GW170817, General Relativistic Magnetohydrodynamic Simulations, and the Neutron Star Maximum Mass (1711.00473v3)

Abstract: Recent numerical simulations in general relativistic magnetohydrodynamics (GRMHD) provide useful constraints for the interpretation of the GW170817 discovery. Combining the observed data with these simulations leads to a bound on the maximum mass of a cold, spherical neutron star (the TOV limit): ${M_{\rm max}^{\rm sph}}\lesssim 2.74/\beta$, where $\beta$ is the ratio of the maximum mass of a uniformly rotating neutron star (the supramassive limit) over the maximum mass of a nonrotating star. Causality arguments allow $\beta$ to be as high as $1.27$, while most realistic candidate equations of state predict $\beta$ to be closer to $1.2$, yielding ${M_{\rm max}^{\rm sph}}$ in the range $2.16-2.28 M_\odot$. A minimal set of assumptions based on these simulations distinguishes this analysis from previous ones, but leads to a similar estimate. There are caveats, however, and they are enumerated and discussed. The caveats can be removed by further simulations and analysis to firm up the basic argument.

• The paper uses GRMHD simulations to constrain the TOV mass limit of nonrotating neutron stars between 2.16 and 2.28 M⊙ using GW170817 data.
• It demonstrates that delayed collapse of a hypermassive neutron star supports jet formation driven by amplified magnetic fields crucial for sGRB production.
• The findings refine neutron star equation-of-state constraints and motivate further studies on spin configurations, mass ratios, and remnant collapse dynamics.

Overview of "GW170817, General Relativistic Magnetohydrodynamic Simulations, and the Neutron Star Maximum Mass"

This research paper explores the consequences of the gravitational wave event GW170817 on the understanding of neutron star masses, employing general relativistic magnetohydrodynamic (GRMHD) simulations. The key focus is on constraining the maximum mass of a cold, nonrotating neutron star (often referred to as the Tolman-Oppenheimer-Volkoff or TOV limit) by integrating observational data from GW170817 with numerical simulations.

Theoretical Framework and Assumptions

The paper uses the framework of GRMHD simulations to model the post-merger remnant of the binary neutron star (NSNS) system responsible for GW170817. The primary assumptions include:

1. GW170817 originates from an NSNS merger, excluding the presence of a low-mass black hole due to weak observational evidence.
2. The resulting remnant forms a hypermassive neutron star (HMNS) that undergoes delayed collapse, forming a spinning black hole.
3. The short gamma-ray burst (sGRB) associated with GW170817 is powered by the accretion of matter onto this black hole, leading to a magnetically driven jet.
4. A collimated jet formation, necessary for sGRB production, occurs as a result of the strong magnetic fields.

GRMHD Simulation Insights

The simulations explore how different scenarios for the NSNS merger remnant and describe a set of conditions under which the sGRB phenomena observed in GW170817 can occur. Specifically, they favor scenario (2a) identified in the paper, where the HMNS experiences delayed collapse into a black hole, setting up conditions for jet formation. The RMS simulations show that:

• A significant Poynting flux can be generated when a jet forms, with the conditions being met when the poloidal magnetic fields are sufficiently amplified.
• They posit that prompt collapse immediately after merger does not support jet formation due to the absence of necessary magnetic and poloidal structures.

Implications for Neutron Star Maximum Mass

Using the GRMHD simulations and invoking causality arguments, the paper derives upper bounds for the TOV limit. With causality allowing the parameter $\beta$ (the ratio of the maximum mass for a uniformly rotating neutron star to a nonrotating one) to be as high as 1.27, the authors calculate the TOV mass limit to be between $2.16$ and $2.28 M_\odot$, considering realistic equations of state predictions. Such constraints are significant since they shape our understanding of the neutron star’s composition at extreme densities.

Discussion and Future Directions

The findings have significant implications for the neutron star equation of state and the physics of neutron star mergers. The analysis emphasizes the importance of certain GRMHD simulation parameters like initial magnetic fields and demands further investigation to refine these limits. Moreover, the work highlights potential future avenues such as incorporating detailed studies of different spin configurations, mass ratios, and realistic nuclear equations of state. These investigations will be crucial for addressing current caveats, such as uncertainties in $\beta$ and the specifics of remnant collapse scenarios. Further exploration of these factors could enhance our understanding of the limits on neutron star masses and the mechanism behind sGRB generation.

In conclusion, this paper contributes valuable insights into the astrophysical processes following NSNS mergers, while narrowing constraints on the maximum mass of neutron stars. The combination of observational data from GW170817 with advanced simulations exemplifies the evolving approach in multimessenger astrophysics and the quest for understanding the fundamental properties of dense matter.