Relativistic Simulations of Black Hole–neutron Star Coalescence: The Jet Emerges

Abstract

We perform magnetohydrodynamic simulations in full general relativity (GRMHD) of a binary black hole-neutron star on a quasicircular orbit that undergoes merger. The binary mass ratio is 3 : 1, the black hole initial spin parameter a/m = 0.75 (m is the black hole Christodoulou mass) aligned with the orbital angular momentum, and the neutron star is an irrotational Γ = 2 polytrope. About two orbits prior to merger (at time t = t B ), we seed the neutron star with a dynamically weak interior dipole magnetic field that extends into the stellar exterior. At t = t B the exterior has a low-density atmosphere with constant plasma parameter β ≡ P gas /P mag . Varying β at t B in the exterior from 0.1 to 0.01, we find that at a time ∼ 4000M ∼ 100(M NS /1.4M )ms [M is the total (ADM) mass] following the onset of accretion of tidally disrupted debris, magnetic winding above the remnant black hole poles builds up the magnetic field sufficiently to launch a mildly relativistic, collimated outflowan incipient jet. The duration of the accretion and the lifetime of the jet is ∆t ∼ 0.5(M NS /1.4M )s. Our simulations furnish the first explicit examples in GRMHD which show that a jet can emerge following a black hole -neutron star merger.

Short gamma-ray bursts from binary neutron star mergers: the time-reversal scenario

“…During the short ( 1 s) accretion phase a relativistic jet can be launched via different mechanisms (involving neutrino annihilation and/or magnetic fields), finally producing the SGRB. This picture is supported by recent numerical simulations ( [9,10] and references therein), which have shown that a massive (∼ 0.1 M ) accretion torus is naturally formed when the merger leads to the prompt formation of a spinning BH.…”

Neutron-Star Merger Ejecta as Obstacles to Neutrino-Powered Jets of Gamma-Ray Bursts

“…The GRB is understood as consequence of highly collimated, ultrarelativistic polar outflows or jets of low-density plasma or Poynting flux, whose energy is partly converted to γ-rays at distances of 10 12 -10 13 cm, far away from the compact remnant (e.g., Piran 2004). The annihilation of neutrinoantineutrino pairs radiated by the hot accretion torus as possible energy sources to drive these jets, is discussed (e.g., Eichler et al 1989;Jaroszynski 1993;Woosley 1993;Popham et al 1999;Ruffert & Janka 1999;Di Matteo et al 2002;Birkl et al 2007;Dessart et al 2009;Zalamea & Beloborodov 2011), also considering the energy release by magnetohydrodynamic effects, Poynting flux (e.g., Blandford & Znajek 1977;McKinney & Blandford 2009;Paschalidis et al 2015), and electron-positron pair production (Usov 1992) associated with ultra-strong magnetic fields threading the BHtorus system or MNS and their environments, as possible energy sources.…”

“…For example, the emergence of a jet-favorable magnetic field configuration in NS-NS/BH mergers (Dionysopoulou et al 2015;Kiuchi et al 2015;Paschalidis et al 2015), the efficiency of neutrino production in accretion disks and tori with steadystate accretion rates (e.g., Popham et al 1999;Di Matteo et al 2002;Chen & Beloborodov 2007), the efficiency of neutrino-antineutrino (nn-) annihilation above the poles of a BH (e.g., Birkl et al 2007;Zalamea & Beloborodov 2011;Richers et al 2015), or the jet acceleration and collimation by the interaction with the accretion torus, non-relativistic winds (Aloy et al 2005;Murguia-Berthier et al 2014), and the ejecta from dynamical NS-NS mergers (Nagakura et al 2014;Duffell et al 2015), have all been studied. Also, time-dependent and self-consistent hydrodynamic simulations of BH-torus remnants including some neutrino treatment have been performed to study neutrino emission and annihilation (Ruffert & Janka 1999;Setiawan et al 2004Setiawan et al , 2006Fernández & Metzger 2013;Foucart et al 2015;Just et al 2015a).…”

One-arm spiral instability in hypermassive neutron stars formed by dynamical-capture binary neutron star mergers

“…By combining GW and EM signals from NSNSs one can in principle test relativistic gravity and constrain the behavior of matter at super-nuclear densities. Furthermore, NSNS mergers may offer explanations to long-standing astrophysical puzzles, such as the nature of short-hard gamma ray burst progenitors [14][15][16], and the origin of r-process elements [17]. …”