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 Alfvén 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 (cid:38) 5 × 10 14 G, the spindown timescale can be shorter than than the Kelvin-Helmholtz timescale. For 𝐵 0 (cid:38) 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 (cid:39) 1 . 3 × 10 15 G ( 𝑃 ★ 0 / 400 ms ) − 1 . 4 ( 𝑀 / 1 . 4 M (cid:12) ) 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.