Binary Neutron Star Mergers: A Jet Engine for Short Gamma-Ray Bursts

Abstract

We perform magnetohydrodynamic simulations in full general relativity (GRMHD) of quasi-circular, equal-mass, binary neutron stars that undergo merger. The initial stars are irrotational, = 1 polytropes and are magnetized. We explore two types of magnetic-field geometries: one where each star is endowed with a dipole magnetic field extending from the interior into the exterior, as in a pulsar, and the other where the dipole field is initially confined to the interior. In both cases the adopted magnetic fields are initially dynamically unimportant. The merger outcome is a hypermassive neutron star that undergoes delayed collapse to a black hole (spin parameter/ ~ 0.74) immersed in a magnetized accretion disk. About 4000 ~ 60(/1.625) ms following merger, the region above the black hole poles becomes strongly magnetized, and a collimated, mildly relativistic outflow-an incipient jet-is launched. The lifetime of the accretion disk, which likely equals the lifetime of the jet, is Δ ~ 0.1 (/1.625) s. In contrast to black hole-neutron star mergers, we find that incipient jets are launched even when the initial magnetic field is confined to the interior of the stars.

The Fate of Neutron Star Binary Mergers

“…Binary NS mergers are also some of the best candidates for short gamma-ray bursts (SGRBs; Nakar 2007), which is supported by their presence in old stellar environments, nondetection of an associated SN, and large offsets from their host galaxies (Berger 2014). The two main scenarios for generating the central engine are then the formation of a strongly magnetized torus around a spinning black hole (BH; e.g., Popham et al 1999;Lee & Ramirez-Ruiz 2007;Rezzolla et al 2011;Ruiz et al 2016) or the formation of a long-lived magnetar (e.g., Duncan & Thompson 1992;Dai et al 2006;Rowlinson et al 2013). In addition, post merger, binary NSs have been suggested for a variety of electromagnetic transients that have not been conclusively confirmed.…”

Radio Constraints on Long-Lived Magnetar Remnants in Short Gamma-Ray Bursts

“…Accretion onto the black hole then powers a relativistic transient, a short-duration gamma-ray burst (GRB; Narayan et al 1992;Ruffert & Janka 1999;Aloy et al 2005;Rezzolla et al 2011;Berger et al 2013;Tanvir et al 2013;Berger 2014;Ruiz et al 2016), with a prompt gamma-ray emission duration of  2 s. One of the biggest uncertainties in this canonical picture is how long the NS remnant survives prior to collapse. This depends on the mass of the final remnant and the highly uncertain Equation of State (EOS) of dense nuclear matter Lasky et al 2014;Fryer et al 2015;Lawrence et al 2015).…”

What can we learn from GRBs?

“…bursts within the IGC paradigm [6,[38][39][40], and NS-NS (or NS-BH) binaries for short bursts, as widely accepted and confirmed by strong observational and theoretical evidences [12][13][14][15][16][17][18][19][20][21][22]. These paradigms have led to the classification of GRBs in seven different sub-classes (see figure 1).…”

“…Short bursts are associated to NS-NS or BH-NS mergers [12][13][14][15][16][17][18][19][20][21][22]: their host galaxies are of both early-and latetype, their localization with respect to the host galaxy often indicates a large offset [23][24][25][26][27][28][29] or a location of minimal star-forming activity with typical circumburst medium (CBM) densities of ∼ 10 −5 -10 −4 cm −3 , and no supernovae (SNe) have ever been associated to them.…”