Summary
Collapsing stars constitute the main black hole (BH) formation channel, and are occasionally associated with the launch of relativistic jets that power gamma-ray bursts (GRBs). Thus, collapsars offer an opportunity to infer the natal (before spin-up/down by accretion) BH spin directly from observations. We show that once the BH saturates with large-scale magnetic flux, the jet power is solely dictated by the BH spin and mass accretion rate. Recent core-collapse simulations by Halevi et al. 2022 and GRB observations favor stellar density profiles that yield a typical BH accretion rate, 1e-2 solar mass/second, which is weakly dependent on time. This leaves the BH spin as the main factor that governs the jet power. By comparing the resultant jet power to characteristic GRB luminosities, we find rapidly spinning BHs produce jets with excessive power, so that the majority of BHs associated with jets are born slowly spinning with a dimensionless spin a~0.2, or a~0.5 for wobbling jets. This result could be applied to the entire core-collapse BH population, unless an anti-correlation between the stellar magnetic field and angular momentum is present. We verify our results by carrying out the first 3D general relativistic magnetohydrodynamic simulations of collapsar jets with characteristic GRB energies, powered by slowly spinning BHs. We find that jets of typical GRB power do not retain their energy during the propagation in the star, providing the first numerical indication that many jets might fail to generate a GRB.
The spin of a newly formed black hole (BH) at the center of a massive star evolves from its natal value due to two competing processes: accretion of gas angular momentum that increases the spin, and extraction of BH angular momentum by outflows that decreases the spin. Ultimately, the final, equilibrium spin is set by the balance between both processes. In order for the BH to launch relativistic jets and power a gamma-ray burst (GRB), the BH magnetic field needs to be dynamically important. Thus, we consider the case of a magnetically arrested disk (MAD) driving the spin evolution of the BH. By applying the semi-analytic MAD BH spin evolution model of Lowell et al. 2022 to collapsars, we show that if the BH accretes ~20% of its initial mass, its dimensionless spin inevitably reaches small values, a<0.2. For such spins, and for mass accretion rates inferred from collapsar simulations, we show that our semi-analytic model reproduces the energetics of typical GRB jets, L~1e50 erg/s. We show that our semi-analytic model reproduces the nearly constant power of typical GRB jets. If the MAD onset is delayed, this allows powerful jets at the high end of the GRB luminosity distribution, L~1e52 erg/s, but the final spin remains low, a< 0.3. These results are consistent with the low spins inferred from gravitational wave detections of binary BH mergers.