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tokens: to examine several SNR candidates proposed in the MAGPIS survey. Fig. 21 Spectral index of the SNR G018.8+00.3 (THOR source G18.761+0.287). The black contours represent the continuum emission in steps of 10, 20, 30, and 40 mJy beam. MAGPIS ( Helfand et al. 2006 ) provides 49 new SNR candidates, which are only partly included in the SNR catalog by Green (2014) . Their criteria for a SNR candidate is that they have: 1) a high ratio between 20 cm continuum and 21 m flux density; 2) a counterpart at 90 cm continuum emission; and 3) a distinctive SNR morphology, e.g., shell-type or filled-center ( Dubner Giacani 2015 ). Followup observations for these candidates to determine the distance using H i absorption are presented in Johanson Kerton (2009) . Since MAGPIS has higher spatial resolution, it is more sensitive at detecting the mentioned SNR morphology. However, the THOR survey can help to characterize the radiation and distinguish between thermal and non-thermal radiation. Within the THOR region, we find 33 MAGPIS SNR candidates, which are listed in Table 4 . Only five of them are included in the SNR catalog presented by Green (2014) . In contrast to this, 24 of these MAGPIS SNR candidates have at least one counterpart in the WISE H ii region catalog presented by Anderson et al. (2014) . However, thanks to the high density of H ii regions within the Galactic plane, these can be chance alignments along the line of sight. Table 4 MAGPIS SNR candidates in comparison with THOR sources, Green SNR and WISE H ii regions. Notes. The SNR candidates are given in Helfand et al. (2006) . Since the SNR candidates can be clumpy, several THOR sources can be found within one MAGPIS SNR candidate so we list all corresponding THOR sources. The associated SNR names taken from Green (2014) are given, as well as associated WISE H ii regions given in Anderson et al. (2014) . The given H ii regions can be associated with the SNR, but they can also be foreground or background objects. Figure 22 shows the size for each H ii region. The diameter of the MAGPIS SNR candidates is taken from Helfand et al. (2006) , and the distance is determined via H i absorption and taken from Johanson Kerton (2009) . The spectral index is measured for the peak position of the THOR source (see Sect. 3.7 ) and can vary significantly within the source (see Fig. 22 ). (a) Spectral index map is shown in Fig. 22 . (b) Multiple H ii regions can be found within the region. (c) Determination of the spectral index is uncertain, since the source is not detected in all spectral windows. Fig. 22 Spectral index maps of extended MAGPIS SNR candidates. The color scale represents the spectral index from 1.5 to 0.5, the black contours show the continuum emission in steps of 5, 10, 15, 20, 40, and 100 mJy beam. The red and white circles indicate the sizes of the SNR candidates given in Helfand et al. (2006) and the sizes of the H ii regions given in Anderson et al. (2014) , respectively. In each panel the