Implications from the upper limit of radio afterglow emission of FRB 131104/Swift J0644.5-5111

TL;DR

This study constrains the environment and origin of FRB 131104 and its associated gamma-ray transient by analyzing radio afterglow limits, suggesting a compact binary merger origin over a massive star progenitor.

Contribution

The paper provides detailed constraints on the afterglow parameters and environment of the FRB-gamma-ray transient system, offering insights into its possible progenitors and challenging the cosmological origin based on dispersion measure.

Findings

Ambient medium density is likely very low for typical redshift, favoring a merger environment.

High redshift is inconsistent with radio afterglow limits unless microphysics parameters are very small.

The association between FRB 131104 and Swift J0644.5-5111 remains plausible under certain conditions.

Abstract

A $\gamma$-ray transient, Swift J0644.5-5111, has been claimed to be associated with FRB 131104. The $\gamma$-ray energy output is estimated as $E_\gamma \approx 5\times 10^{51}$\,erg at the nominal $z\approx 0.55$ redshift implied by the dispersion measure of FRB 131104. However, a long-term radio imaging follow-up observations only place an upper limit on the radio afterglow flux of Swift J0644.5-5111. Applying the external shock model, we make a detailed constraint on the afterglow parameters for the FRB 131104/Swift J0644.5-5111 system. We find that for the commonly used microphysics shock parameters (e.g., $\epsilon_e=0.1$, $\epsilon_B=0.01$ and $p=2.3$), if the redshift value inferred from the DM value is correct to order of magnitude (i.e., $z>0.1$), the ambient medium number density should be $\leq 10^{-3}~\rm{cm^{-3}}$, which is the typical value for a compact binary merger environment but disfavors a massive star origin. Assuming a typical ISM density, one would require that the redshift of the FRB much smaller than the value inferred from DM value ($z\ll0.1$), implying a non-cosmological origin of DM. The constraints are much looser if one adopts smaller $\epsilon_B$ and $\epsilon_e$ values, as observed in some GRB afterglows. The FRB 131104/Swift J0644.5-5111 association remains plausible. We critically discuss possible progenitor models for the system.