Particle acceleration by pulsars: Acceleration by DC electric fields

Models of particle acceleration inside the light cylinder of a pulsar are reviewed. The models involve a steady component of electric field parallel to the magnetic field, in the frame rotating with the neutron star. The pulsed incoherent radiation (in particular X- and γ- radiation) observed from four pulsars puts the most serious constraints on the models; the energy to explain the observations must be put into particles at low enough altitude that the magnetic field remains rigid. Models with acceleration occurring near the stellar surface are incapable of providing the energy for the Crab pulsar, and only barely able for the Vela pulsar. A high-altitude ''outer gap'' can explain the Crab energy; outer gaps may not form for longer period pulsars. If the same mechanism is responsible for the similar γ-ray pulse profiles of all four γ-ray pulsars, outer gaps become unlikely. An alternative high-altitude acceleration model, involving a ''slot gap'', is mentioned; it too has difficulties but potentially can explain the pulse shapes