The fundamental plane for radio magnetars
  High magnetic fields are a distinguishing feature of neutron stars and the
existence of sources (the soft gamma repeaters and the anomalous X-ray pulsars)
hosting an ultra-magnetized neutron star (or magnetar) has been recognized in
the past few decades. Magnetars are believed to be powered by magnetic energy
and not by rotation, as with normal radio pulsars. Until recently, the radio
quietness and magnetic fields typically above the quantum critical value
(Bq~4.4x10^{13} G), were among the characterizing properties of magnetars. The
recent discovery of radio pulsed emission from a few of them, and of a low
dipolar magnetic field soft gamma repeater, weakened further the idea of a
clean separation between normal pulsars and magnetars. In this Letter we show
that radio emission from magnetars might be powered by rotational energy,
similarly to what occurs in normal radio pulsars. The peculiar characteristics
of magnetars radio emission should be traced in the complex magnetic geometry
of these sources. Furthermore, we propose that magnetar radio activity or
inactivity can be predicted from the knowledge of the star's rotational period,
its time derivative and the quiescent X-ray luminosity.
