First 100 ms of a long-lived magnetized neutron star formed in a binary neutron star merger

Abstract

The recent multimessenger observation of the short gamma-ray burst (SGRB) GRB 170817A together with the gravitational wave (GW) event GW170817 provides evidence for the long-standing hypothesis associating SGRBs with binary neutron star (BNS) mergers. The nature of the remnant object powering the SGRB, which could have been either an accreting black hole (BH) or a long-lived magnetized neutron star (NS), is, however, still uncertain. General relativistic magnetohydrodynamic (GRMHD) simulations of the merger process represent a powerful tool to unravel the jet launching mechanism, but so far most simulations focused the attention on a BH as the central engine, while the long-lived NS scenario remains poorly investigated. Here, we explore the latter by performing a GRMHD BNS merger simulation extending up to ∼100 ms after merger, much longer than any previous simulation of this kind. This allows us to (i) study the emerging structure and amplification of the magnetic field and observe a clear saturation at magnetic energy Emag ∼ 10 51 erg, (ii) follow the magnetically supported expansion of the outer layers of the remnant NS and its evolution into an ellipsoidal shape without any surrounding torus, and (iii) monitor density, magnetization, and velocity along the axis, observing no signs of jet formation. We also argue that the conditions at the end of the simulation disfavor later jet formation on subsecond timescales if no BH is formed. Furthermore, we examine the rotation profile of the remnant, the conversion of rotational energy associated with differential rotation, the overall energy budget of the system, and the evolution of the GW frequency spectrum. Finally, we perform an additional simulation where we induce the collapse to a BH ∼70 ms after merger, in order to gain insights on the prospects for massive accretion tori in case of a late collapse. We find that a mass around ∼0.1 M remains outside the horizon, which has the potential to power a SGRB via the Blandford-Znajek mechanism when accreted.1 Hereafter, long-lived refers to remnants with lifetime 100 ms.

Collimated outflows from long-lived binary neutron star merger remnants

“…We note, however, that starting from ∼ 40 ms after merger the dominant amplification mechanism (i.e. the MRI) is well resolved, giving us confidence that the following magnetohydrodynamic evolution is not severely affected by the lack of resolution and that our results are, at least qualitatively, reliable (see analogous case discussed in Ciolfi et al 2019). An important aspect common to both models is the absence of an accretion disk around the massive NS remnant.…”

“…The alternative scenario, based on a long-lived NS remnant as SGRB central engine, remained instead largely unexplored until Ciolfi et al (2017) started a first systematic investigation. Along the same line, a new simulation extending up to ∼ 100 ms after merger was recently presented in Ciolfi et al (2019). No signs of jet formation were found, nor favourable indications that a jet would be launched at later times.…”

“…Numerical codes, methods, and setup are the same adopted in Ciolfi et al (2017Ciolfi et al ( , 2019, with the following exceptions. First, we extended the computational domain up to ∼ 3400 km along all axes.…”

“…In Fig. 3 we show the magnetic field structure between 150 and 250 ms after merger, together with representative isodensity surfaces (see Ciolfi et al 2019;Kawamura et al 2016 for details on magnetic field line visualization). Along the spin axis, a jet-like helical structure emerges from the dense environment surrounding the massive NS remnant.…”

Magnetic, thermal and rotational evolution of isolated neutron stars

“…A different mechanism is based on the interplay between compression and convection in the hot-bubble region between the proto-NS and the stalled shock (Obergaulinger et al 2015). Another relatively recent idea gaining popularity is the formation during a NS-NS merger (Ciolfi et al 2019). In any case, all the viable mechanisms involve some degree of turbulence, so that the outcome is plausibly different from a perfectly ordered dipole.…”