Understanding the low magnetic field magnetar, SGR 0418+5279, from a magnetized core model

Abstract

Abstract We consider the newly found low magnetic field magnetar, SGR 0418+5279, which exhibits flares, in the context of a model recently proposed by us in which magnetars owe their strong magnetic fields to a high baryon density, magnetized core. We calculate the characteristic core size which will give rise to a surface polar field of about 1013 G, observed for this magnetar. We then estimate the time of transport of the magnetic field to the crust by ambipolar diffusion, and find this time to be roughly consistent with the spin-down age of SGR 0418+5279. Our model suggests that a precise post-flare timing analysis for this magnetar would show a persistent increase in the spin-down rate of ν˙, as observed, for example, in PSR 1846-0258, and in due course a decrease in the braking index, consistent with a post-flare increase in the surface field.