On the radioemission from Magnetars

Abstract

We discuss a model for radio pulsars and magnetars, which explain different observational properties of these objects. Recently developed model of the partially screened polar gap for radio pulsars assumes that the magnetic field at the stellar surface is very strong and differs essentially from the pure dipolar structure. This assumption is well supported by recent calculations of the cohesive energy for the neutron star surface, which show that the accelerating gap above polar cap can form provided that the local surface magnetic field exceeds 1014 G. In addition, X‐ray observations of radio pulsars make it clear that the hot spot surface is much smaller than that of the canonical one obtained from the dipolar geometry. The only natural explanation of this observational feature is that the hot spot is defined by the crust anchored magnetic field anomalies, dominating the surface magnetic field in radio pulsars. The open field lines of the dipolar field reconnect with the actual surface magnetic field lines forming a much smaller polar cap. The flux conservation law implies that the magnetic field at the surface is few times 1014 G, with the curvature radius of field lines near the polar cap is much smaller than that of dipolar field lines. The high curvature of the field lines, which is an essential condition for creation of dense electron‐positron plasma necessary for the radio emission generation, depends on the ratio of the dipolar and crust anchored fields. In the case of the magnetars this ratio is near unity, because dipolar field estimated by the pulsar spin‐down rate is the same order as the actual surface magnetic field. Therefore, although magnetars have large enough surface magnetic field to create the inner gap accelerator, the curvature radius of field lines is too large to magnetic pair creation in quantities large enough for generation of coherent radio emission. Thus, magnetars should be radio quiet as it is for majority of these objects.