Black Hole Spin-down in Collapsars in 3D Neutrino Transport GRMHD Simulations

Collapsars -- massive stars whose cores promptly collapse into black holes (BHs) -- can power long-duration gamma-ray bursts (LGRBs) via relativistic, collimated, electromagnetically-driven outflows, or jets. Their power depends on the BH magnetic field strength and spin. To survive the infalling stellar material, jets need the central BH to attain dynamically important magnetic fields that can suppress the mass inflow and lead to a magnetically arrested disk (MAD). Previous work found that non-radiative MADs can spin down their BHs to an equilibrium spin, $a_{\rm eq}^\text{nr}=0.035-0.07$. Such low spins result in extremely low power jets that may struggle to escape out of the star. However, the dense and hot collapsar disks emit neutrinos that cool the disk, reduce its thickness, and increase the angular momentum supply to the BH. Using 3D two-moment neutrino-transport general relativistic magnetohydrodynamic simulations, we show for the first time that successful collapsar jets powered by neutrino-cooled disks still rapidly spin down their BHs, although to a higher $a_{\rm eq}\approx 0.13$. This value is consistent with LIGO/Virgo/KAGRA inferred spins, is $2-4$x higher than for non-radiative MADs, and results in $4-16$x more powerful LGRB jets, which are more capable of drilling out of the progenitor star. This value of $a_{\rm eq}$ holds across a wide range of progenitor structures and mass accretion rates, $\dot{m} \sim(0.1-10)M_{\odot}/\rm{s}$. We find that for typical LGRB durations, $t\gtrsim30$~s, such BHs consume sufficient mass to reach $a_{\rm eq} \approx 0.13$ by LGRB's end. However, shorter or lower-$\dot{m}$ LGRBs can leave behind more rapidly spinning BHs.