Top panel: Phase–folded light curves of epoch X2. Light curves are extracted at different energy bands: 0.3–0.5 keV, 0.5–0.7 keV, 0.7–1.0 keV, and 1.0–2.0 keV. For clarity, only the 0.3–0.5 keV and1.0–2.0 keV light curves are shown. Dotted lines indicate the best-fit sinusoidal functions to the data (χ²/degree of freedom < 2 for each energy band). Middle panel: Phase–folded light curves of epoch X3. Bottom panel: Time lags of the light curves in different energy bands with respect to the reference band of 0.3–0.5 keV. A positive lag indicates that the given band lags behind the reference band in X–ray signals. The date points for epoch X3 are shifted by +1 ks in time difference for clarity. The J1231 data show that the hard band (1.0–2.0 keV) lags the soft band (0.3–0.5 keV) in time (1.4 ± 0.9 ks at X2, and 2.0 ± 0.8 ks at X3). J1231 is thus unlike QPEs, which typically show a ‘hard–rise–soft–decay’ mode, where the hard band leads the soft band during each eruption.