Spectral energy distribution of GRB 150309A afterglow. Top panel: Spectral energy distribution at T₀ +5.2 h using the simultaneous X-ray data as well as the NIR upper limits (and the K-band detection) reported in this paper. The solid red line indicates the X-ray spectral slop obtained using the best-fit time-averaged PC mode spectrum, and the pink shaded region within the dashed red lines indicates the associated uncertainty with the X-ray spectral slop. The solid black line indicates the joint NIR-X-ray spectral index (β_K − X). The left and right sides of the Y-axis, representing the flux density and magnitude values, are in the AB system. Bottom panel: Ratio of X-ray observed data and best-fit model. The horizontal cyan line indicates the ratio equal to one.