tokens: from the synchrotron emission by accelerated electrons but instead from a thermal process; the low-energy photon index would then relate to the spectral slope in the Rayleigh–Jeans regime of the blackbody component ( α = +1). 5.2. Coarse Time-resolved Spectral Analysis In this section, we describe the spectral evolution with time for the defined time intervals. Values from all fits are shown in Table 1 . As described below, strong spectral evolution is seen in the prompt phase. In the first time interval a , the Band function or a power-law function with exponential cutoff (CUTPL) provides a good fit to the data and there is no statistical difference between Band and CUTPL. This is also consistent with the fact that in the case of fitting the Band function alone the high-energy photon index β is very low ( β −3). Furthermore, when we adopt CUTPL to represent the obtained spectrum there is no need for any additional spectral component, as there is no statistical improvement by adding an extra component such as the blackbody or a power-law function (ΔPGstat ∼ 1) as shown in Table 1 . In the second and third time intervals, b and c , if we choose only the Band function, we find the same residuals as obtained in Section 5.1 (e.g., the low-energy bump). We see that the best-fit model is Band+PL and there is clear statistical improvement with respect to the case of the Band function alone (ΔPGstat = 86 and 157 in intervals b and c , respectively). Compared with the spectrum in the initial interval, the Band component low-energy index becomes harder. Interestingly, in these intervals the obtained low-energy photon indices are too large ( α = +0.2 to +0.4) to be explained by non-thermal synchrotron emission as described in Section 5.1 . Together with the small high-energy photon indices ( β ≤ −3), this indicates a narrow spectral component originating in thermal emission from the photosphere. From the theoretical point of view, Beloborodov ( 2010 ) pointed out that synchrotron emission and temporal variability lead to a softer spectrum ( α = +0.4) than the Rayleigh–Jeans index ( α = +1). In addition, a relativistically expanding conical jet structure implies a dispersion in the Doppler factor, which can broaden the photon spectrum (Asano Mészáros 2013 ). Here, we choose the diskpbb model implemented in XSPEC as a modified blackbody model to reproduce the thermal photospheric emission. The diskpbb model represents a multi-temperature blackbody spectrum where the local disk temperature T ( r ) at a distance r from a compact object is proportional to . While this model describes optically thick radiation from a geometrically thin accretion disk, we introduce this model phenomenologically to reproduce our modified blackbody spectrum. If we adopt this model, the photon index in the Rayleigh–Jeans range is represented as 2–2/ p and the diskpbb model can obtain a broader spectrum than a pure blackbody. The diskpbb model plus the power-law function,
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