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

Abstract

In the hot environments of the anomalous X-ray pulsars and the soft gamma-ray repeaters, as indicated by their luminous, pulsed, quiescent X-ray emission, $γ$-rays generated from the inner gaps may have shorter attenuation lengths via two photon pair production 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 super-strong magnetic fields, as conjectured by Baring & 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 which are much dimmer in X-rays, if $γ$-rays with both polarization modes split.