Properties of M15 X-2 outburst detected by NICER in the 0.3–10 keV energy band. Top panel: the light curve. Bottom panel: Hardness ratio of the count rates between the 2–10 and 0.3–2.0 keV energy bands. The X-ray bursts have been removed from the light curve, but the detection time of each burst is marked with a grey vertical line. For the fifth burst, only the tail was observed, so the onset is an approximation based on Fig. 3.