The mergers of neutron star (NS) binary systems are multi-messenger events and provide a wide range of observables, which can be used to explore and constrain the incompletely known Equation of State (EoS) at very high densities as well as stellar properties of neutron stars. These events can result in the formation of a black-hole (BH) due to a gravitational collapse.
For a sufficiently high total binary mass the gravitational collapse to a BH occurs immediately after merging.
This is called a prompt collapse and occcurs if the total mass exceeds a so called threshold mass.
The threshold mass depends on the EoS and parameters of the binary systems like the mass ratio and intrinsic spin.
In NS merger simulations the NSs are often assumed to be irrotational since their spin period are considered slow compared to the orbital period at merging of about 2ms.
But spins of NSs in binary system have been measured in the range of a few tens of milliseconds and therefore should also be included in simulations to study the effect of such spins.
The rotation of the NSs in binary systems changes the total angular momentum of the system depending on the orientation of the spin angular momentum relative to the orbital angular momentum.
Spin of NSs can have an significant impact on the observables and dynamics of the merger like the gravitational wave (GW) signal, dynamical ejecta, kilonova emission as well as properties of the remnant and thus also affect the threshold mass.
To systematically investigate this influence on the threshold mass we discuss simulations of NS mergers with multiple spin configurations for equal-mass systems and different EoSs.
We find a linear dependence of the threshold mass on the spin and observe an increase of 0.2-0.25 solar masses for the considered parameter space.