Black hole discharge: Very-high-energy gamma rays from black hole-neutron star mergers

Abstract

With mass ratio larger than ∼ 5, star disruption is not expected for a black hole merging with a neutron star during the final plunge phase. In the late inspiral stage, the black hole is likely charged as it cuts through the magnetic field carried by the neutron star, leaving a temporarily charged black hole after merger. The unstable charged state of the remnant black hole rapidly neutralizes by interacting with the surrounding plasma and photons, which we investigate in first principle by numerically solving a coupled set of Boltzmann equations. The resulting basic picture is as follows. Electrons and positrons are accelerated in the BH electric field, which then lose energy to surrounding soft photons via Compton scattering; more electrons and positrons will be created from pair production as the hard photons colliding with soft photons, or through the Schwinger process in strong electromagnetic fields. The cascade stops when the charged black hole accretes enough opposite charges and becomes neutralized. We find that ∼ 10% (which depends on the soft photon energy and number density) of the total electric energy is carried away to infinity in a time interval ∼ 1 ms by very-high-energy (> 50 GeV, the low energy detection threshold of the MAGIC telescope) gamma rays whose spectrum is approximately a power law with spectral index ∼ −2.3.

Numerical-relativity simulations of the quasicircular inspiral and merger of nonspinning, charged black holes: Methods and comparison with approximate approaches

“…Studying charged black holes in Einstein-Maxwell theory not only provides a way to capture some features of specific modified theories in a controlled environment, but also simulations of such systems have direct astrophysical and fundamental physics applications. First, while black holes are expected to be electrically neutral [25][26][27][28][29], there is no definitive observational support for this expectation. Therefore, this assumption must be tested.…”

Numerical-relativity simulations of the quasi-circular inspiral and merger of non-spinning, charged black holes: methods and comparison with approximate approaches

“…Studying charged black holes in Einstein-Maxwell theory not only provides a way to capture some features of specific modified theories in a controlled environment, but also simulations of such systems have direct astrophysical and fundamental physics applications. First, while black holes are expected to be electrically neutral [25][26][27][28][29], there is no definitive observational sup-port for this expectation. Therefore, this assumption must be tested.…”

Multimessenger signals from black hole-neutron star mergers without significant tidal disruption

“…This allows us to explore the most dynamical part of the merger, when the interaction of the BH with the NS's magnetosphere will be strongest. It also allows us to study the post-merger dissipation of the magnetosphere, which may also source an associated electromagnetic transient (Lehner et al 2012;Lyutikov & McKinney 2011;Pan & Yang 2019). We treat the plasma dynamics with a resistive magnetohydrodynamics (MHD) approach (Palenzuela 2013) that can interpolate between the fluid pressure dominated regions inside the NS, and the magnetically dominated regime in the tenuous plasma surrounding the binary.…”