Long-Term Evolution of Slowly Rotating Collapsar in Special Relativistic Magnetohydrodynamics

TL;DR

This study uses special relativistic MHD simulations to explore the long-term evolution of slowly rotating collapsars, revealing critical angular momentum thresholds for jet formation and different jet launching mechanisms, with implications for gamma-ray burst models.

Contribution

It provides the first long-term (up to 10 seconds) special relativistic MHD simulations of slowly rotating collapsars, highlighting the importance of initial angular momentum and magnetic field strength in jet formation.

Findings

Jets are launched only if initial angular momentum exceeds a critical threshold.

Strong magnetic fields lead to magnetic tower-driven outflows.

Weaker magnetic fields produce outflows via inflows from equator to poles.

Abstract

We present our numerical results of two-dimensional magnetohydrodynamic (MHD) simulations of the collapse of rotating massive stars in light of the collapsar model of gamma-ray bursts (GRBs). Pushed by recent evolution calculations of GRB progenitors, we focus on lower angular momentum of the central core than the ones taken mostly in previous studies. By performing special relativistic simulations including both realistic equation of state and neutrino coolings, we follow a unprecedentedly long-term evolution of the slowly rotating collapsars up to $\sim$ 10 s, accompanied by the formation of jets and accretion disks. Our results show that for the GRB progenitors to function as collapsars, there is a critical initial angular momentum, below which matter is quickly swallowed to the central objects, no accretion disks and no MHD outflows are formed. When larger than the criteria, we find the launch of the MHD jets in the following two ways. For models with stronger initial magnetic fields, the magnetic pressure amplified inside the accretion disk can drive the MHD outflows, which makes the strong magnetic explosions like a 'magnetic tower' (type II). For the models with weaker initial magnetic fields, the magnetic tower stalls first and the subsequent MHD outflows are produced by some eventual inflows of the accreting material from the equator to the polar regions (type I). Regardless of type I or II, the jets can attain only mildly relativistic speeds with the explosion energy less than $10^{49} \erg$, which could possibly be related to the X-ray flashes. Due to high opacity for neutrinos inside the disk, we find that the luminosities of $\nu_e$ and $\bar{\nu}_e$ become almost comparable, which is advantageous for making the energy deposition rate larger.