Correlation of Burst Behaviour with Magnetar Age

We analyze a wide set of historical magnetar burst observations detected with five different instruments, calibrating these to the energy range of Fermi-GBM observations for consistency. We find a striking correlation between a magnetar's characteristic age and both its typical burst energy and its burst activity level. Arguing that this bursting behaviour also correlates with true age, we interpret it as the result of a reducing high-stress volume of the crust in an aging magnetar: previous giant flares cause relaxation of large regions of its crust and inhibit burst clustering, whilst the reducing burst energy reflects the progressively shallower region of the crust where Hall drift can build stresses effectively, as the field decays through the range $\sim 10^{12}-10^{13}\,\mathrm{G}$. Low-energy bursts from very young magnetars may represent failures of weak regions of the crust that have only recently solidified.