Limits on Einstein's Equivalence Principle from the first localized Fast Radio Burst FRB 150418

Abstract

Fast Radio Bursts have recently been used to place limits on Einstein's Equivalence Principle via observations of time delays between photons of different radio frequencies by \citet{wei15}. These limits on differential post-Newtonian parameters ($\Delta \gamma<2.52\times10^{-8}$) are the best yet achieved but still rely on uncertain assumptions, namely the relative contributions of dispersion and gravitational delays to the observed time delays and the distances to FRBs. Also very recently, the first FRB host galaxy has likely been identified, providing the first redshift-based distance estimate to FRB 150418 \citep{kea16}. Moreover, consistency between the \omegaigm\ estimate from FRB 150418 and \omegaigm~expected from $\Lambda$CDM models and WMAP observations leads one to conclude that the observed time delay for FRB 150418 is highly dominated by dispersion, with any gravitational delays small contributors. This points to even tighter limits on $\Delta \gamma$. In this paper, the technique of \citet{wei15} is applied to FRB 150418 to produce a limit of $\Delta \gamma < 1 - 2\times10^{-9}$, approximately an order of magnitude better than previous limits and in line with expectations by \citet{wei15} for what could be achieved if the dispersive delay is separated from other effects. Future substantial improvements in such limits will depend on accurately determining the contribution of individual ionized components to the total observed time delays for FRBs.