1-2 time steps before the peak. This corresponds to the presence of a weak red-shifted component for the Mg [ii]{} line, that can be observed earlier than for other lines (cf. the top panels of Fig. \[fig:spectraB\], \[fig:spectraB\_16\] and \[fig:spectraB\_52\]). It is however difficult to say whether this is due to a differences in opacity between the lines or to some artifact of the PSF (see Appendix \[sec:appendixB\]).

HXR spectra {#sec:fermi}
===========

Modern radiative-hydrodynamical models of the flaring lower solar atmosphere like the flare RADYN code [@2015ApJ...809..104A] usually assume that the flare energy is transported from the corona by means of a beam of accelerated, mildly relativistic electrons. The parameters characterizing the electrons beam can be derived by analyzing the HXR spectra originating from the flaring atmosphere, assuming that the bremsstrahlung emission derives from the impact of such beam onto the dense chromosphere [e.g. @1971SoPh...18..489B].

To this end, we analyzed the HXR data acquired by the Fermi/Gamma-Ray Burst Monitor (GBM). As mentioned in Section \[sec:flare\_description\], Fermi fully covered the impulsive phase of the flare. GBM continuously observes the whole unocculted sky, recording two different data types: CTIME data, with a fine time resolution (0.256 s; 64 ms on flare trigger), 