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

Long-duration gamma-ray bursts (GRBs) accompany the collapse of massive stars and carry information about the central engine. However, no 3D models have been able to follow these jets from their birth via black hole (BH) to the photosphere. We present the first such 3D general-relativity magnetohydrodynamic simulations, which span over six orders of magnitude in space and time. The collapsing stellar envelope forms an accretion disk, which drags inwardly the magnetic flux that accumulates around the BH, becomes dynamically important, and launches bipolar jets. The jets reach the photosphere at similar to 10(12) cm with an opening angle theta ( j ) similar to 6 degrees and a Lorentz factor Gamma( j ) less than or similar to 30, unbinding greater than or similar to 90% of the star. We find that (i) the disk-jet system spontaneously develops misalignment relative to the BH rotational axis. As a result, the jet wobbles with an angle theta ( t ) similar to 12 degrees, which can naturally explain quiescent times in GRB lightcurves. The effective opening angle for detection theta ( j ) + theta ( t ) suggests that the intrinsic GRB rate is lower by an order of magnitude than standard estimates. This suggests that successful GRBs are rarer than currently thought and emerge in only similar to 0.1% of supernovae Ib/c, implying that jets are either not launched or choked inside most supernova Ib/c progenitors. (ii) The magnetic energy in the jet decreases due to mixing with the star, resulting in jets with a hybrid composition of magnetic and thermal components at the photosphere, where similar to 10% of the gas maintains magnetization sigma greater than or similar to 0.1. This indicates that both a photospheric component and reconnection may play a role in the prompt emission.

Automated summary

This study presents the first 3D general-relativity magnetohydrodynamic simulations that can track the formation process of long-duration gamma-ray bursts (GRBs). The study finds that the collapsing stellar envelope forms an accretion disk that launches bipolar jets, and also explains some observed phenomena in GRB lightcurves. Furthermore, the study suggests that successful GRBs are rarer than currently thought and proposes a new mechanism for the production of GRBs.