Far-infrared SED of XID403 (red big symbols and solid line) compared to that of the local starburst galaxy Arp220 (small black dots and dashed line; Rangwala et al. 2011) shifted to z = 4.75 and renormalized. The FIR photometry of XID403 is reported in Table 1. The FIR SED was fitted with a general modified black-body by fixing the emissivity index to β = 2 and the rest-wavelength below which the dust is optically thick to λ₀ = 200  μm, as commonly observed in high-z SMGs (Conley et al. 2011; Fu et al. 2012; Riechers et al. 2013) and also in Arp220. The best-fit temperature is T_d = 58.5 ± 5.3 K. The best-fit dust mass is M_d = 4.9 ± 0.7 × 10⁸M_⊙ (see text for details).