# Introduction

## Introduction

The LIGO and Virgo Collaborations recently reported the first direct detection of a gravitational-wave (GW) signal and demonstrated that it was produced by the inspiral and coalescence of a binary black hole (BHBH) system (Abbot et al. 2016). This breakthrough marks the beginning of the era of GW astronomy. GW signals are expected to be generated not only by BHBH binaries but also by neutron star--neutron star (NSNS) and black hole--neutron star (BHNS) binaries.

Merging NSNSs and BHNSs are not only important sources of GWs but also the two most popular candidate progenitors of short gamma-ray bursts (sGRBs) (Eichler et al. 1989; Narayan et al. 1992; Mochkovitch et al. 1993). NSNSs and BHNSs may also generate other detectable, transient electromagnetic (EM) signals prior to (Hansen & Lyutikov 2001; McWilliams & Levin 2011; Paschalidis et al. 2013; Palenzuela et al. 2013; Ponce et al. 2014) and following (Metzger & Berger 2012; Metzger et al. 2015) merger. Combining GW and EM signals from these mergers could test relativistic gravity and constrain the NS equation of state (EOS). Moreover, association of a GW event with an sGRB (the holy grail of "multimessenger astronomy") would provide convincing evidence for the compact binary coalescence model. However, the interpretation of EM and GW signals from such mergers will rely on theoretical understanding of these events, which requires simulations in full general relativity (GR) to treat the strong dynamical fields and high velocities arising in these scenarios. There have been multiple studies of compact binary mergers. [See textbook by Baumgarte & Shapiro (2010) for a review; for NSNSs, see Faber & Rasio (2012) and Paschalidis et al. (2012); Gold et al. (2012); East & Pretorius (2012); Neilsen et al. (2014); Dionysopoulou et al. (2015); Sekiguchi et al. (2015); Dietrich et al. (2015); and Palenzuela et al. (2015) for recent results.] These earlier studies have advanced our knowledge of EOS effects, neutrino transport, ejecta properties, and magnetospheric phenomena. However, few studies focused on the potential for NSNSs to power sGRBs.

Recent work by Paschalidis et al. (2015b) (hereafter PRS) demonstrated that mergers of magnetized BHNS systems can launch jets and be the engines that power sGRBs. The key ingredient for generating a jet was found to be the initial endowment of the NS with a dipole B field that extends into the NS exterior, as in a pulsar magnetosphere. By contrast, if the initial magnetic field is confined to the interior of the NS, no jet is observed (Etienne et al. 2012b; Kiuchi et al. 2015).

The primary motivation for this paper is to answer the question: Can NSNS mergers produce jets in the same way as BHNS systems, or does this mechanism require an initial BH? Recently, it was shown that neutrino annihilation may not be strong enough to power jets (Just et al. 2015), so MHD processes must play a major role for jet formation. Previous ideal GRMHD simulations by Rezzolla et al. (2011) suggest that NSNS mergers may launch a relativistic jet, while those by Kiuchi et al. (2014) focusing on different initial configurations, show otherwise. Both of these studies have considered only scenarios where the B field is initially confined to the interior of the two NSs.

Here we describe the results of ideal GRMHD simulations of NSNSs in which we follow PRS and allow an initially strong, but dynamically unimportant, dipole B field to extend from the interior of the NSs into the exterior. We call this configuration the pulsar model (hereafter P). The existence of pulsars suggests that this may be the astrophysically most common case. As in PRS, we define an incipient jet as a collimated, mildly relativistic outflow which is at least partially magnetically dominated ($b^2/(2\rho_0)>1$, where $b^2 = B^2/4 \pi$ and $\rho_0$ is the rest-mass density). We find that the P configuration launches an incipient jet. To study the impact of the initial B-field geometry and to compare with previous studies, we also perform simulations where the field is confined to the interior of the NSs (hereafter the I model), keeping its strength at the stellar center the same as in the P case. In contrast to BHNS systems, we find that interior-only initial B fields also lead to jet formation in NSNSs. Throughout this work, geometrized units ($G=c=1$) are adopted unless otherwise specified.

Simulations were performed on the Blue Waters supercomputer at UIUC. The Illinois GRMHD code, which implements the BSSN formulation of GR with moving box adaptive mesh refinement, was used for all simulations.

To identify references, see arXiv:1604.02455