Long-term X-ray light curves of the source X1–X6 (from top to bottom). The magenta, green, and blue points indicate Chandra, XMM-Newton, and Swift/XRT data, respectively. Down-pointing arrows indicate 3σ upper limits. Swift/XRT upper limits are in grey. XMM-Newton/pn and XMM-Newton/MOS upper limits are shown by two different arrows. The count rates on which this figure is built are reported in Table B.1.