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.