Delayed Radio Flares in Neutrino-associated Blazars: The Case of TXS 0506+056

Abstract

Radio flares have been postulated to be associated with the production of astrophysical neutrinos. For example, TXS 0506+056 exhibits a 2-3 yr delay between the 2017 IceCube-170922A/$γ$-ray flare and a GHz radio maximum. We quantitatively test if the delayed radio flare originates from the same compact region where neutrinos and $γ$-rays are produced as it expands downstream and synchrotron self-absorption (SSA) is reduced. Starting from the 2017 flare blob parameters, we model the expanding production region and its evolving radio emission with LeHaMoC in a fully time-dependent framework, and compare our 1.2-22 GHz light curves to RATAN-600 data. We study different scenarios with increasing levels of sophistication, including continuous injection and energy re-dissipation on pc scales. While a simple expanding blob scenario fails to reproduce the radio data, a downstream dissipation episode of particles in the optically thin regime, followed by jet deceleration, successfully describes the radio evolution. Within our one-zone time-dependent framework, the delayed radio flare is unlikely to come from an expanding neutrino production zone becoming transparent to radio emission. Additional ingredients are needed, such as re-dissipation downstream with a subsequent Doppler-factor decline. The radio flare is powered by leptonic synchrotron emission and is largely insensitive to the proton population relevant for neutrino production, implying that the delayed radio flare mainly probes downstream dissipation and beaming in certain jet configurations rather than being a genuine feature associated with the neutrino production.

Figures (8)

• Figure 1: Multiwavelength behavior and schematic model. Top panel: Long-term light curve showing the weekly Fermi-LAT $\gamma$-ray flux from the Fermi-LAT Light Curve Repository 2023ApJS..265...31A, based on the LAT likelihood analysis described therein together with the radio flux densities (points colored by observing frequency). The vertical dashed line marks the time of the high-energy neutrino arrival. Bottom panel: Schematic of the expanding blob model. A compact emitting region of initial radius $R'_0$ corresponding to the neutrino/$\gamma$- ray flare conditions inferred by Keivani2018ApJ, propagates along the jet and expands. The outer radius $R'_{\rm rf}$ corresponds to the optically thin radio-emitting stage constrained by RATAN-600 and MOJAVE observations.
• Figure 2: SED at the neutrino epoch computed with LeHaMoC, using the Keivani2018ApJ LMBB2b leptohadronic solution. Different line styles represent different photon-production processes, grey curves denote secondary/hadronic components, while colored curves highlight the main leptonic contributions. The thick black solid line shows the overall photon spectrum, while the thick orange solid line shows the all-flavor neutrino spectrum.
• Figure 3: Profiles of the comoving non-thermal pair number density $n'_{\rm e}$ (left axis, solid black) together with the Doppler factor $\delta(R')$ (right axis, blue dashed) as a function of the radius of the emitting region $R'$. The dotted black line shows the asymptotic $R'^{-2}$ scaling at large radii. Vertical dashed lines mark the transition to the reacceleration phase at $t'_{\rm reacc}$ and the deceleration time $t'_{\rm dec}$. Shaded regions indicate the expansion and re-dissipation phases.
• Figure 4: Multi-frequency RATAN-600 radio light curves of TXS 0506+056 at 1.2–22 GHz (markers with 1$\sigma$ uncertainties), shown with vertical offsets of 2 Jy between adjacent bands for clarity. The solid curves correspond to Scenario C (re-dissipation followed by jet deceleration). The vertical dashed line marks the onset of the deceleration phase, after which the Doppler factor is assumed to decrease as a power law. For each band, the thin horizontal dashed line indicates the estimated quiescent/background level.
• Figure 5: Representative synthetic multi-frequency (1.2-22 GHz) light curves for Scenario A (dotted and dashed lines) and B (solid line), same as Fig. \ref{['fig:sigma_vs_p']}.