Magnetized Hypermassive Neutron-Star Collapse: A Central Engine for Short Gamma-Ray Bursts

A hypermassive neutron star (HMNS) is a possible transient formed after the merger of a neutron-star binary. In the latest axisymmetric magnetohydrodynamic simulations in full general relativity, we find that a magnetized HMNS undergoes “delayed” collapse to a rotating black hole (BH) as a result of angular momentum transport via magnetic braking and the magnetorotational instability. The outcome is a BH surrounded by a massive, hot torus with a collimated magnetic field. The torus accretes onto the BH at a quasisteady accretion rate $\sim 10M_{\odot }/\mathrm{s}$; the lifetime of the torus is $\sim 10\mathrm{ms}$. The torus has a temperature $\gtrsim {10}^{12}\mathrm{K}$, leading to copious ($\nu \overline{\nu }$) thermal radiation that could trigger a fireball. Therefore, the collapse of a HMNS is a promising scenario for generating short-duration gamma-ray bursts and an accompanying burst of gravitational waves and neutrinos.

Figure 1

Upper four panels: Snapshots of the density contours for $\rho$ (solid curves) and velocity vectors. The contours are drawn for $\rho ={10}^{15–0.4i}\mathrm{g}/{\mathrm{cm}}^3$($i=0–9$). In the last panel, a curve with $\rho ={10}^{11}\mathrm{g}/{\mathrm{cm}}^3$ is also drawn. The (red) circle near the center in the last two panels denotes an apparent horizon. The scale of the velocity is indicated in the upper left corner. The lower four panels denote the magnetic field (contours of the toroidal component of the vector potential $A_{\phi }$) at the same times as the upper panels. The solid contour curves are drawn for $A_{\phi }=0.8(1–0.1i)A_{\phi ,\max ,0}$ ($i=0–9$) and the dotted curves are for $A_{\phi }=0.08(1–0.2i)A_{\phi ,\max ,0}$ ($i=1–4$). Here, $A_{\phi }=A_{\phi ,\max ,0}$ is the maximum value of $A_{\phi }$ at $t=0$.

Figure 2

Evolution of the surface density and averaged thermal energy per nucleon of the torus as functions of cylindrical radius at $t/P_c=33.4$ (dashed curves), 39.3 (long-dashed curves), 44.5 (dotted-dashed curves), and 49.9 (solid curves).