2019MNRAS.487.1426B

We constrain the formation rate of Galactic magnetars directly from observations. Combining spin-down rates, magnetic activity, and association with supernova remnants, we put a 2σ limit on their Galactic formation rate at 2.3¡-¡20 kyr^–1. This leads to a fraction 0.4_–0.28^+0.6 of neutron stars being born as magnetars. We study evolutionary channels that can account for this rate as well as for the periods, period derivatives, and luminosities of the observed population. We find that their typical magnetic fields at birth are 3 x 10¹⁴-10¹⁵ G, and that those decay on a timescale of ∼10⁴ yr, implying a maximal magnetar period of P_max ≃ 13 s. A sizable fraction of the magnetars' energy is released in outbursts. Giant Flares with E >= 10⁴⁶ erg are expected to occur in the Galaxy at a rate of ∼5 kyr^–1. Outside our Galaxy, such flares remain observable by Swift up to a distance of ∼100 Mpc, implying a detection rate of ∼5 yr^–1. The specific form of magnetic energy decay is shown to be strongly tied to the total number of observable magnetars in the Galaxy. A systematic survey searching for magnetars could determine the former and inform physical models of magnetic field decay.