The early evolution of magnetar rotation - I. Slowly rotating 'normal' magnetars

In the seconds following their formation in core-collapse supernovae, "proto"-magnetars drive neutrino-heated magneto-centrifugal winds. Using a suite of two-dimensional axisymmetric MHD simulations, we show that relatively slowly rotating magnetars with initial spin periods of P-*0 = 50-500 ms spin down rapidly during the neutrino Kelvin-Helmholtz cooling epoch. These initial spin periods are representative of those inferred for normal Galactic pulsars, and much slower than those invoked for gamma-ray bursts and super-luminous supernovae. Since the flow is non-relativistic at early times, and because the Alfven radius is much larger than the proto-magnetar radius, spindown is millions of times more efficient than the typically-used dipole formula. Quasi-periodic plasmoid ejections from the closed zone enhance spindown. For polar magnetic field strengths B-0 greater than or similar to 5 x 10(14) G, the spindown timescale can be shorter than than the Kelvin-Helmholtz timescale. For B-0 greater than or similar to 10(15) G, it is of order seconds in early phases. We compute the spin evolution for cooling proto-magnetars as a function of B-0, P-*0, and mass (M). Proto-magnetars born with B-0 greater than similar or equal to 1.3 x 10(15)G(P-*0/400ms)(-1.4)(M/1.4M(circle dot))(2.2) spin down to periods >1 s in just the first few seconds of evolution, well before the end of the cooling epoch and the onset of classic dipole spindown. Spindown is more efficient for lower M and for larger P-*0. We discuss the implications for observed magnetars, including the discrepancy between their characteristic ages and supernova remnant ages. Finally, we speculate on the origin of 1E 161348-5055 in the remnant RCW 103, and the potential for other ultra-slowly rotating magnetars.