前例のないガンマ線バースト、長寿命ジェットで説明される(Unprecedented gamma-ray burst explained by long-lived jet)

2023-08-31 ノースウェスタン大学

＜関連情報＞

• https://news.northwestern.edu/stories/2023/08/unprecedented-gamma-ray-burst-explained-by-long-lived-jet/
• https://iopscience.iop.org/article/10.3847/2041-8213/aceeff

ブラックホールと中性子星の合体による数秒の相対論的ジェットの大規模進化(Large-scale Evolution of Seconds-long Relativistic Jets from Black Hole–Neutron Star Mergers)

Ore Gottlieb, Danat Issa, Jonatan Jacquemin-Ide, Matthew Liska, Francois Foucart, Alexander Tchekhovskoy, Brian D. Metzger, Eliot Quataert, Rosalba Perna, Daniel Kasen
The Astrophysical Journal Letters  Published 2023 August 31
DOI:10.3847/2041-8213/aceeff

Abstract

We present the first numerical simulations that track the evolution of a black hole–neutron star (BH–NS) merger from premerger to r ≳ 10¹¹ cm. The disk that forms after a merger of mass ratio q = 2 ejects massive disk winds (3–5 × 10⁻²M_⊙). We introduce various postmerger magnetic configurations and find that initial poloidal fields lead to jet launching shortly after the merger. The jet maintains a constant power due to the constancy of the large-scale BH magnetic flux until the disk becomes magnetically arrested (MAD), where the jet power falls off as L_j ∼ t⁻². All jets inevitably exhibit either excessive luminosity due to rapid MAD activation when the accretion rate is high or excessive duration due to delayed MAD activation compared to typical short gamma-ray bursts (sGRBs). This provides a natural explanation for long sGRBs such as GRB 211211A but also raises a fundamental challenge to our understanding of jet formation in binary mergers. One possible implication is the necessity of higher binary mass ratios or moderate BH spins to launch typical sGRB jets. For postmerger disks with a toroidal magnetic field, dynamo processes delay jet launching such that the jets break out of the disk winds after several seconds. We show for the first time that sGRB jets with initial magnetization σ₀ > 100 retain significant magnetization (σ ≫ 1) at r > 10¹⁰ cm, emphasizing the importance of magnetic processes in the prompt emission. The jet–wind interaction leads to a power-law angular energy distribution by inflating an energetic cocoon whose emission is studied in a companion paper.