Magnetic Ergostars, Jet Formation and Gamma-Ray Bursts: Ergoregions versus Horizons

Abstract

We perform the first fully general relativistic, magnetohydrodynamic simulations of dynamically stable hypermassive neutron stars with and without ergoregions to assess the impact of ergoregions on launching magnetically--driven outflows. The hypermassive neutron stars are modeled by a compressible and causal equation of state and are initially endowed with a dipolar magnetic field extending from the stellar interior into its exterior. We find that, after a few Alfvén times, magnetic field lines in the ergostar (star that contains ergoregions) and the normal star have been tightly wound in both cases into a helical funnel within which matter begins to flow outward. The maximum Lorentz factor in the outflow is ${\Gamma }_L\sim 2.5$, while the force-free parameter holds at $B^2/8\pi {\rho }_0\lesssim 10$. These values are incompatible with highly relativistic, magnetically-driven outflows (jets) and short $\gamma$-ray bursts. We compare these results with those of a spinning black hole surrounded by a magnetized, massless accretion disk that launches a bona fide jet. Our simulations suggest that the Blandford-Znajek mechanism for launching relativistic jets only operates when a black hole is present, though the Poynting luminosity in all cases is comparable. Therefore, one cannot distinguish a magnetized, accreting black hole from a magnetized hypermassive neutron star in the so-called mass-gap based solely on the value of the observed Poynting luminosity. These results complement our previous studies of supramassive remnants and suggest that it would be challenging for either normal neutron stars or ergostars in a hypermassive state to be the progenitors of short $\gamma$-ray bursts.

7 pages, 3 figures. Matches published version

References in corpus (17)

• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• Multi-messenger Observations of a Binary Neutron Star Merger
• Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A
• A decade of Gamma-Ray Bursts observed by $Fermi$-LAT: The 2${}^{nd}$ GRB catalog
• Estimating the Contribution of Dynamical Ejecta in the Kilonova Associated with GW170817
• High resolution numerical-relativity simulations for the merger of binary magnetized neutron stars
• Fully General Relativistic Simulations of Black Hole-Neutron Star Mergers
• Relativistic simulations of black hole-neutron star coalescence: the jet emerges
• General relativistic simulations of compact binary mergers as engines of short gamma-ray bursts
• Binary Black-Hole Mergers in Magnetized Disks: Simulations in Full General Relativity
• First 100 ms of a long-lived magnetized neutron star formed in a binary neutron star merger
• Relativistic magnetohydrodynamics in dynamical spacetimes: A new AMR implementation
• GRMHD simulations of prompt-collapse neutron star mergers: the absence of jets
• General relativistic simulations of black hole-neutron star mergers: Effects of tilted magnetic fields
• Effects of spin on magnetized binary neutron star mergers and jet launching
• Black holes, disks and jets following binary mergers and stellar collapse: The narrow range of EM luminosities and accretion rates
• The role of the ergosphere in the Blandford-Znajek process

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• PIC methods in astrophysics: Simulations of relativistic jets and kinetic physics in astrophysical systems
• Multimessenger Binary Mergers Containing Neutron Stars: Gravitational Waves, Jets, and $\mathbf{\gamma }$-Ray Bursts
• Magnetohydrodynamic simulations of self-consistent rotating neutron stars with mixed poloidal and toroidal magnetic fields
• Jetlike structures in low-mass binary neutron star merger remnants
• General-relativistic resistive-magnetohydrodynamics simulations of self-consistent magnetized rotating neutron stars
• Ergoregion instability in bosonic stars: scalar mode structure, universality, and weakly nonlinear effects