Magnetar-like flares behind the high-energy emission in LS 5039

Abstract

LS 5039 hosts a high-mass star, and a compact object that might be a strongly magnetized neutron star (NS). This scenario requires a mechanism to power its persistent and strong nonthermal emission. We investigate a mechanism in which the nonsteady interaction structure of the stellar and the NS winds can regularly excite NS magnetospheric activity, releasing extra energy and fueling the source nonthermal emission. The NS wind shocked by the stellar wind can recurrently touch the NS magnetosphere, triggering magnetic instabilities whose growth can release extra energy into the NS wind in a cyclic manner. To illustrate and study the impact of these cycles on the two-wind interaction structure on different scales, we performed relativistic hydrodynamics simulations in 2D and 3D with periods of an enhanced power in the NS wind along the orbit. We also used analytical tools to characterize processes near the NS relevant for the nonthermal emission. As the NS wind termination shock touches the magnetosphere energy dissipation occurs, but the whole shocked two-wind structure is eventually driven away halting the extra energy injection. However, due to the corresponding drop in the NS wind ram pressure, the termination shock propagates back toward the magnetosphere, resuming the process. These activity cycles excite strong waves in the shocked flows, intensifying their mixing and the disruption of their spiral-like structure produced by orbital motion. Further downstream, the shocked winds can become a quasi-stable, relatively smooth flow. The recurrent interaction between the NS magnetosphere and shocked wind can fuel a relativistic outflow powerful enough to explain the nonthermal emission of LS 5039. A magnetospheric multipolar magnetic field much stronger than the dipolar one may provide the required energetics, and help to explain the lack of evidence of a recent supernova remnant.