Properties of a hypothetical cold pulsar wind in LS 5039

Abstract

Context. LS 5039 is a powerful high-mass gamma-ray binary that probably hosts a young non-accreting pulsar. However, despite the wealth of data available, the means by which the non-thermal emitter is powered are still unknown. Aims. We use a dynamical-radiative numerical model, and multiwavelength data, to constrain the properties of a hypothetical pulsar wind that would power the non-thermal emitter in LS 5039. Methods. We ran simulations of an ultrarelativistic (weakly magnetized) cold e±-wind that Compton scatters stellar photons and that dynamically interacts with the stellar wind. The effects of energy losses on the unshocked e±-wind dynamics, and the geometry of the two-wind contact discontinuity, are computed for different wind models. The predicted unshocked e±-wind radiation at periastron, when expected to be the highest, is compared to LS 5039 data. Results. The minimum possible radiation from an isotropic cold e±-wind overpredicts the X-ray to gamma-ray fluxes at periastron by a factor of ∼3. In the anisotropic (axisymmetric) wind case X-ray and ≳100 MeV data are not violated by wind radiation if the wind axis is at ≲20−40° from the line of sight (chance probability of ≲6−24%), depending on the anisotropic wind model, or if the wind Lorentz factor ∈102 − 103, in which case the wind power can be higher, but it requires e±-multiplicities of ∼106 and 109 for a 10−2 s and 10 s pulsar period, respectively. Conclusions. The studied model predicts that a weakly magnetized cold pulsar e±-wind in LS 5039 should be strongly anisotropic, with either a wind Lorentz factor ∈102 − 103 and very high multiplicities or with a fine-tuned wind orientation. A weakly magnetized, cold baryon-dominated wind would be a possible alternative, but then the multiplicities should be rather low, while the baryon-to-e± energy transfer should be very efficient at wind termination. A strongly magnetized cold wind seems to be the most favorable option as it is consistent with recent research on pulsar winds and does not require fine-tuning of the pulsar wind orientation, and the wind multiplicity and Lorentz factor are less constrained.

Modelling the correlated keV/TeV light curves of Be/gamma-ray binaries

“…In addition, the inhomogeneity of stellar outflow will cause additional flux variabilities (Bosch-Ramon 2013; Paredes-Fortuny et al 2015;de la Cita et al 2017). Also, the anisotropy of the pulsar wind might also affect the radiation (Kong et al 2012;Bosch-Ramon 2021). We also ignore the effect of pair absorption on TeV flux, and this process might reduce the TeV photons, especially around periastron and SUPC (Dubus 2006b).…”

The impact of the stellar wind on the non-thermal processes in gamma-ray binaries

“…( 6)]. For a realistic geometry that scales consistently with hydrodynamical simulations, we model the Coriolis shock as a spherical cap centered at the star with radius ∼ 3.3𝑟 cor [4,5].…”

Investigating the puzzling radio structures of the gamma-ray binary LS 5039

“…In both cases, the emitter is considered to be the inside part of the previously defined outflow, which mainly consists of shocked pulsar material. Therefore, no contribution to the outflow emission from the unshocked wind material is taken into account (see [15] for a study of the potential contribution from the unshocked pulsar wind) . We assume a leptonic origin of the radiation and neglect the hadronic contribution, although the latter may be relevant in some cases (e.g., [16], for a review in the similar case of microquasars jets).…”