2013MNRAS.429..113H

It has recently been proposed that radio emission from magnetars can be evaluated using a `fundamental plane' in parameter space between pulsar voltage gap and ratio of X-ray luminosity Lx to rotational energy loss rate {Edot}. In particular, radio emission from magnetars will occur if Lx/{Edot}<1 and the voltage gap is large, and there is no radio emission if Lx/{Edot}>1. Here we clarify several issues regarding this fundamental plane, including demonstrating that the fundamental plane is not uniquely defined. We also show that, if magnetars and all other pulsars are different manifestations of a unified picture of neutron stars, then pulsar radio activity (inactivity) appears to be determined by the ratio Lx/{Edot} ≳ 1 (Lx/{Edot} ≳ 1), although observational bias and uncertainty in the ratio for some sources may still invalidate this conclusion. Finally, we comment on the use of other pulsar parameters that are constructed from the three observables: spin period, P, period derivative, {dot}P, and Lx.