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

Abstract

Long-duration γ-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 ∼1012 cm with an opening angle θ j ∼ 6° and a Lorentz factor Γ j ≲ 30, unbinding ≳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 θ t ∼ 12°, which can naturally explain quiescent times in GRB lightcurves. The effective opening angle for detection θ j + θ 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 ∼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 ∼10% of the gas maintains magnetization σ ≳ 0.1. This indicates that both a photospheric component and reconnection may play a role in the prompt emission.

Collapsar Black Holes Are Likely Born Slowly Spinning

“…In this letter, we constrain the natal BH spin from observations for the first time. Many previous studies have focused on rapidly spinning BHs as the central engines powering relativistic GRB jets (Komissarov & Barkov 2009;Janiuk & Yuan 2010;Tchekhovskoy et al 2011;Aloy & Obergaulinger 2021;Gottlieb et al 2022aGottlieb et al , 2022bBavera et al 2022;Fujibayashi et al 2022). Here, we argue analytically (Section 2) and numerically (Section 3) that GRB observables suggest that the majority of BHs form with a low spin.…”

“…The simulations showed that if the magnetic field amplitude is below a certain threshold, which depends on the stellar core density, then the jets fail to launch. Interestingly, Gottlieb et al (2022aGottlieb et al ( , 2022b found that even when the magnetic field is set at the minimum threshold for jet launching, the emerging jet is orders of magnitudes more powerful than the observed characteristic GRB luminosity. 3 This begs the question of what prelaunching conditions are required for powering typical GRBs.…”

A structured jet explains the extreme GRB 221009A

“…This value matches the spectral measurements of the early high-energy emission (23,24), suggesting that the synchrotron component extends to the gigaelectronvolt range. Thus, we can use the high energy flux, assuming that it is afterglow-dominated, as a proxy for the blast-wave kinetic energy (25), obtaining E K,iso ≈ 10 55 (1 + Y ) erg, where Y is the Compton parameter of the gigaelectronvolt-emitting electrons. A similarly high value is obtained by assuming a typical gamma-ray efficiency η γ ≈ 20%.…”

“…The standard assumption that GRB jets have a constant energy dE K / d Ω within the core of the jet is likely an oversimplification, and a structured jet naturally arises as the GRB breaks out of its stellar environment ( 32 , 55 ). Here, we consider a GRB jet with a broken power-law structure (Fig.…”

Multicollision Internal Shock Lepto-hadronic Models for Energetic Gamma-Ray Bursts (GRBs)

“…Possible extensions of the presented work may concern theoretical aspects (e.g., improvement of the modeling) or connection to new experimental results. First of all, a refined outflow model informed by magnetohydrodynamic simulations (as presented for example in Gottlieb et al 2021Gottlieb et al , 2022bGottlieb et al , 2022a might provide a more realistic picture of the physical conditions in the plasma, such as locations of energy dissipation and magnetization. Alternatively, one could lift the assumption of a top-hat jet model made here, and apply our work to a structured jet, similar to the one inferred by observations of GRB 170814A (Troja et al 2019).…”