Large-scale Evolution of Seconds-long Relativistic Jets from Black Hole–Neutron Star Mergers

• Published in: Astrophysical journal. Letters Vol. 954; no. 1; p. L21
• Main Authors: Gottlieb, Ore, Issa, Danat, Jacquemin-Ide, Jonatan, Liska, Matthew, Foucart, Francois, Tchekhovskoy, Alexander, Metzger, Brian D., Quataert, Eliot, Perna, Rosalba, Kasen, Daniel, Duez, Matthew D., Kidder, Lawrence E., Pfeiffer, Harald P., Scheel, Mark A.
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01.09.2023; IOP Publishing
• Subjects: ASTRONOMY AND ASTROPHYSICS; Astrophysical black holes; Black holes; Compact binary stars; Deposition; Disks; Emission analysis; Energy distribution; Evolution; Gamma ray bursts; Gamma rays; Jets; Luminosity; Magnetic fields; Magnetic flux; Magnetization; Magnetohydrodynamical simulations; Mass ratios; Neutron stars; Neutrons; Numerical simulations; Relativistic jets; Star mergers; Stellar mass black holes; Stellar mergers; Wind
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/aceeff

We present the first numerical simulations that track the evolution of a black hole–neutron star (BH–NS) merger from premerger to r ≳ 10 11 cm. The disk that forms after a merger of mass ratio q = 2 ejects massive disk winds (3–5 × 10 −2 M ⊙ ). 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 σ 0 > 100 retain significant magnetization ( σ ≫ 1) at r > 10 10 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.