Modelling the high-energy emission from gamma-ray binaries using numerical relativistic hydrodynamics

Abstract

Context. Detailed modelling of the high-energy emission from gamma-ray binaries has been propounded as a path to pulsar wind physics. Aims. Fulfilling this ambition requires a coherent model of the flow and its emission in the region where the pulsar wind interacts with the stellar wind of its companion. Methods. We have developed a code that follows the evolution and emission of electrons in the shocked pulsar wind based on inputs from a relativistic hydrodynamical simulation. The code is used to model the well-documented spectral energy distribution and orbital modulations from LS 5039. Results. The pulsar wind is fully confined by a bow shock and a back shock. The particles are distributed into a narrow Maxwellian, emitting mostly GeV photons, and a power law radiating very efficiently over a broad energy range from X-rays to TeV gamma rays. Most of the emission arises from the apex of the bow shock. Doppler boosting shapes the X-ray and very high energy (VHE) lightcurves, constraining the system inclination to i ≈ 35• . There is tension between the hard VHE spectrum and the level of X-ray to MeV emission, which requires differing magnetic field intensities that are hard to achieve with constant magnetisation σ and Lorentz factor Γ p of the pulsar wind. Our best compromise implies σ ≈ 1 and Γ p ≈ 5 × 10 3 , so respectively higher and lower than the typical values in pulsar wind nebulae. Conclusions. The high value of σ derived here, where the wind is confined close to the pulsar, supports the classical picture that has pulsar winds highly magnetised at launch. However, such magnetisations will require that further investigations are based on relativistic MHD simulations.

Mass loading of bow shock pulsar wind nebulae

“…Even for Lorentz factor as low as 10 3 , a value estimated from the modelling of some pulsars (see, e.g., Dubus et al 2015), λ ML,ph remains smaller than d0. It will be shown in §3 that Eq.…”

A model for high-energy emission of the Intrabinary shock in pulsar binaries

“…Of course, the shock flow can extend to a large distance and may make other structures [8]. We note that the IBS emission becomes rapidly weaker with distance s due to gradually reducing B and seed photon density (for SSC and EC).…”

“…Due to these emissions, the energy distribution of the particles evolves in time and space, so does the emitted photon spectrum. Furthermore, the IBS emission will be Lorentz boosted because some of the particles are bulk accelerated in the flow as shown in magnetohydrodynamic simulations [7,8], and so the Lorentz beaming also needs to be considered. Of course, the stellar and the pulsar emission can contaminate the IBS emission.…”

“…Around the CD, shocks are � e-mail: hjan@chungbuk.ac.kr formed of which the relativistic pulsar wind shock is of interest in regard to the high-energy emission in these systems [6]. In the pulsar wind shock, particles are stochastically accelerated and are believed to be bulk accelerated as they flow [7,8]; Lorentz beaming is expected which may be observed as spikes in the orbital light curve [9,10].…”

“…We then discuss plasma properties in the IBS of the source and potential use of the model for other pulsar binaries. Note that similar models have been used for other sources such as the gamma-ray binary LS 5039 [8] and the pulsar binary PSR J2215+5135 [9].…”