# Binary neutron star mergers: Effects of spin and post-merger dynamics

## TL;DR

This study systematically explores how neutron star spin affects the gravitational wave and electromagnetic signals from binary neutron star mergers, revealing weak influence on post-merger frequency but significant effects on ejecta and disk mass.

## Contribution

It provides the first detailed analysis of neutron star spin effects up to a dimensionless spin of 0.33 using fully general-relativistic simulations.

## Key findings

- Post-merger gravitational wave frequency is weakly affected by spin.
- Spin influences the ejecta mass, velocity, and angular distribution.
- The one-arm instability's occurrence depends on spin orientation.

## Abstract

Spin can have significant effects on the electromagnetic transients accompanying binary neutron star mergers. The measurement of spin can provide important information about binary formation channels. In the absence of a strong neutron star spin prior, the degeneracy of spin with other parameters leads to significant uncertainties in their estimation, in particular limiting the power of gravitational waves to place tight constraints on the nuclear equation of state. Thus detailed studies of highly spinning neutron star mergers are essential to understand all aspects of multimessenger observation of such events. We perform a systematic investigation of the impact of neutron star spin---considering dimensionless spin values up to $a_{\rm NS}=0.33$---on the merger of equal mass, quasicircular binary neutron stars using fully general-relativistic simulations. We find that the peak frequency of the post-merger gravitational wave signal is only weakly influenced by the neutron star spin, with cases where the spin is aligned (antialigned) with the orbital angular momentum giving slightly lower (higher) values compared to the irrotational case. We find that the one-arm instability arises in a number of cases, with some dependence on spin. Spin has a pronounced impact on the mass, velocity, and angular distribution of the dynamical ejecta, and the mass of the disk that remains outside the merger remnant. We discuss the implications of these findings on anticipated electromagnetic signals, and on constraints that have been placed on the equation of state based on multimessenger observations of GW170817.