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Fig. 1.


Panel a: AAVSO V-band (red dots) and B-band (blue dots) light curves covering the period late 2004–early 2018. The vertical dashed line shows the time of maximum optical brightness. The black solid line shows a curve from a moving average with a mean of 228 days (the orbital period). Panel b: Swift BAT 14–50 keV light curve with 100-day bins. It is evident that the BAT flux started to decay almost simultaneously with the increase in the optical flux, which started around December 2014; it became too faint for its detection around the optical maximum. Panel c: Swift BAT softness ratio (15–25/25–100 keV) with 100-day bins. This ratio steeply increased owing to the softening of the X-ray emission. Panel d: Swift XRT 0.3–10 keV count rate. Panel e: Swift UVOT UVM2-magnitude light curve. Stars indicate magnitudes determined from the CCD readout streak. Even during the rise to optical maximum, the UV flux has already increased dramatically. Panel f: Swift XRT hardness ratio (2–10/0.3–2 keV). Since the beginning of the optical brightening, the X-ray emission has become about 100 times softer than before.

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