Inferring the population properties of binary neutron stars with gravitational-wave measurements of spin

The recent LIGO-Virgo detection of gravitational waves from a binary neutron star inspiral event GW170817 and the discovery of its accompanying electromagnetic signals mark a new era for multimessenger astronomy. In the coming years, advanced gravitational-wave detectors are likely to detect tens to hundreds of similar events. Neutron stars in binaries can possess significant spin, which is imprinted on the gravitational waveform via the effective spin parameter chi(eff). We explore the astrophysical inferences made possible by gravitational-wave measurements of chi(eff). First, using a fiducial model informed by radio observations, we estimate that approximate to 15-30% of binary neutron stars should have spins measurable at greater than or similar to 90% confidence level by advanced detectors assuming the spin axis of the recycled neutron star aligns with the total orbital angular momentum of the binary. Second, using Bayesian inference, we show that it is possible to tell whether or not the spin axis of the recycled neutron star tends to be aligned with the binary orbit using greater than or similar to 30 detections. Finally, interesting constraints can be placed on neutron star magnetic field decay after greater than or similar to 300 detections, if the spin periods and magnetic field strengths of Galactic binary neutron stars are representative of the merging population.