2018MNRAS.480.1353D

The viscosity-driven 'spin-flip' instability in newly born magnetars with interior toroidal magnetic fields is re-examined. We calculate the bulk viscosity coefficient (ζ) of cold, npeµ matter in neutron stars (NS), for selected values of the nuclear symmetry energy and in the regime where β-equilibration is slower than characteristic oscillation periods. We show that: (i) ζ is larger than previously assumed and the instability time-scale correspondingly shorter; (ii) for a magnetically induced ellipticity ε_B <= 4 x 10^–3, typically expected in newborn magnetars, spin-flip occurs for initial spin periods <=2-3 ms, with some dependence on the NS equation of state (EoS). We then calculate the detectability of GW signals emitted by newborn magnetars subject to 'spin-flip', by accounting also for the reduction in range resulting from realistic signal searches. For an optimal range of ε_B ∼ (1 - 5) x 10^–3, and birth spin period <=2 ms, we estimate an horizon of >=4, and >=30 Mpc, for Advanced and third generation interferometers at design sensitivity, respectively. A supernova (or a kilonova) is expected as the electromagnetic counterpart of such GW events. Outside of the optimal range for GW emission, EM torques are more efficient in extracting the NS spin energy, which may power even brighter EM transients.