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

TL;DR

The paper proposes that magnetar-like, cyclic flares from a strongly magnetized NS in LS 5039 can regulate energy transfer to the stellar-wind–NS-wind interaction, sustaining the observed nonthermal emission with $L_{NT}\sim 10^{36}$ erg s$^{-1}$. Using 2D and 3D relativistic hydrodynamics simulations plus analytical estimates near the NS, the authors show that periodic enhancements of the NS wind periodically touch the magnetosphere, triggering energy release and driving waves that energize the shocked winds. They find that the energy injection can produce a relativistic outflow consistent with LS 5039’s emission, and that a multipolar, magnetar-strength field is needed to supply the requisite energetics while reconciling the lack of a recent SNR. The framework suggests magnetar-like activity in gamma-ray binaries could power persistent nonthermal emission over extended timescales and informs expectations for particle acceleration and observational signatures.

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.

Figures (7)

• Figure 1: Example of the time profile of a flaring period; $\delta\rho$ is the density on top of the regular pulsar wind density during flaring periods, $x= 2\pi t N_{fl}/T_{orb}$ the dimensionless time (see text for details -Sect. \ref{['sec:setwind']}-), and $\Delta =0.1$ the width of the pulse.
• Figure 2: Colored density maps on the orbital plane, with colored arrows representing the modulus of the 3-velocity vector for the 2D models. The flaring period rates are $N_f=90$ (top left, 2Dn90e10), 30 (top right, 2Dn30e10), and 10 (middle left, 2Dn10e10) flaring periods per orbit, keeping the total energy budget of the system constant. In the model 2Dn30e30, we increased the power of the flaring period by a factor of 3, while keeping $N_f= 30$ (middle right panel). The non-flaring cases are presented at the bottom left (nfeta0.05) and right (nfeta0.5).
• Figure 3: Colored density maps with colored arrows representing the modulus of the 3-velocity vector for the 3D model (3Dn30e10) for various cuts: $XY$ (top; orbital plane), $XZ$ (middle), and $ZY$ (bottom).
• Figure 4: Colored density maps with colored arrows representing the modulus of the 3-velocity vector for the 3D model (3Dn30e10) for cuts on the orbital plane, at orbital phases $\phi = 0.5435$ (top left), $\phi = 0.575$ (top right), $\phi = 0.5812$ (middle left), $\phi = 0.6033$ (middle right), $\phi = 0.6467$ (bottom left), and $\phi = 0.6576$ (bottom right).
• Figure 5: Zoom-in of three snapshots of the cycle followed by the interaction structure: weak pulsar wind (left), enhanced pulsar wind (middle), and the end of energy injection (right).