Formation Rates and Evolution Histories of Magnetars

Abstract

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 2$\sigma$ limit on their Galactic formation rate at $2.3-20\mbox{kyr}^{-1}$. 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 $3\times 10^{14}-10^{15}$G, and that those decay on a time-scale of $\sim 10^4$years, implying a maximal magnetar period of $P_{\rm max}\approx 13$s. A sizable fraction of the magnetars' energy is released in outbursts. Giant Flares with $E\geq 10^{46}$ erg are expected to occur in the Galaxy at a rate of $\sim 5\mbox{kyr}^{-1}$. Outside our Galaxy, such flares remain observable by {\it Swift} up to a distance of $\sim 100$~Mpc, implying a detection rate of $\sim 5\mbox{ yr}^{-1}$. 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.