in several of the Fermi GBM energy bands. The dashed lines frame the interval corresponding to the period of maximum UV emission analyzed in the paper.[]{data-label="fig:hxr_intensity"}](hxr_intensity_oct.eps){width="8cm"}

A unique set of observations was acquired by IRIS, that was running a flare watch program on NOAA 12158, using the sit and stare (SNS) mode and the standard flare line list (OBSID 3860259453). High-cadence (9.4 s) flare spectra were obtained for many hours before the flare, and all the way to the end of the impulsive phase. Several spectral windows were acquired within the 1332 - 1358 Å, 1389 - 1407 Å (FUV), and 2783 - 2834 Å (NUV) intervals [see e.g. Fig. 3 in @2017ApJ...836...12K]. Exposure times were 8 s for both NUV and FUV channels up until the start of the flare; afterward, NUV exposures were reduced to 2.4 s to avoid saturation. Simultaneous slit-jaw images (SJI) were obtained at a cadence of $\sim$19 s, alternating between two channels: 2796 Å centered on Mg [ii]{}; and 1400 Å centered on Si [iv]{}. The $120" \times 120"$ field of view (FOV) of IRIS is shown as a white box in Figure \[fig:irisaia\]; the IRIS spectrograph slit is indicated with a dotted white 