Neutron star bulk viscosity, "spin-flip" and GW emission of newly born magnetars

TL;DR

This paper re-examines the bulk viscosity and spin-flip instability in newborn magnetars, showing they can emit detectable gravitational waves if certain conditions are met, with implications for multimessenger astronomy.

Contribution

It provides new calculations of bulk viscosity and spin-flip timescales, and estimates GW detectability from newborn magnetars with realistic parameters.

Findings

Bulk viscosity is larger than previously assumed.

Spin-flip occurs for initial spin periods under 2-3 ms in typical magnetars.

Detectable GW signals are possible within 4-30 Mpc depending on detector sensitivity.

Abstract

The viscosity-driven "spin-flip" instability in newborn magnetars with interior toroidal magnetic fields is re-examined. We calculate the bulk viscosity coefficient ($\zeta$) of cold, $npe \mu$ matter in neutron stars (NS), for selected values of the nuclear symmetry energy and in the regime where $\beta$-equilibration is slower than characteristic oscillation periods. We show that: i) $\zeta$ is larger than previously assumed and the instability timescale correspondingly shorter; ii) for a magnetically-induced ellipticity $\epsilon_B \lesssim 4 \times 10^{-3}$, typically expected in newborn magnetars, spin-flip occurs for initial spin periods $\lesssim 2-3$ 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 $\epsilon_B \sim (1-5) \times 10^{-3}$, and birth spin period $\lesssim 2$ ms, we estimate an horizon of $\gtrsim 4$ Mpc, and $\gtrsim 30$ 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.