Doppler map of the Hα emission lines. We plot the gravitational free-fall gas stream trajectory (lower curve), the Keplerian disk velocity along the stream (upper curve) and the Roche lobe of the donor star using K₁ = 72.7639 km s⁻¹ (Deneva et al. 2016), i = 76° (Strader et al. 2019; Yap et al. 2019), M_NS = 1.76 M_⊙ and q = 0.209. The plus signs represent, from top to bottom, the companion and the NS, while the ‘×’ sign represents the center of mass. The map was computed for a systemic velocity γ = −10.5 km s⁻¹.