The blazar TXS 0506+056 associated with a high-energy neutrino: insights into extragalactic jets and cosmic ray acceleration

Abstract: A neutrino with energy of $\sim$290 TeV, IceCube-170922A, was detected in coincidence with the BL Lac object TXS~0506+056 during enhanced gamma-ray activity, with chance coincidence being rejected at $\sim 3\sigma$ level. We monitored the object in the very-high-energy (VHE) band with the MAGIC telescopes for $\sim$41 hours from 1.3 to 40.4 days after the neutrino detection. Day-timescale variability is clearly resolved. We interpret the quasi-simultaneous neutrino and broadband electromagnetic observations with a novel one-zone lepto-hadronic model, based on interactions of electrons and protons co-accelerated in the jet with external photons originating from a slow-moving plasma sheath surrounding the faster jet spine. We can reproduce the multiwavelength spectra of TXS 0506+056 with neutrino rate and energy compatible with IceCube-170922A, and with plausible values for the jet power of $\sim 10^{45} - 4 \times 10^{46} {\rm erg \ s^{-1}}$. The steep spectrum observed by MAGIC is concordant with internal $\gamma\gamma$ absorption above a few tens of GeV entailed by photohadronic production of a $\sim$290 TeV neutrino, corroborating a genuine connection between the multi-messenger signals. In contrast to previous predictions of predominantly hadronic emission from neutrino sources, the gamma-rays can be mostly ascribed to inverse Compton up-scattering of external photons by accelerated electrons. The X-ray and VHE bands provide crucial constraints on the emission from both accelerated electrons and protons. We infer that the maximum energy of protons in the jet co-moving frame can be in the range $\sim 10^{14}$ to $10^{18}$ eV.

• The paper reports the detection of a ~290 TeV neutrino coinciding with enhanced gamma-ray activity, supporting a multi-messenger emission scenario.
• It employs 41 hours of MAGIC telescope observations to capture day-scale variability and a steep gamma-ray spectrum indicative of internal absorption.
• The study uses a one-zone lepto-hadronic model to constrain jet power (10^45 to 4×10^46 erg/s) and illuminate mechanisms for neutrino and ultra-high-energy cosmic ray production.

Overview of the Blazar TXS 0506+056 and High-Energy Neutrino Observations

This paper presents a detailed analysis of the blazar TXS 0506+056, associated with the high-energy neutrino IceCube-170922A. The collaboration focuses on its multi-messenger nature, linking neutrinos to gamma-ray activity. This research provides insights into the dynamics of extragalactic jets and cosmic ray acceleration, employing the MAGIC telescopes for extensive observation and analysis.

Main Findings

1. Neutrino Detection and Association: The detection of a ~290 TeV neutrino coincident with enhanced gamma-ray activity from TXS 0506+056 indicates a likely physical association, with a chance coincidence being unlikely at a 3σ level.
2. VHE Observations: MAGIC telescopes monitored TXS 0506+056 for ~41 hours post-neutrino detection, finding significant variability on a day-timescale. The gamma-ray emission was captured over various states, showing a spectrum above ~80 GeV.
3. Theoretical Modeling: The paper employs a one-zone lepto-hadronic model, interpreting the interaction of electrons and protons with external photons in the jet. This explains the multi-wavelength spectrum and neutrino emission, suggesting jet powers in the range of 10^45 to 4×10^46 erg/s.
4. Gamma-Ray Spectrum: The steep gamma-ray spectrum observed by MAGIC can be attributed to internal γγ absorption, consistent with photo-hadronic processes that produce the detected neutrino.
5. Jet Structure: The analysis implies a structured jet design, whereby a slower sheath impacts photon interactions. This enhances neutrino production efficacy without requiring excessive proton acceleration energy.
6. Numerical Results: The neutrino event rate estimated from the model aligns well with IceCube observations, supporting the association between the blazar activity and high-energy neutrinos.

Implications

• Model Support: The correlation of enhanced gamma-ray and neutrino activities supports the model of structured jets in blazars conducive to high-energy particle acceleration.
• Neutrino Production Mechanisms: This analysis corroborates the photo-hadronic production channels within jets, offering a unified model for interpreting multi-messenger signals.
• UHECR Acceleration: The inferred maximum proton energy (10^14 to 10^18 eV) provides constraints on models of ultra-high-energy cosmic ray (UHECR) acceleration, potentially positioning AGN jets as viable sites for these phenomena.

Future Prospects

Further multi-messenger observations and advances in neutrino observatories could refine the models connecting AGN jet physics and neutrino production. Enhanced VLBI observations may verify jet sheath structures, contributing to a more comprehensive understanding of blazar dynamics and high-energy astrophysics. This has broad implications for astrophysical models, better correlating observed EM phenomena with neutrino signals, thereby refining our understanding of cosmic high-energy processes.