Numerical simulations of jet launching and breakout from collapsars

Simulate the entire GRB dynamics since its production until the jet's break-out from the star envelope is computationally expensive. In this work, we aim to investigate jet production at the black hole horizon and its subsequent evolution beyond the envelope boundary. We perform 2.5-dimensional GRMHD simulations using, as initial conditions, remapped massive star progenitors previously evolved with a state-of-the-art stellar evolutionary codes. Specifically, we adopt the Wolf-Rayet star models 12TH and 16TI from the Woosley \& Heger framework, and we evolve our own Long GRB progenitor with the MESA code. A dipolar magnetic field is imposed, with variations in field strength to explore its influence. We find that the jets are magnetically dominated, with their thermal and kinetic components remaining comparable during propagation through the stellar envelope. The cocoon is thermally dominated in most cases. Jet launching is successful when the magnetic field has a dipolar configuration and spin $a=0.9$, in contrast, a hybrid magnetic field does not result in jet formation. We observe disk formation in cases without a magnetized jet, however, the formation of a well-defined funnel is not evident and the expansion of such wind does not break the star for longer times. Our simulations show that a collapsar launches a successful, relativistic jet only when a strong, large-scale dipolar field, driving a magnetically arrested disk. Such jets collimate, accelerate, and break out of progenitor stars in $1.5\lesssim t\lesssim3.5$ s, while weaker or non-magnetised flows stall inside the star. The resulting jets have luminosities of $L_{j}\sim10^{49}$--$10^{53}\,$erg\,s$^{-1}$ and the final energy structure reveals that magnetisation distribution and progenitor stratification as the key determinants of jet structure and success.