Light curve of J1257 during the XMM-Newton observation 0691610201. Top panel: Light curve in the soft (0.3–1 keV) and hard (1–10 keV) band. Bottom panel: Hardness ratio of the hard count rate over the soft count rate. The red (dashed purple) line shows the result of the fit of the hardness ratio with a constant outside (within) the high-flux phase (highlighted in blue). The corresponding shaded region shows the 1σ confidence interval. The bin time in both panels is 1500 s.