Numerical simulations of jet launching and breakout from collapsars

TL;DR

This study addresses how relativistic jets are launched in collapsars by self-consistently coupling accretion dynamics to jet launching via the Blandford–Znajek mechanism. Using axisymmetric (2.5-D) GRMHD simulations with a Kerr BH of spin $a\sim0.9$, the authors model three pre-collapse progenitors and vary peak magnetic field strengths $B_0$ in the range $10^{12}$–$10^{14}$ G and field geometries to follow jet formation from horizon to breakout at $R_\star\sim10^{11}$ cm. They find that jets form only for strong dipolar fields, achieving magnetic flux $\Phi_{BH}\gtrsim10^{25}$ G cm$^2$ and entering a magnetically arrested-disk state with $\phi_{MAD}\gtrsim15$, producing jet luminosities $L_j\sim5\times10^{49}$–$7\times10^{52}$ erg s$^{-1}$ and breakout times $t_{bo}\sim1.5$–$3.5$ s; the inner magnetized core and surrounding wings, along with the progenitor density structure, determine the final jet morphology and energy partition. The results illustrate the critical influence of the progenitor's inner density gradient and magnetic topology on jet dynamics and have implications for prompt emission and afterglow predictions, while also highlighting the need for fully 3D modeling and neutrino physics to capture turbulence and jet wobbling.

Abstract

Collapsing stripped-envelope massive stars are progenitors of Long Gamma-Ray Bursts (LGRBs). Powerful relativistic jets drill through the stellar envelope before the gamma emission. Previous hydrodynamical studies imposed jets artificially, neglecting accretion dynamics, while the central engine simulations have reproduced jet launching via the Blandford-Znajek mechanism focusing on the inner core regions. However, both the central engine and the progenitor structure are crucial to determining the jet's evolution. In this study, we present axisymmetric (2.5-D) GRMHD simulations that self-consistently follow jet formation from the black-hole horizon to breakout at the stellar surface ($R_\star \sim 10^{11}$~cm). The setup assumes a Kerr black hole with spin $a \sim 0.9$ in the centre of three progenitor models, varying the magnetic-field strength and geometry. Relativistic jets are successfully launched by a strong dipolar magnetic field ($B_0 \gtrsim 10^{12}$-$10^{14}$~G) from magnetically arrested disks. These jets, initially magnetically dominated, convert energy into thermal and kinetic during their propagation. We found breakout times within $1.5 \lesssim t_{\rm bo} \lesssim 3.5$~s and luminosities $L_j \sim 5\times10^{49}-7\times10^{52}$~erg\,s$^{-1}$. Our results highlight the role of the initial magnetic field strength and its geometry, emphasizing the progenitor's density distribution as a key factor impacting the final structure and dynamics of LGRB jets.

Figures (11)

• Figure 1: A comparison of the density profiles of the progenitor stars. The three models are characterised by three key regions where density changes abruptly: the central region ($r\lesssim 10^8$ cm), from the boundary of the core until $r\sim 10^9$ cm, and the drop near the stellar surface.
• Figure 2: Initial distribution of magnetisation $\sigma$ at $t=0$. The magnetic field has a dipole-like configuration in most models. In model m2-B0 the field is vertical in the core, and outside it is dipole-like. The initial strength of the magnetic field is given in Table \ref{['tab:models_performed']}.
• Figure 3: The map of the magnetisation and magnetic field lines at $t\sim 3.9$ s. We define the jet core as the high magnetised material $\sigma \geq 1$, while the jet wings the region where $10^0>\sigma > 10^{-6}$.
• Figure 4: Radial profiles of density after five seconds of evolution. Each panel displays slices taken at different polar angles. The top panel corresponds to the $z$-axis ($\theta=0^\circ$), while the middle and bottom panels show cuts at $\theta=45^\circ$ and $\theta=90^\circ$, respectively. Continuous lines show the initial condition and dashed lines show the stratification at $t=5$ s.
• Figure 5: The central engine activity for all models. The top panel shows the accretion rate, the middle panel presents the magnetic flux, and the bottom panel displays the MAD parameter in Lorentz-Heaviside units.