Mergers of double neutron stars with one high-spin component: brighter kilonovae and fallback accretion, weaker gravitational waves
arxiv(2023)
Abstract
Neutron star (NS) mergers where both stars have negligible spins are commonly
considered as the most likely “standard” case. In globular clusters, however,
the majority of NSs have been spun up to millisecond (ms) periods and, based on
observed systems, we estimate that a non-negligible fraction of all double NS
mergers (∼ 4±2 %) contains one component with a spin of a (few) ms. We
use the Lagrangian numerical relativity code to simulate mergers where one
star has no spin and the other has a dimensionless spin parameter of
χ=0.5. Such mergers exhibit several distinct signatures compared to
irrotational cases. They form only one, very pronounced spiral arm and they
dynamically eject an order of magnitude more mass of unshocked material at the
original, very low electron fraction. One can therefore expect particularly
bright, red kilonovae. Overall, the spinning case collisions are substantially
less violent and they eject smaller amounts of shock-generated
semi-relativistic material. Therefore, the ejecta produce a weaker blue/UV
kilonova precursor signal, but – since the total amount is larger –
brighter kilonova afterglows months after the merger. The spinning cases
also have significantly more fallback accretion and thus could power late-time
X-ray flares. Since the post-merger remnant loses energy and angular momentum
significantly less efficiently to gravitational waves, such systems can delay a
potential collapse to a black hole and are therefore candidates for
merger-triggered gamma-ray bursts with longer emission time scales.
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