NICER light curve and hardness ratio of J234402. Top: 0.3–2 keV NICER light curve. The light curve shows significant variability on a timescale of days. The sharp turns up and down are consistent with the rapid variability observed in the XMM-Newton and eROSITA data. The dashed line indicates the cut-off between the low-state and high-state intervals at 3.78 counts s⁻¹, as discussed in Sect. 2.4.2. Bottom: hardness ratio (HR) in NICER light curve. We define the HR using the 0.3–0.5 keV and the 0.5–2 keV count rates (R_{0.3 − 0.5} and R_{0.5 − 2}, respectively), as defined in the text. We detect no significant change over time.