Light curves and 3σ upper limits of XMMU J122939.7+075333 in luminosity units for all the XMM-Newton and Chandra observations in Table 1. The conversion from 0.5–2 keV count rates into luminosity in the 0.5–2 keV energy band was based on the best-fit parameters of the average spectrum for each observation. The upper limits were derived from the 0.3–2 keV count-rate limits in Table 1 assuming the best-fit spectral parameters of the first flare detected in 2016 (Table 2). Although Dage et al. (2019) report a 3σ detection in the 2019 observation, we set an upper limit because most of the counts were detected above 2 keV. The horizontal bars indicate either the bin size of the light curve (ranging from 1 to 10 ks) or the observation exposure time (for upper limits and observations with a single time bin). The start dates of the observations and the instrument are indicated in each panel.