We perform the first magnetohydrodynamic simulation tracking the magnetosphere of a collapsing magnetar. The collapse is expected for massive rotating magnetars formed in merger events, and it may occur many hours after the merger. The results suggest a novel gamma-ray burst (GRB) scenario, which creates a delayed high-energy counterpart of the merger gravitational waves. The collapse launches an outgoing magnetospheric shock, and a hot magnetized outflow forms behind the shock. The outflow is modulated by the ring-down of the nascent black hole, imprinting its kilohertz quasi-normal modes on the GRB tail.
Comment: 7 pages, 4 figures