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We combine the long-term optical light curve of the Seyfert 1 galaxy NGC5548with the X-ray light curve measured by the Rossi X-ray Timing Explorer over 6years, to determine the relationship between the optical and X-ray continua.The X-ray light curve is strongly correlated with the optical light curve onlong (~year) time-scales. The amplitude of the long-term optical variability inNGC5548 is larger than that of the X-ray variability (after accounting for thehost galaxy contribution), implying that X-ray reprocessing is not the mainsource of the optical/X-ray correlation. The correlated X-ray and opticalvariations in NGC5548 may be caused by instabilities in the inner part of theaccretion flow, where both the X-ray and optical emission regions may belocated.Comment: Accepted for publication in ApJ Letter

Correlated Long-Term Optical and X-Ray Variations in NGC 5548