  LS~5039 is a powerful gamma-ray binary that probably hosts a non-accreting
pulsar. Despite the wealth of data available, the power source of the
non-thermal emitter is still unknown. We use a dynamical-radiative numerical
model and multiwavelength data to constrain the properties of a pulsar wind
that may power the non-thermal emitter in LS~5039. We ran simulations of an
ultrarelativistic (low-$B$) cold $e^\pm$-wind that Compton scatters stellar
photons and that dynamically interacts with the stellar wind. The effects of
energy losses on the unshocked $e^\pm$-wind dynamics, and the geometry of the
two-wind contact discontinuity, are computed for different wind models. The
predicted unshocked $e^\pm$-wind radiation at periastron, when expected to be
highest, is compared to LS~5039 data. The minimum possible radiation from an
isotropic cold $e^\pm$-wind overpredicts the X-ray to gamma-ray fluxes at
periastron by a factor of $\sim 3$. In the anisotropic wind case X-ray and
$\gtrsim 100$ MeV data are not violated by wind radiation if the wind axis is
at $\lesssim 20-40^\circ$ from the line of sight (probability of $\lesssim
6-24$\%), depending on the anisotropic wind model, or if the wind Lorentz
factor $\in 10^2-10^3$, in which case the wind power can be higher, but it
requires $e^\pm$-multiplicities of $\sim 10^6$ and $10^9$ for a $10^{-2}$~s and
10~s pulsar period, respectively. The studied model predicts that a low-$B$
cold pulsar $e^\pm$-wind in LS~5039 should be strongly anisotropic, with either
a wind Lorentz factor $\in 10^2-10^3$ and very high multiplicities or with a
fine-tuned wind orientation. A low-$B$, cold baryon-dominated wind would be
possible, but then the multiplicities should be rather low, while the
baryon-to-$e^\pm$ energy transfer should be very efficient at wind termination.
A strongly magnetized cold wind seems to be the most favorable (least
constrained) option.
