Hardness ratios of candidates and confirmed SNRs in our sample. The energy bands used to calculate the hardness ratios are for HR1 : soft (= 0.3–0.7keV) and medium (0.7–1.1 keV), for HR2: medium (0.7–1.1 keV) and hard (1.1–2.3 keV), and for HR3: hard (1.1–2.3keV) and harder (2.3–4.0 keV). The sources are separated according to the explosion type as reported in Maggi et al. (2016). The SNRs tend to be separated according to the progenitor type of the remnant. The lines describe the expected HR values assuming different models. In particular we considered Fe-rich thermal plasma emission (VNEI with Fe = 2Z_⊙) plotted with red lines and O-rich thermal plasma emission (VNEI with O = 2Z_⊙) plotted with blue lines with different ionisation timescales τ (τ = 10¹⁰ s cm⁻³ and τ = 10¹² s cm⁻³) and different temperatures (kT = 0.3 keV and kT = 0.5 keV). In addition, we also plotted power law models in green with different photon indices T (r = 1,2,3). The dashed and dotted black lines indicate different column densities N_H = 0.0,0.5 and 1.0 cm⁻². The contours in HR2-HR3 represent the Gaussian kernel-density estimation for core-collapse (CC) and type Ia SNRs. The contours indicate 68, 90, and 95% probability.