On the jet-ejecta interaction in 3D GRMHD simulations of binary neutron star merger aftermath

Abstract

Short γ-ray burst (sGRB) jets form in the aftermath of a neutron star merger, drill through disk winds and dynamical ejecta, and extend over 4-5 orders of magnitude in distance before breaking out of the ejecta. We present the first 3D general-relativistic magnetohydrodynamic sGRB simulations to span this enormous scale separation. They feature three possible outcomes: jet+cocoon, cocoon, and neither. Typical sGRB jets break out of the dynamical ejecta if: (i) the bound ejecta isotropic equivalent mass along the pole at the time of the BH formation is 10 −4 M , setting a limit on the delay time between the merger and BH formation, otherwise the jets perish inside the ejecta and leave the jet-inflated cocoon to power a low-luminosity sGRB; and (ii) postmerger remnant disk contains strong large-scale vertical magnetic field, 10 15 G; and (iii) if the jets are weak ( 10 50 erg), the ejecta isotropic equivalent mass along the pole must be small ( 10 −2 M ). Generally, the jet structure is shaped by the early interaction with disk winds rather than the dynamical ejecta. As long as our jets break out of the ejecta, they retain a significant magnetization ( 1), suggesting that magnetic reconnection is a fundamental property of sGRB emission. The angular structure of the outflow isotropic equivalent energy after breakout consistently features a flat core followed by a steep power-law distribution (slope 3), similar to hydrodynamic jets. In the cocoon-only outcome, the dynamical ejecta broadens the outflow angular distribution and flattens it (slope ∼ 1.5).

Black Hole to Photosphere: 3D GRMHD Simulations of Collapsars Reveal Wobbling and Hybrid Composition Jets

“…Thus, the jet structure is a hybrid composition of mildly magnetized and thermal parts, whereas its extended (radial and angular) structure is remarkably consistent with those found in hydrodynamic jets. In a companion paper, Gottlieb et al (2022b), we show that the picture is different for short GRBs jets that propagate in light ejecta. Those jets are subject to weak mixing and thus can retain σ  1 at the photosphere.…”

The Structure of Gamma Ray Burst Jets

“…On top of this, the idealized nature of the progenitor models employed in most of these studies can affect the results by introducing exact symmetries that are not present in nature. The steady advancement of computational methods and resources has led recently to many important works that investigated some of these limitations (e.g., [144][145][146][147][148][149][150][151][152]). These include three-dimensional GRMHD simulations that self-consistently cover jet launch (usually within the Blandford-Znajek paradigm), propagation and breakout [150][151][152], even though these still feature idealized initial conditions and do not include a treatment of neutrinos, whose contribution to cooling, transport of momentum and energy can have prominent effects on the central regions.…”

“…The steady advancement of computational methods and resources has led recently to many important works that investigated some of these limitations (e.g., [144][145][146][147][148][149][150][151][152]). These include three-dimensional GRMHD simulations that self-consistently cover jet launch (usually within the Blandford-Znajek paradigm), propagation and breakout [150][151][152], even though these still feature idealized initial conditions and do not include a treatment of neutrinos, whose contribution to cooling, transport of momentum and energy can have prominent effects on the central regions. Yet, these simulations currently constitute some of the most detailed investigations that can shed light on the GRB jet structure.…”

Observational characteristics of radiation-mediated shocks in photospheric gamma-ray burst emission

“…Our model assumes negligible magnetic fields in the region where the RMS occurs. As magnetic fields likely play a crucial role in the launching of the jet, this assumption depends on how quickly the magnetic field is dissipated (e.g., Gottlieb et al 2022). If there exists moderate magnetization across the RMS transition layer, a collisionless sub-shock can form in the immediate downstream (Lundman & Beloborodov 2019).…”