New Limits on the Photon Mass with Radio Pulsars in the Magellanic Clouds

TL;DR

This paper improves the upper limit on the photon mass by analyzing radio pulsar dispersion measures in the Magellanic Clouds, achieving a constraint significantly tighter than previous optical-based limits.

Contribution

It demonstrates that pulsar dispersion measurements in nearby galaxies can set more stringent bounds on photon mass than earlier methods.

Findings

Photon mass limit as low as 2.0×10⁻⁴⁵ g

Limit is four orders of magnitude better than Crab Nebula pulsar constraint

Comparable to limits from certain fast radio bursts if their extragalactic origin is confirmed

Abstract

A conservative constraint on the rest mass of the photon can be estimated under the assumption that the frequency dependence of dispersion from astronomical sources is mainly contributed by the nonzero photon mass effect. Photon mass limits have been earlier set through the optical emissions of the Crab Nebula pulsar, but we prove that these limits can be significantly improved with the dispersion measure (DM) measurements of radio pulsars in the Large and Small Magellanic Clouds. The combination of DM measurements of pulsars and distances of the Magellanic Clouds provide a strict upper limit on the photon mass as low as $m_{\gamma} \leq2.0\times10^{-45}~\rm{g}$, which is at least four orders of magnitude smaller than the constraint from the Crab Nebula pulsar. Although our limit is not as tight as the current best result ($\sim10^{-47}~\rm{g}$) from a fast radio burst (FRB 150418) at a cosmological distance, the cosmological origin of FRB 150418 remains under debate; and our limit can reach the same high precision of FRB 150418 when it has an extragalactic origin ($\sim10^{-45}~\rm{g}$).