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X-ray lightcurve of the powerful, radio-quiet AGN PHL1092 (18). Note the persistent large variability. A remarkable flare on day 8 shows a change in luminosity exceeding 1042 erg⋅s−2. This requires a mass-to-energy conversion efficiency exceeding 0.6 and cannot be associated with stellar processes. Mildly relativistic motions close to a black hole have probably amplified the apparent variability.
