On the Radio Quiescence of Anomalous X-Ray Pulsars and Soft Gamma-Ray Repeaters

Abstract

In the hot environments of the anomalous X-ray pulsars (AXPs) and the soft gamma-ray repeaters (SGRs), as indicated by their luminous, pulsed, quiescent X-ray emission, gamma rays generated from the inner gaps may have shorter attenuation lengths via two-photon pair production rather than via magnetic photon splitting. The AXP/SGR environments may not be pairless, even if photon splitting could completely suppress one-photon pair production in superstrong magnetic fields, as conjectured by Baring and Harding. Two-photon pair production more likely occurs near the threshold, which tends to generate low-energy pairs that are not energetic enough to power radio emission in the observed bands. However, emission in longer wavelengths may not be prohibited, in principle, if these objects are indeed magnetars. The so-called photon-splitting deathlines are still valid for high magnetic field pulsars that are much dimmer in X-rays, if gamma rays with both polarization modes split.