Origin and evolution of magnetars
  We present a population synthesis study of the observed properties of the magnetars, which allows for X-ray selection effects, investigating the hypothesis that they are drawn from a population of progenitors that are more massive than those of the normal radio pulsars. We assume that the anomalous X-ray emission is caused by the decay of a toroidal or tangled up field that does not partake in the spin down of the star.   We find that we can explain the observed properties, such as the period and field distributions and the Period - Period derivative diagram, if we suitably parametrise the time evolution of the anomalous X-ray luminosity as an exponentially decaying function of time.   The magnetic flux of the neutron stars is required to be a strong function of the progenitor mass with the magnetars arising from the mass range 20-45 solar masses.   Unlike with the radio pulsars, the magnetars only weakly constrain the birth spin period, due to their rapid spin-down. Our model predicts a birthrate of about 0.15-0.3 per century. 
Lilia Ferrario and D.T. Wickramasinghe