Direct emission of strong radio pulses during magnetar flares

Abstract

ABSTRACT The emission of intense radio pulses by flaring magnetars is investigated. Small-scale current gradients can be imprinted into a strongly magnetized outflow by the same processes that source fireball radiation in the closed magnetosphere. This structure arises from a combination of crustal yielding, internal tearing, and turbulent cascade. We consider the quasi-linear development of weak, small-scale currents as (i) they are stretched out and frozen by relativistic expansion, and then (ii) pass through a shock. In particular, we derive the amplitudes of the ordinary and fast waves that emerge downstream of a relativistically magnetized shock in response to a mode that is frozen into the upstream flow (a frozen Alfvén wave or entropy wave). An upstream mode with comoving wavelength exceeding the skin depth can linearly convert to a secondary mode propagating above the plasma frequency. A simple and accurate treatment of shocks with extreme magnetization is developed, and the formation of internal shocks in the outflow from a bursting, rotating magnetar is outlined. The emission process described here does not require a strong shock or cool e± pairs (in contrast with the electromagnetic maser shock instability). In some cases, a high-frequency wave is reflected back to the observer, but with a minuscule amplitude that makes it subdominant to other emission channels. The dominant secondary electromagnetic mode is superluminal at emission, is subject to weak induced scattering within the outflow, and can reach the observer in the radio band.