Investigating the Temporal Evolution of Gamma-Ray Burst Central Engine Parameters Based on Numerical Simulations

Abstract

A hyperaccreting stellar-mass black hole (BH) has been proposed as the candidate central engine of gamma-ray bursts (GRBs). Comparing the predictions from the central engine models with the temporal behavior of GRBs is of great interest. In this paper, using the open-source GRMHD HARM-COOL code, we evolve several 2D magnetized hyperaccreting BH models with realistic equation of state in a fixed curved space-time background. We extend the code to include the calculation of neutrino annihilation power. We then study the time evolution of BH central engine parameters, i.e., the neutrino annihilation power, the Blandford-Znajke (BZ) power, and the initial magnetization $σ_0$. We find that the neutrino power is generally consistent with previous analytical results. Usually, the neutrino annihilation process tends to launch a thermal ``fireball'', while the BZ jet is Poynting-flux-dominated. Our results, especially the evolution characteristics of $σ_0$ may help to understand the complex GRB spectral behavior.

Figures (4)

• Figure 1: The time evolution of the accretion rate for the system with BH mass of $M_{\rm bh}=3M_\odot$ and the initial disk mass of $M_{\rm t}=0.1 M_\odot$. We consider four cases: $a_\bullet=0.1$ (top left), $a_\bullet=0.6$ (top right), 0.9 (bottom left) and 0.98 (bottom right).
• Figure 2: The time evolution of the neutrino annihilation power for a BH with mass $m_\bullet=3$, and spin $a_\bullet=0.1$ (top left), $a_\bullet=0.6$ (top right), 0.9 (bottom left) and 0.98 (bottom right).
• Figure 3: The time evolution of the BZ power $\dot{E}_{\rm B}$ (solid blue lines) for a BH with mass $m_\bullet=3$, and spin $a_\bullet=0.1$ (top left), $a_\bullet=0.6$ (top right), 0.9 (bottom left) and 0.98 (bottom right). For comparisons, we also show the results of $\dot{E}_{\rm B}^{\rm BZ77}$ (dotted blue lines), $\dot{E}^{\rm TNM}_{\rm B}$ (dashed blue lines), and the neutrino power $\dot{E}_{\nu \bar{\nu}}$ (red lines).
• Figure 4: The time evolution of the magnetization parameter $\sigma_0$ for a BH with mass $m_\bullet=3$, and spin $a_\bullet=0.1$ (black), $a_\bullet=0.6$ (red), $a_\bullet=0.9$ (green), and $a_\bullet=0.98$ (blue). The solid, dotted and dashed lines represent $\sigma_0$ calculated with $\dot{E}_{\rm B}$, $\dot{E}_{\rm B}^{\rm BZ77}$ and $\dot{E}^{\rm TNM}_{\rm B}$, respectively.