LIMITS ON EINSTEIN'S EQUIVALENCE PRINCIPLE FROM THE FIRST LOCALIZED FAST RADIO BURST FRB 150418

Fast radio bursts (FRBs) have recently been used to place limits on Einstein's Equivalence Principle via observations of time delays between photons of different radio frequencies by Wei et al. These limits on differential post-Newtonian parameters (Delta gamma < 2.52 x 10(-8)) are the best yet achieved, but they 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. Moreover, consistency between the Omega(IGM) estimate from FRB 150418 and Omega(IGM), 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 being small contributors. This points to even tighter limits on Delta gamma. In this paper, the technique of Wei et al. is applied to FRB 150418 to produce a limit of Delta gamma < 1-2 x 10(-9), approximately an order of magnitude better than previous limits and in line with expectations by Wei et al. 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.