Estimated shock radii and time period for shock oscillations as functions of interstellar medium density. The two solid lines delimit the time interval. (Here we assume that the oscillations end when the shock velocity reaches 130 km s⁻¹.) The two horizontal, dotted lines represent the two cases simulated in which we find oscillations (n_H = 5.5 cm⁻³ and 50 cm⁻³, both with E₅₁ = 0.931).