[Paper Review] Variability and Optical Polarization Can Probe the Neutrino and Electromagnetic Emission Mechanisms of TXS~0506+056

H. C. Zhang, Ke Fang+1 more|arXiv (Cornell University)|Jul 29, 2018

Astrophysics and Cosmic Phenomena11 references14 citations

TL;DR

This paper investigates the emission mechanisms of the flaring blazar TXS 0506+056 using multi-wavelength spectral fitting and variability/polarization signatures to distinguish between proton synchrotron (PS) and electron inverse Compton (IC) scenarios for high-energy emission and neutrino production. It finds that optical polarization variability and multi-wavelength co-variations can distinguish the two models: PS predicts low, stable optical polarization (<10%) and orphan fast variability in the low-energy band, while IC predicts highly variable optical polarization (>20%) and co-variability across bands.

ABSTRACT

The association of the high-energy neutrino event IceCube-170922A with the flaring blazar TXS~0506+056 indicates that hadronic processes may operate in a blazar jet. We perform semi-analytical spectral fitting of the multi-wavelength emission to obtain estimates of the jet physical parameters, and find that the multi-wavelength emission can be explained by either a proton synchrotron scenario or an electron inverse Compton scattering scenario. In the proton synchrotron scenario, a strong magnetic field of $10-100$~G is required, implying that the particle acceleration is likely driven by magnetic energy dissipation such as magnetic reconnection events. The inverse Compton scenario implies a magnetic field of $0.1-1$~G. Thus the particle acceleration is likely driven by the kinetic energy dissipation such as shocks. We also discuss the neutrino production in the context of single-zone and multi-zone models based on the above two scenarios. We demonstrate that the variability and optical polarization signatures can be used to distinguish the two scenarios due to their drastically different magnetic field. Specifically, the proton synchrotron scenario may show orphan fast variability in the low-energy spectral component on top of the active state, with an optical polarization degree $\lesssim 10\%$ throughout the active state. The inverse Compton scattering scenario instead predicts co-variability of the low- and high-energy components on both short and long time scales, as well as a strongly variable optical polarization degree that can reach $\gtrsim 20\%$. Our results suggest that optical polarization measurements and well-resolved multi-wavelength light curves can be used to understand the electromagnetic and high-energy neutrino emissions by TXS~0506+056 and similar events in the future.

Motivation & Objective

• To determine whether the high-energy emission in TXS 0506+056 arises from proton synchrotron (PS) or electron inverse Compton (IC) processes based on multi-wavelength spectral fitting.
• To investigate how neutrino production mechanisms differ in PS and IC scenarios under single-zone and multi-zone models.
• To identify observable signatures—specifically variability and optical polarization—that can distinguish between the PS and IC scenarios.
• To provide testable predictions for future observations using optical polarization and multi-wavelength light curves.

Proposed method

• Semi-analytical spectral fitting of the multi-wavelength spectral energy distribution (SED) of TXS 0506+056 to constrain jet physical parameters.
• Modeling of high-energy emission via proton synchrotron (PS) and electron inverse Compton (IC) mechanisms, with distinct magnetic field requirements (10–100 G for PS, 0.1–1 G for IC).
• Analysis of variability patterns in the low- and high-energy bands under both PS and IC scenarios to identify distinguishing light curve behaviors.
• Assessment of optical and γ-ray polarization signatures, including degree and variability, based on magnetic field strength and emission mechanism.
• Comparison of one-zone and two-zone models for neutrino flux production, incorporating external radiation fields and time-dependent effects.
• Use of generic model features—particularly magnetic field strength—to derive observable signatures independent of specific parameter sets.

Experimental results

Research questions

• RQ1 Can the multi-wavelength SED of TXS 0506+056 be explained by either proton synchrotron or electron inverse Compton emission mechanisms?
• RQ2 What are the distinct variability patterns in the optical and high-energy bands expected under the proton synchrotron versus inverse Compton scenarios?
• RQ3 How do optical polarization degrees and their variability differ between the proton synchrotron and inverse Compton models?
• RQ4 Can the observed neutrino flux from IceCube-170922A be reconciled with the SED in both PS and IC scenarios, and what role does the emission region geometry play?
• RQ5 What observational signatures—specifically in polarization and variability—can distinguish between hadronic and leptonic emission mechanisms in blazar jets?

Key findings

• The proton synchrotron (PS) scenario requires a strong magnetic field of 10–100 G, implying particle acceleration via magnetic energy dissipation such as magnetic reconnection.
• The inverse Compton (IC) scenario requires a weaker magnetic field of 0.1–1 G, suggesting particle acceleration via kinetic energy dissipation such as shocks.
• The PS scenario predicts fast, orphan variability in the low-energy spectral component without a corresponding high-energy counterpart, with optical polarization degree ≤10% throughout the active state.
• The IC scenario predicts co-variability of low- and high-energy components on both short and long timescales, with optical polarization degree that can reach >20%.
• The γ-ray polarization degree is expected to be low in the PS scenario (similar to optical), while the IC scenario predicts nearly unpolarized γ-ray emission.
• In a one-zone model, the average neutrino flux is only ~1% of the observed IceCube level, but a two-zone model with an intense external UV field can produce sufficient neutrino flux, with neutrino detection preceding the 100 GeV flare in both PS and IC scenarios.