arXiv subjects

Sean E. Li

Publications and source records attributed to Sean E. Li.

4 recordsLinked to original sources

arXiv2026

Strong Black Hole Natal Kicks in Magnetized Accretion-Powered Explosions

Asymmetric stellar explosions impart an impulse, or natal kick, to their compact remnants by linear momentum conservation. Black hole (BH) natal kicks are often assumed to be weaker than neutron star kicks because of greater mass accretion, and they are harder to constrain observationally because isolated BHs are electromagnetically faint. Using 3D general-relativistic magnetohydrodynamic simulations lasting up to ~30 s, we show that BHs formed from the collapse of rapidly rotating massive stars (collapsars) threaded by strong large-scale magnetic fields can acquire large natal kicks 10^2--10^3 km/s. Asymmetric electromagnetic outflows, magnetic-flux eruptions, and the gravitational pull of aspherical ejecta shape the kick magnitude, and their relative contributions depend on the progenitor structure, magnetic-flux history, and BH spin. More rapidly spinning BHs receive stronger, more nearly spin-aligned kicks, primarily through the gravitational pull of asymmetric jet-driven ejecta. Delayed transitions to the magnetically arrested state produce more asymmetric outflows and can generate even stronger recoils. Because the large-scale magnetic flux required in our models is also a key ingredient of relativistic gamma-ray burst jets and their associated energetic, jet-driven supernovae, natal kicks may be a natural consequence of magnetized collapsars. Such kicks could substantially alter BH retention in dense stellar environments, binary survival, spin-orbit misalignment, and the viability of magnetized collapsars in producing highly spinning BHs within the pair-instability mass gap.

astro-ph.HE↗

arXiv2025

Spinning into the Gap: Direct-Horizon Collapse as the Origin of GW231123 from End-to-End GRMHD Simulations

GW231123, the most massive binary black hole (BH) merger observed to date, involves component BHs with masses inside the pair-instability mass gap and unusually high spins. This challenges standard formation channels such as classical stellar evolution and hierarchical mergers. However, stellar rotation and magnetic fields, which have not been systematically incorporated in prior models, can strongly influence the BH properties. We present the first self-consistent simulations tracking a massive, low-metallicity helium star from helium core burning through collapse, BH formation, and post-BH formation accretion using 3D general-relativistic magnetohydrodynamic (GRMHD) simulations. Starting from a $250\,M_\odot$ helium core, we show that collapse above the pair-instability mass gap, aided by rotation and magnetic fields, drives mass loss through disk winds and jet launching. This enables the formation of highly spinning BHs within the mass gap and reveals a BH spin-mass correlation. Strong magnetic fields extract angular momentum from the BH through magnetically driven outflows, which in turn suppress accretion, resulting in slowly spinning BHs within the mass gap. In contrast, stars with weak fields permit nearly complete collapse and spin-up of the BH to $ a\approx1$. We show that massive low-metallicity stars with moderate magnetic fields naturally produce BHs whose masses and spins match those inferred for GW231123, and are also consistent with those of GW190521. The outflows may impart a BH kick, which can induce spin-orbit misalignment and widen the post-collapse orbit, delaying the merger. The outflows launched during collapse may power short-lived, high-luminosity jets comparable to the most energetic $γ$-ray bursts, offering a potential observational signature of such events in the early universe.

astro-ph.HE↗

arXiv2023

Dynamical perturbations of black-hole punctures: effects of slicing conditions

While numerous numerical relativity simulations adopt a 1+log slicing condition, shock-avoiding slicing conditions form a viable and sometimes advantageous alternative. Despite both conditions satisfying similar equations, recent numerical experiments point to a qualitative difference in the behavior of the lapse in the vicinity of the black-hole puncture: for 1+log slicing, the lapse appears to decay approximately exponentially, while for shock-avoiding slices it performs approximately harmonic oscillation. Motivated by this observation, we consider dynamical coordinate transformations of the Schwarzschild spacetime to describe small perturbations of static trumpet geometries analytically. We find that the character of the resulting equations depends on the (unperturbed) mean curvature at the black-hole puncture: for 1+log slicing it is positive, predicting exponential decay in the lapse, while for shock-avoiding slices it vanishes, leading to harmonic oscillation. In addition to identifying the value of the mean curvature as the origin of these qualitative differences, our analysis provides insight into the dynamical behavior of black-hole punctures for different slicing conditions.

gr-qc↗

arXiv2023

Bona-Massó slices of Reissner-Nordström spacetimes

Motivated by recent numerical relativity simulations of charged black holes and their interactions, we explore the properties of common slicing conditions in Reissner-Nordström spacetimes. Specifically, we consider different choices for the so-called Bona-Massó function, and construct static and spherically symmetric slices of the Reissner-Nordström spacetime satisfying the corresponding slicing conditions. For some of these functions the construction is entirely analytical, while for others we use numerical root-finding to solve quartic equations. Our solutions are parameterized by the charge-to-mass ratio $λ= Q/M$ and approach a unique slice, independent of the Bona-Massó functions considered here, in the extremal limit $λ\to 1$.

gr-qc↗