Prediction for X-ray bubble luminosity as a function of shock-front radius for three values of the interstellar gas density at R = 100 pc (n_H, 0) from the analytic model in Nims et al. (2015), assuming n_H ∝ R ⁻¹ (solid lines), compared to the upper limits from our observations (solid triangles). Different upper limits are shown because the predicted wind spectrum depends on density. The errors on the prediction come from bolometric correction uncertainty () when scaling to the bolometric luminosities of HE 0227–0913 and HE 0351+0240. Except for the lowest densities, our upper limits fall below the prediction.