Combined Limit on the Photon Mass with Nine Localized Fast Radio Bursts

Abstract

A nonzero-mass hypothesis for the photon can produces a frequency-dependent dispersion of light, which results in arrival-time differences of photons with different frequencies originating from a given transient source. Extragalactic fast radio bursts (FRBs), with their low frequency emissions, short time durations, and long propagation distances, are excellent astrophysical probes to constrain the rest mass of the photon $m_γ$. However, the derivation of a limit on $m_γ$ is complicated by the similar frequency dependences of dispersion expected from the plasma and nonzero photon mass effects. If a handful measurements of redshift for FRBs are available, the different redshift dependences of the plasma and photon mass contributions to the dispersion measure (DM) might be able to break dispersion degeneracy in testing the photon mass. For now, nine FRBs with redshift measurements have been reported, which can turn this idea into reality. Taking into account the DM contributions from both the plasma and a possible photon mass, we use the data on the nine FRBs to derive a combined limit of $m_γ\leq7.1\times10^{-51}\;{\rm kg}$, or equivalently $m_γ\leq4.0\times10^{-15}\; {\rm eV}/c^{2}$ at 68\% confidence level, which is essentially as good as or represents a factor of 7 improvement over previous limits obtained by the single FRBs. Additionally, a reasonable estimation for the DM contribution from the host galaxy, $\rm DM_{host}$, can be simultaneously achieved in our analysis. The rapid progress in localizing FRBs will further tighten the constraints on both $m_γ$ and $\rm DM_{host}$.