Constraining the Environmental Properties of FRB 131104 Using the Unified Dynamical Afterglow Model

Abstract

Multiband observations of the fast radio burst (FRB) 131104 show that this burst may be associated with a gamma-ray transient entitled Swift J0644.5−5111. Follow-up observations for potential X-ray and radio counterparts of FRB 131104/Swift J0644.5−5111 got null results and provided the upper limits of the emission flux at 5.5 GHz, 7.5 GHz, U -band, and X-ray band. By assuming this association and using these upper limits, environmental properties (the fraction of energy in a magnetic field ε B and the number density n ) of the progenitor system of FRB 131104/Swift J0644.5−5111 were constrained in the context of the standard afterglow model that neglects the nonrelativistic effect and jet effect by several groups. In this paper, we adopt a unified afterglow model that takes into account the nonrelativistic effect and jet effect and use the upper limits of four bands (5.5 GHz, 7.5 GHz, U -band, and X-ray) to obtain more stringent constraints on the parameter space spanned by ε B and n . We thus suggest that FRB 131104/Swift J0644.5−5111 might originate from a black hole–neutron star merger event. Moreover, we calculate multiband emissions from a kilonova powered by the radioactivity of r -process elements synthesized in the ejected neutron-rich material and find that the U -band emission from the putative kilonova is significantly lower than the upper limit of the observations.