Tracing colliding winds in the UV line orbital variability of gamma-ray binaries

Gamma-ray binaries emit most of their radiated power beyond ~10 MeV. The non-thermal emission is thought to arise from the interaction of the relativistic wind of a rotation-powered pulsar with the stellar wind of its massive (O or Be) companion star. A powerful pulsar creates an extended cavity, filled with relativistic electrons, in the radiatively-driven wind of the massive star. As a result, the observed P Cyg profiles of UV resonant lines from the stellar wind should be different from those of single massive stars. We propose to use UV emission lines to detect and constrain the colliding wind region in gamma-ray binaries. We compute the expected orbital variability of P Cyg profiles depending upon the interaction geometry (set by the ratio of momentum fluxes from the winds) and the line-of-sight to the system. We predict little or no variability for the case of LS 5039 and PSR B1259-63, in agreement with currently available HST observations of LS 5039. However, variability between superior and inferior conjunction is expected in the case of LS I+61 303.

💡 Research Summary

The paper investigates how the interaction between a rotation‑powered pulsar wind and the radiatively driven wind of a massive O or Be companion in gamma‑ray binaries modifies the classic UV P Cygni line profiles. In the pulsar‑wind scenario the relativistic wind excavates a low‑density cavity within the stellar wind. Because resonant UV photons are scattered only by the remaining wind, the geometry and size of this cavity imprint characteristic changes on the absorption and emission components of the line.

The authors construct a semi‑analytical model. The stellar wind follows a β‑law (v(r)=v∞(1−R⋆/r)β) with typical mass‑loss rates >10⁻⁷ M⊙ yr⁻¹. The pulsar wind is treated as a highly relativistic, isotropic outflow with spin‑down power Ė. The stagnation point where the two ram pressures balance is given by p⋆=pₚ, leading to a dimensionless momentum ratio η=Ė/(Ṁ v(Rs) c). For η<1 the stellar wind dominates and the cavity is a narrow cone around the pulsar; for η>1 the pulsar wind dominates and the cone opens beyond 90°, potentially enveloping the star. The opening half‑angle ψ of the contact discontinuity (CD) is taken from hydrodynamic simulations (Bogovalov et al. 2008) via an empirical formula. The line‑of‑sight angle α between the cavity axis and the observer is expressed in terms of orbital inclination i, true anomaly t, and argument of periastron ω.

To compute line profiles the authors adapt Lucy’s (1971) formalism for P Cygni formation. The key modification is the inclusion of a factor fν(r) that represents the fraction of a constant‑velocity surface Sν not occulted by the cavity. The normalized flux at frequency ν is obtained by integrating over the stellar surface, weighting each element by fν(r) and the standard Sobolev escape probability. They define a profile area A=∫|P(v)−1| dv, which quantifies the total deviation of the line from the continuum, and normalize it by the single‑star case to obtain A₀.

A systematic parameter study shows that A₀ decreases (i.e., the line becomes weaker) when the cavity is close to the star (small orbital separation s) and when the cavity axis points toward the observer (small α). The effect is strongest near η≈1, where ψ≈90° and the cavity blocks roughly half the wind that would otherwise contribute to the line. For very large η the cavity can engulf the star, but physical limits (the pulsar wind would strike the stellar surface) make such extreme values unrealistic.

Applying the model to three well‑studied gamma‑ray binaries yields distinct predictions. LS 5039 has η≈1 and α≈90° throughout its orbit, so the cavity never significantly obscures the wind; the model predicts A₀≈0.95 and essentially no orbital variability, consistent with existing HST UV spectra. PSR B1259‑63, with a highly eccentric orbit and a long period, also shows little variability because the line of sight rarely aligns with the cavity. In contrast, LS I+61 303 has η in the range 0.3–1 and α varying from 0° (inferior conjunction) to 180° (superior conjunction). The model predicts a substantial change in A₀ (≈0.6–0.9) between these phases, i.e., a noticeably shallower absorption trough and reduced red‑shifted emission at superior conjunction.

The authors discuss observational prospects: the COS instrument on HST provides a spectral resolution R≈3500 (≈40–90 km s⁻¹), sufficient to detect changes in A₀ of 0.05–0.1. Therefore, time‑resolved UV spectroscopy of LS I+61 303 across its orbit would be a decisive test of the pulsar‑wind scenario. They also acknowledge model simplifications: only single lines are treated (no doublet scattering), ionization gradients (Strömgren zones) are ignored, and the cavity is approximated as a perfect cone rather than the complex, possibly turbulent structure seen in full 3‑D simulations. Incorporating these effects, as well as radiative braking and non‑axisymmetric wind structures, would refine the predictions.

In conclusion, UV P Cygni line variability offers a novel diagnostic of wind–wind interaction in gamma‑ray binaries. The lack of variability in LS 5039 and PSR B1259‑63 supports the presence of a pulsar wind that does not significantly alter the line‑forming region, while the predicted strong variability in LS I+61 303 provides a clear observational target to confirm or refute the pulsar‑wind model versus alternative microquasar jet scenarios.