Long-term X-ray light curve of J0054. The magenta and green data refer to Chandra and XMM-Newton data, respectively; the down-pointing arrows indicate 3σ upper limits, while the diamond markers indicate detections (with 1σ error bars). Swift/XRT detections are shown as blue diamonds, while the upper limits are indicated by down-pointing grey arrows. Count rates for the upper limits and for the Swift/XRT detections were converted to fluxes assuming the same spectral shape of the Chandra 2014 detection. Years are on the X-axis and flux in the 0.3–10 keV band is on the Y-axis.