Left: relation of the median energy to the temperature of a black-body model (left-hand curves and y-axis, blue in the color version of this plot) and to the photon index of a power-law model (right curves and y-axis, red). Simulations for the Chandra ACIS-S detector with Cycle 13 (2012) response. An absorption column density of 10²¹ cm⁻² was assumed. Solid lines show models without background, dotted lines show models with 50% contribution of background counts. Parameter ranges are relevant to super-soft sources and X-ray binaries, and the two types of spectra are clearly separated at about Ẽ ≈ 1.0−1.3 keV. Right: verification of the source classification procedure in M 51. Distribution of sources with median energy Ẽ ≤ 2 keV over the photon index of the power-law model. The blue and red dashed histograms show the distributions of sources classified as soft and hard, respectively, and the black solid line indicates the total distribution.