Simulating the Magnetorotational Collapse of Supermassive Stars: Incorporating Gas Pressure Perturbations and Different Rotation Profiles

University of Illinois at Urbana-Champaign

Abstract

Collapsing supermassive stars (SMSs) with masses $M \gtrsim 10^{4-6}M_{\odot}$ have long been speculated to be the seeds that can grow and become supermassive black holes (SMBHs). We previously performed general relativistic magnetohydrodynamic (GRMHD) simulations of marginally stable $\Gamma = 4/3$ polytropes uniformly rotating at the mass-shedding limit and endowed initially with a dynamically unimportant dipole magnetic field to model the direct collapse of SMSs. These configurations are supported entirely by thermal radiation pressure and reliably model SMSs with $M \gtrsim 10^{6}M_{\odot}$. We found that around $90\%$ of the initial stellar mass forms a spinning black hole (BH) remnant surrounded by a massive, hot, magnetized torus, which eventually launches a magnetically-driven jet. SMSs, therefore, could be sources of ultra-long gamma-ray bursts (ULGRBs). Here we perform GRMHD simulations of $\Gamma \gtrsim$ 4/3, polytropes to account for the perturbative role of gas pressure in SMSs with $M \lesssim 10^{6}M_{\odot}$. We also consider different initial stellar rotation profiles. Each star is initially seeded with a dynamically weak dipole magnetic field that is either confined to the stellar interior or extended from its interior into the stellar exterior. We calculate the gravitational wave burst signals for the different cases. We find that the mass of the black hole remnant is $90\% - 99\%$ of the initial stellar mass, depending sharply on $\Gamma - 4/3$, as well as on the initial stellar rotation profile. After $t \sim 250 - 550M \simeq 1 - 2 \times 10^3$ $(M/10^6M_{\odot})$s following the appearance of the BH horizon, an incipient jet is launched and it lasts for $\sim 10^4 - 10^5 (M/10^6M_{\odot})$s, consistent with the duration of long gamma-ray bursts. Our numerical results suggest that the Blandford-Znajek mechanism powers the incipient jet. They are also in rough agreement with our recently proposed universal model that estimates accretion rates and electromagnetic (Poynting) luminosities that characterize magnetized BH-disk remnant systems that launch a jet. This model helps explain why the outgoing electromagnetic luminosities computed for vastly different BH-disk formation scenarios all reside within a narrow range $(\sim 10^{52 \pm 1}$ erg s$^{-1})$, roughly independent of $M$.

arXiv:1807.07970

Rendering

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University of Illinois at Urbana-Champaign

Introduction
Case A: n3-EXTINT
Case B: n295-EXTINT
Case C: n29-EXTINT
Case D: n29-EXTINT, 0.75 Spin
Gravitational Waves