Fig. 1.
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Swift/XRT long-term light curve (top panel) and hardness ratio curve of NGC 55 ULX-1 (bottom panel). The hardness ratio is computed as the ratio of the counts in the 2−10 keV to 0.3−2 keV energy bands. The values for the four on-axis deep XMM-Newton observations are shown with horizontal lines and the times of the events with X markers followed by the corresponding ObsID.
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