Radio-interferometric monitoring of FRB 131104: A coincident AGN flare, but no evidence for a cosmic fireball

TL;DR

This study used radio interferometry to monitor FRB 131104, finding no evidence of a cosmic fireball and identifying a variable AGN unrelated to the gamma-ray transient, challenging some progenitor models.

Contribution

It provides the first long-term radio monitoring of FRB 131104's localization region, setting upper limits on afterglow emissions and identifying an unrelated variable AGN.

Findings

No radio afterglow consistent with standard progenitor models.

Discovery of a variable radio source identified as a narrow-line AGN.

Probabilistic analysis suggests the gamma-ray transient is not associated with the FRB.

Abstract

The localization of fast radio bursts (FRBs) has been hindered by the poor angular resolution of the detection observations and inconclusive identification of transient or variable counterparts. Recently a $\gamma$-ray pulse of $380$ s duration has been associated with the fast radio burst FRB 131104. We report on radio-continuum imaging observations of the original localization region of the FRB, beginning three days after the event and comprising 25 epochs over 2.5 yr. Besides probabilistic arguments that suggest that the association between the $\gamma$-ray transient and the FRB is not compelling, we provide upper limits on a putative radio afterglow of this transient that are at odds with standard models for its progenitor. We further report the discovery of an unusual variable radio source spatially and temporally coincident with FRB 131104, but not spatially coincident with the $\gamma$-ray event. The radio variable flares by a factor of $3$ above its long term average within $10$ d of the FRB at 7.5 GHz, with a factor-of-$2$ increase at $5.5$ GHz. Within our observations, the variable has persisted with only modest modulation and never approached the flux density observed in the days after the FRB. We identify an optical counterpart to the variable. Optical and infrared photometry, and deep optical spectroscopy, suggest that the object is a narrow-line radio AGN.