Spectrum of XID 42, XID 539 and XID 666 (green shaded histogram), expressed as the mean brightness within the regions contained in the blue ellipses of Fig. 1. The black solid line represents the Gaussian line fitting. Due to its double-peaked feature, we fit the line of XID 666 with two Gaussian components, indicated in the plot with the blue and red dashed lines. The channel width Δv is reported for each spectrum.