2017ApJ...842...23Y

Pulsars are magnetized rotating compact objects. They spin down due to magnetic dipole radiation and wind emission. If a photon has nonzero mass, the spin-down rate will be lower than in the zero-mass case. We show that an upper limit of the photon mass, i.e., m_γ h/Pc², may be placed if a pulsar with period P is observed to spin down. Recently, a white dwarf (WD)-M dwarf binary, AR Scorpii, was discovered to emit pulsed broadband emission. The spin-down luminosity of the WD can comfortably power non-thermal radiation from the system. Applying our results to the WD pulsar with P = 117 s, we obtain a stringent upper limit of the photon mass between m_γ 6.3×10^–50 g, assuming a vacuum dipole spindown, and m_γ 9.6×10^–50 g, assuming spindown due to a fully developed pulsar wind.