A Non-Orbital Gravitational Plasma Trap With A Possible Electromagnetic Signature

We introduce a setup in which a neutral plasma can be trapped at the center of a binary black hole (BBH) system. If the binary system travels through a uniform cosmic background magnetic field that is relatively aligned perpendicular to the plane of rotation of the BBH, then the charged particle trajectories would be confined in three dimensions around the center of the trap, approximately at a spherical region with 10% of the orbital radius of the system. The cyclotron motion due to magnetic field confines the charged particles on the plane of rotation and gravitational attraction confines the particles in the perpendicular direction. The trap can be filled when BBH system goes through a denser HII region and each black hole accretes mass through Bondi-Hoyle-Lyttleton process. After the BBH system leaves the denser region, the mass that is close to the black holes will be accreted while the matter close to the center stays. The full trap can have a novel EM signature in two ways: (1) n=10^14 cm^-3 is optically thick enough to result in dimming of background stars; (2) the rapid accretion of the trap around the time of merger. Such possibilities of EM signature make the setup observationally verifiable in addition to being a novel theoretical configuration.