Mergers of double neutron stars with one high-spin component: brighter kilonovae and fallback accretion, weaker gravitational waves

Understand

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 ($\sim 4\pm2\;\%$) contains one component with a spin of a (few) ms.
• We use the Lagrangian numerical relativity code \SpB to simulate mergers where one star has no spin and the other has a dimensionless spin parameter of $\chi=0.5$.
• Such mergers exhibit several distinct signatures compared to irrotational cases.