Tight Constraint on Photon Mass from Pulsar Spindown

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(gamma) <= h/Pc-2, 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(gamma) < 6.3 10(-50) g, assuming a vacuum dipole spindown, and m(gamma) < 9.6 x 10(-50) g, assuming spindown due to a fully developed pulsar wind.Initiative Postdocs Supporting Program [BX201600003]; National Basic Research Program (973 Program) of China [2014CB845800]; China Postdoctoral Science Foundation [2016M600851]; KIAA-CAS FellowshipSCI(E)ARTICLE184