Introduction

Introduction

Black hole-neutron star (BHNS) binaries are promising sources of detectable gravitational waves (GWs) by ground based laser interferometers such as aLIGO, VIRGO, GEO, and KAGRA. Moreover, mergers of BHNSs have ben suggested as the central engines that power short-hard gamma ray bursts (sGRBs). The GW signal from the inspiral and merger, as well as possible radioactively powered electromagnetic (EM) signals from BHNSs and the effect of different nuclear equations of state, have been explored in full general relativity (GR). Studies of magnetized BHNS mergers in full GR magnetohydrodynamics (MHD) also have been carried out. Magnetospherically-powered precursor EM signals from BHNS systems have been simulated as well.

While all these studies have made great leaps forward in modeling BHNSs and advancing our theoretical understanding, prior to this study there were no self-consistent calculation in full GR that starts from the late BHNS inspiral and demonstrates that jets can be launched after NS tidal disruption. This step is crucial to establishing BHNS systems as viable central engines for sGRBs and solidifying their role as multimessenger systems.

In early GRMHD studies of BHNS mergers the B-fields were confined to the NS interior. However, NSs are expected to be endowed with dipole B-fields that extend into the NS exterior (as is required by current theories of pulsar magnetospheres). A more realistic initial configuration for a magnetized BHNS merger should reflect this theory. This initial configuration introduces two new features: poloidal B-field lines attached to fluid elements thread the BH prior to tidal disruption, and following disruption, while the B-field in the disk remains predominantly toroidal, the initially poloidal B-field in the exterior maintains a strong poloidal component threading the low-density debris and tying the disk to the BH poles.

We perform ideal GRMHD simulations of BHNS systems and show that they can launch incipent jets provided the NS is initially endowed with a dipolar B-field extending into the exterior.