Neutron star bulk viscosity, "spin-flip" and GW emission of newly born magnetars
Date Issued
2018
Author(s)
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.
Volume
480
Issue
1
Start page
1353
Issn Identifier
0035-8711
Ads BibCode
2018MNRAS.480.1353D
Rights
open.access
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