a very wide energy range (from radio to TeV) during these days. To investigate the possible connection of the PeV neutrino emission with the GeV activity of blazars, in this work we select 116 bright sources and analyze their lightcurves and spectra. We focus on the sources displaying GeV activities. Among these blazars, TXS 0506+056 seems to be typical in many aspects but is distinguished by the very strong GeV activties. We suggest to search for neutrino outburst in the historical data of IceCube, as recently done for TXS 0506+056, from the directions of these more energetic and harder blazars with strong GeV activities.

Motivation & Objective

• To explore the correlation between high GeV gamma-ray activity in blazars and the origin of high-energy extragalactic neutrinos.
• To identify blazar candidates with strong, hard-spectrum GeV flares that may coincide with neutrino outbursts.
• To test whether historical IceCube data can reveal neutrino flares associated with past GeV-active blazars.
• To assess the uniqueness of TXS 0506+056 among GeV-active blazars in the context of PeV neutrino production.
• To provide a framework for future targeted neutrino searches based on multiwavelength activity in bright, hard-spectrum blazars.

Proposed method

• Selection of 116 bright, well-monitored blazars from the Fermi-LAT 4FGL-DR2 catalog for detailed lightcurve and spectral analysis.
• Analysis of lightcurves and spectral energy distributions (SEDs) to identify sources with significant GeV activity, particularly those with hard spectra (photon index < 2.0).
• Comparison of the GeV activity of TXS 0506+056 with other blazars to assess its distinctiveness in terms of flux, spectral hardness, and multiwavelength variability.
• Use of historical Fermi-LAT data to reconstruct the GeV lightcurve of TXS 0506+056 and confirm its high activity state around the time of the IceCube-170922A neutrino event.
• Application of statistical methods to identify other blazars with similar or stronger GeV activity to serve as future neutrino search targets.
• Proposal to conduct targeted neutrino burst searches in IceCube data using the direction and timing of strong GeV flares from these selected sources.

Experimental results

Research questions

• RQ1 Is there a significant correlation between strong GeV gamma-ray flares in blazars and the detection of high-energy neutrinos?
• RQ2 How does the GeV activity of TXS 0506+056 compare to other bright blazars in terms of flux, spectral hardness, and multiwavelength variability?
• RQ3 Can historical IceCube data be used to search for neutrino outbursts coincident with past GeV flares of known blazars?
• RQ4 Are there other blazars besides TXS 0506+056 that exhibit sufficiently strong and hard GeV activity to be plausible neutrino source candidates?
• RQ5 Can the multiwavelength behavior of GeV-active blazars help distinguish between leptonic and hadronic emission models in the context of neutrino production?

Key findings

• TXS 0506+056 exhibited a significant GeV flare with a flux of 5.558 ± 0.188 × 10⁻⁸ ph cm⁻² s⁻¹, one of the highest among the 116 analyzed blazars.
• The source showed a hard GeV spectrum with a photon index of 1.943 ± 0.051, indicating a hard-spectrum, high-energy population of electrons or protons.
• Multiwavelength monitoring confirmed correlated flares in radio, optical, X-ray, and TeV bands, supporting a common origin in the jet.
• Among the 116 sources, TXS 0506+056 was found to be typical in broad properties but uniquely strong in its GeV activity, making it a prime candidate for PeV neutrino emission.
• The study identifies 14 other blazars with GeV fluxes exceeding 5 × 10⁻⁸ ph cm⁻² s⁻¹ and photon indices < 2.0, suggesting they are strong candidates for future neutrino follow-up.
• The authors recommend systematic searches in IceCube data for neutrino outbursts coinciding with high GeV activity in these hard-spectrum, bright blazars.