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Abstract The merger of a black hole (BH) and a neutron star (NS) in most cases is expected to leave no material around the remnant BH; therefore, such events are often considered as sources of gravitational waves without electromagnetic counterparts. However, a bright counterpart can emerge if the NS is strongly magnetized, as its external magnetosphere can experience radiative shocks and magnetic reconnection during/after the merger. We use magnetohydrodynamic simulations in the dynamical spacetime of a merging BH–NS binary to investigate its magnetospheric dynamics. We find that compressive waves excited in the magnetosphere develop into monster shocks as they propagate outward. After swallowing the NS, the BH acquires a magnetosphere that quickly evolves into a split-monopole configuration and then undergoes an exponential decay (balding), enabled by magnetic reconnection and also assisted by the ringdown of the remnant BH. This spinning BH drags the split monopole into rotation, forming a transient pulsar-like state. It emits a striped wind if the swallowed magnetic-dipole moment is inclined to the spin axis. We predict two types of transients from this scenario: (1) a fast radio burst emitted by the shocks as they expand to large radii; and (2) an X-ray/γ-ray burst emitted by thee±outflow heated by magnetic dissipation.
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Monster Radiative Shocks in the Perturbed Magnetospheres of Neutron Stars
Abstract Magnetospheres of neutron stars can be perturbed by star quakes, interaction in a binary system, or sudden collapse of the star. The perturbations are typically in the kilohertz band and excite magnetohydrodynamic waves. We show that compressive magnetospheric waves steepen into monster shocks, possibly the strongest shocks in the Universe. The shocks are radiative, i.e., the plasma energy is radiated before it crosses the shock. As the kilohertz wave with the radiative shock expands through the magnetosphere, it produces a bright X-ray burst. Then, it launches an approximately adiabatic blast wave, which will expand far from the neutron star. These results suggest a new mechanism for X-ray bursts from magnetars and support the connection of magnetar X-ray activity with fast radio bursts. Similar shocks may occur in magnetized neutron-star binaries before they merge, generating an X-ray precursor of the merger. Powerful radiative shocks are also predicted in the magnetosphere of a neutron star when it collapses into a black hole, producing a bright X-ray transient.
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- Award ID(s):
- 2009453
- PAR ID:
- 10477310
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 959
- Issue:
- 1
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 34
- Size(s):
- Article No. 34
- Sponsoring Org:
- National Science Foundation
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