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

Simone Dall'Osso; Luigi Stella; Cristiano Palomba

doi:10.1093/mnras/sty1706

paper

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

arXiv:1806.11164 · doi:10.1093/mnras/sty1706

Abstract

The viscosity-driven "spin-flip" instability in newborn magnetars with interior toroidal magnetic fields is re-examined. We calculate the bulk viscosity coefficient ( $ζ$ ) of cold, $n p e μ$ 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 timescale correspondingly shorter; ii) for a magnetically-induced ellipticity $ε_{B} ≲ 4 \times 1 0^{- 3}$ , 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 $ε_{B} \sim (1 - 5) \times 1 0^{- 3}$ , and birth spin period $≲ 2$ ms, we estimate an horizon of $≳ 4$ Mpc, 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.

10 pages, 4 figures, accepted for publication in MNRAS

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

Abstract

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