Robust Limits on Photon Mass from Statistical Samples of Extragalactic
  Radio Pulsars
  The photon zero-mass hypothesis has been investigated for a long time using
the frequency-dependent time delays of radio emissions from astrophysical
sources. However, the search for a rest mass of the photon has been hindered by
the similarity between the frequency-dependent dispersions due to the plasma
and nonzero photon mass effects. Considering the contributions to the observed
dispersion measure from both the plasma and nonzero photon mass effects, and
assuming the dispersion induced by the plasma effect is an unknown constant, we
obtain a robust limit on the photon mass by directly fitting a combination of
the dispersion measures of radio sources. Using the observed dispersion
measures from two statistical samples of extragalactic pulsars, here we show
that at the 68\% confidence level, the constraints on the photon mass can be as
low as $m_{\gamma}\leq1.51\times10^{-48}~\rm kg\simeq8.47\times10^{-13}~{\rm
eV}/c^{2}$ for the sample of 22 radio pulsars in the Large Magellanic Cloud and
$m_{\gamma}\leq1.58\times10^{-48}~\rm kg\simeq8.86\times10^{-13} {\rm
eV}/c^{2}$ for the other sample of 5 radio pulsars in the Small Magellanic
Cloud, which are comparable with that obtained by a single extragalactic
pulsar. Furthermore, the statistical approach presented here can also be used
when more fast radio bursts with known redshifts are detected in the future.
