Lyman-α line profile of GJ 3470 averaged over the three visits. The black histogram shows the observed spectra, fit over points with black error bars. The orange band shows the regions that are too contaminated by geocoronal emission to be included in the fit. The green line is the best fit for the intrinsic stellar line profile. It yields the blue profile after absorption by the interstellar medium, whose profile is plotted as a dotted black line (ISM absorption in the range 0–1 has been scaled to the vertical axis range). The red line shows the line profile fit to the data, after convolution with the STIS LSF. Gray bands delimitate the 1σ envelopes of the best-fit profiles. The theoretical intrinsic stellar line profile also corresponds to the profile of the ratio β between radiation pressure and stellar gravity, which is reported on the right axis in the top panel. Bottom panel: zoom of the observedLyman-α line and its best-fit model.