Bounding the Photon Mass with the Dedispersed Pulses of the Crab Pulsar and FRB 180916B

Tight limits on the photon mass have been set through analyzing the arrival time differences of photons with different frequencies originating from the same astrophysical source. However, all these constraints have relied on using the first-order Taylor expansion of the dispersion due to a nonzero photon mass. In this work, we present an analysis of the nonzero photon mass dispersion with the second-order derivative of Taylor series. If the arrival time delay corrected for all known effects (including the first-order delay time due to the plasma and photon mass effects) is assumed to be dominated by the second-order term of the nonzero photon mass dispersion, a conservative upper limit on the photon mass can be estimated. Here we show that the dedispersed pulses with the second-order time delays from the Crab pulsar and the fast radio burst FRB 180916B pose strict limits on the photon mass, i.e., $m_{\gamma,2} \leq5.7\times10^{-46}\;{\rm kg}\simeq3.2\times10^{-10}\; {\rm eV}/c^{2}$ and $m_{\gamma,2} \leq6.0\times10^{-47}\;{\rm kg}\simeq3.4\times10^{-11}\; {\rm eV}/c^{2}$, respectively. This is the first time to study the possible second-order photon mass effect.