Light curve of the outburst as seen by NICER with each point corresponding to one observation. The luminosity of each observation is derived from the spectral fits described in Sect. 3. The daily MAXI light-curve of the same period is shown in green. Observations with quasi-simultaneous NuSTAR points are marked in blue. Overlaid are five of the pulse profiles at different times over the outburst. The purple markers at the top indicate the times of periastron passage using the orbital solution by Wilson et al. (2002).