Interstellar Scintillation and the Radio Counterpart of the Fast Radio Burst FRB150418

TL;DR

This paper investigates whether the radio counterpart of FRB150418 is a true associated galaxy or a result of interstellar scintillation, emphasizing the importance of scintillation effects in FRB counterpart searches.

Contribution

The study demonstrates that the observed radio variability can be explained by interstellar scintillation of an AGN, challenging the initial association of the radio source with the FRB.

Findings

Observed variability consistent with scintillating AGN emission

Numerical simulations support scintillation as the cause of variability

Scintillation effects are critical in interpreting FRB radio counterparts

Abstract

Keane et al. (2016) have recently reported the discovery of a new fast radio burst, FRB150418, with a promising radio counterpart at 5.5 and 7.5 GHz -- a rapidly decaying source, falling from 200-300 $\mu$Jy to 100 $\mu$Jy on timescales of $\sim$6 d. This transient source may be associated with an elliptical galaxy at redshift $z=0.492$, providing the first firm spectroscopic redshift for a FRB and the ability to estimate the density of baryons in the intergalactic medium via the combination of known redshift and radio dispersion of the FRB. An alternative explanation, first suggested by Williams & Berger (2016b), is that the identified counterpart may instead be a compact AGN. The putative counterpart's variation may then instead be extrinsic, caused by refractive scintillation in the ionized interstellar medium of the Milky Way, which would invalidate the association with FRB150418. We examine this latter explanation in detail and show that the reported observations are consistent with scintillating radio emission from the core of a radio-loud active galactic nucleus (AGN) having a brightness temperature $T_{\rm b} \gtrsim 10^9 {\rm K}$. Using numerical simulations of the expected scattering for the line of sight to FRB150418, we provide example images and light curves of such an AGN at 5.5 and 7.5 GHz. These results can be compared with continued radio monitoring to conclusively determine the importance of scintillation for the observed radio variability, and they show that scintillation is a critical consideration for continued searches for FRB counterparts at radio wavelengths.