Bounding the photon mass with gravitationally lensed fast radio bursts

TL;DR

This paper demonstrates how gravitational lensing of fast radio bursts can be used to set the most stringent upper limits on the photon mass to date, surpassing previous constraints from other astrophysical observations.

Contribution

It introduces a method to constrain photon mass using time delays and flux ratios in lensed FRBs, providing tighter bounds than previous methods.

Findings

Upper limit of photon mass: < 5.3 × 10^{-42} kg for certain shear

Derived bounds vary with external shear strength

Most stringent photon mass limit from gravitational lensing to date

Abstract

The gravitational time delays of macro-lenses can be used to constrain the rest mass of the photon with high accuracy. Assuming a point-mass $+$ external shear lens model, we prove that an upper limit of the photon mass can be derived directly from two observables--the time delay $\Delta t$ and the leading-to-trailing flux ratio $R$ of strongly lensed fast radio bursts (FRBs). Using the observed values of $\Delta t$ and $R$ of a lensed FRB candidate, i.e., FRB 20190308C, as a reference, we obtain a strict upper limit of the photon mass between $m_\gamma < 5.3 \times {10}^{-42}\,\rm kg$, for a given external shear strength of $\gamma' = 0.01$, and $m_{\gamma} < 2.1 \times 10^{-41}-2.4 \times 10^{-42}\,\text{kg}$, within the external shear range of $0<\gamma'<1$. This provides the most stringent limit to date on the photon mass through gravitational lensing time delays, improving by 1 to 2 orders of magnitude the previous results obtained from lensed active galactic nuclei.