Kenta Matsuo | Searcharxiv

arXiv2026

Motion of a charged test particle around a static black hole in a monopole magnetic field

We study the motion of a charged test particle in the spacetime with a spherically symmetric black hole which is immersed in a monopole magnetic field. We show that the radial motion of the charged test particle is governed by completely the same equation as that in the case of no magnetic field. This result implies that the black hole will acquire the electric charge if it is surrounded by the collisionless plasma composed of protons and electrons which obey the Maxwell velocity distribution. The drastically different situation appears in the tangential motions of charged test particles due to the magnetic field. The trajectory of a charged test particle in the black hole with the magnetic field of the order of 10 Gauss near the black hole, is confined on a very thin cone as long as the specific angular momentum of the particle is not much larger than the gravitational radius of the black hole times the speed of light. This result leads to a possibility that a plasma lump can hover over the black hole and is very hot, in the monopole magnetic field.

gr-qc↗

arXiv2024

Electrification of a non-rotating black hole

Zajacek et al made an interesting theoretical prediction on the electrification of a non-rotating black hole; if a non-rotating black hole is surrounded by plasma composed of protons and electrons, it will acquire electric charge due to the large difference of the inertial mass of a proton and that of an electron. Furthermore they revealed the effects of the electric charge of the black hole on the surrounding plasma. Since their results mainly rely on non-relativistic analyses, we study the same subject through relativistic analyses in this paper. By investigating a test particle in the Schwarzschild spacetime, we find that if initial velocities of protons and electrons far from a black hole follow the Maxwell distribution, the black hole can acquire electric charge whose value depends on the ratio of temperature of the proton and that of the electron. We also show that if the black hole acquires the electric charge, the radii of the innermost stable circular orbit (ISCO) and the specific energy of a charged particle on ISCO can be very different from those of a neutral particle. In contrast to the result obtained by Zajacek et al, we find that the ISCO radii of a proton and an electron are necessarily larger than that of a neutral test particle as long as the black hole acquires the charge. The large ISCO radius might lead to a larger angular diameter of a black hole shadow and a different estimate of the released energy due to the accretion of plasma from the estimate based on the assumption of electric neutrality of the central black hole.

gr-qc↗