Relativistic Hydrodynamics in Close Binary Systems: Analysis of Neutron-Star Collapse

TL;DR

This paper explains how relativistic effects in close binary neutron star systems lead to star compression and collapse, emphasizing the importance of unconstrained hydrodynamics and velocity-dependent terms.

Contribution

It demonstrates that velocity-dependent relativistic hydrodynamic terms cause neutron star compression in binary systems, a phenomenon overlooked by previous constrained analyses.

Findings

Velocity-dependent terms increase self-gravity and cause compression.

Unconstrained hydrodynamics leads to a nonsynchronized, compressed state.

Tidal effects are less significant than relativistic compression terms.

Abstract

We discuss the underlying relativistic physics which causes neutron stars to compress and collapse in close binary systems as has recently been observed in numerical (3+1) dimensional general relativistic hydrodynamic simulations. We show that compression is driven by velocity-dependent relativistic hydrodynamic terms which increase the self gravity of the stars. They also produce fluid motion with respect to the corotating frame of the binary. We present numerical and analytic results which confirm that such terms are insignificant for uniform translation or when the hydrodynamics is constrained to rigid corotation. However, when the hydrodynamics is unconstrained, the neutron star fluid relaxes to a compressed nonsynchronized state of almost no net intrinsic spin with respect to a distant observer. We also show that tidal decompression effects are much less than the velocity-dependent compression terms. We discuss why several recent attempts to analyze this effect with constrained hydrodynamics or an analysis of tidal forces do not observe compression. We argue that an independent test of this must include unconstrained relativistic hydrodynamics to sufficiently high order that all relevant velocity-dependent terms and their possible cancellations are included.