Jet-environment interaction after delayed collapse in binary neutron star mergers

TL;DR

This study uses GRMHD simulations of equal-mass binary neutron star mergers to investigate how delayed collapse of a metastable neutron-star remnant into a black hole shapes jet formation and propagation. By comparing collapse times of $t_{collapse}\\approx 25$ ms and $50$ ms (and a no-collapse magnetar case), the work shows that the pre-collapse outflow environment dramatically influences jet breakout, energetics, and final Lorentz factors, with earlier collapse enabling a faster, cleaner BH-driven jet and later collapse producing a denser cocoon that hampers breakout. The results favor a black-hole central engine for short gamma-ray bursts in BNS mergers and reveal that jet variability and potential electromagnetic precursors can arise from jet–environment interactions during propagation through an evolving magnetized outflow. A key methodological advance is the use of an ultra-low density floor $\\rho_{atm}\\propto r^{-6}$, enabling reliable jet evolution to $\\sim 10^4$ km and robust connection to GRB phenomenology.

Abstract

We present general relativistic magnetohydrodynamic simulations of binary neutron star (BNS) mergers, where the collapse of the metastable massive neutron star (MNS) remnant leads to the production of an incipient jet having terminal Lorentz factor and Poynting-flux luminosity compatible with a short gamma-ray burst (GRB). We consider different MNS lifetimes of about 25 and 50 ms, long enough for massive polar outflows to emerge before black hole (BH) formation. The interaction of the following BH-driven jet with such polar outflows, responsible for shock heating and possible electromagnetic signatures, is self-consistently captured for the first time. Exploiting an unprecedentedly low numerical density floor scaling as r^-6, we explore the jet propagation up to distances of ~10^4 km. Comparing the outcome of different MNS lifetimes, we find that the latter, by strongly affecting the propagation environment, plays a major role in determining the final properties of the escaping jet. Finally, we consider a non-collapsing case, where the MNS-driven outflow is found too dense and slow to be compatible with a GRB jet, thus favoring a BH central engine scenario.

Figures (11)

• Figure 1: 3D rendering of magnetic field-line structure and rest-mass density (red-to-grey colors) for case A, at about $50$ ms (left) and 75 ms (right) after merger. The inset shows a meridional view of the magnetization $b^2/(2\rho)$ at the jet base (vertical scale of up to $\pm 250$ km).
• Figure 2: Evolution of total magnetic energy (top) and Poynting-flux luminosity across the $r\!=\!295$ km spherical surface (bottom), for all cases. The vertical light blue line marks the merger time, whereas vertical black lines mark the collapse time for case A, C, and D, respectively.
• Figure 3: Meridional view of $-u_t$ (left), internal energy density $\rho\epsilon$ (center), and effective specific entropy $s_{\rm eff}$ (right) at $t\!\simeq\!50$ ms after merger for case A ($z\!>\!0$, top) and C ($z\!<\!0$, bottom).
• Figure 4: Meridional view of $-u_t$ at 75 ms after merger for cases A (top-left; $z>0$), B (top-center; $z>0$), C (bottom-left; $z<0$), D (bottom-center; $z<0$), and at 162 ms after merger for case C on larger spatial scales (right).
• Figure 5: Rest-mass density profiles along the z-axis (south side) for cases C and D, at 1 ms or 26 ms after BH formation.