SIMBAD references

We combine the long-term optical light curve of the Seyfert 1 galaxy NGC 5548 with the X-ray light curve measured by the Rossi X-Ray Timing Explorer over 6 years, to determine the relationship between the optical and X-ray continua. The X-ray light curve is strongly correlated with the optical light curve on long (∼1 yr) timescales. The amplitude of the long-term optical variability in NGC 5548 is larger than that of the X-ray variability (after accounting for the host galaxy contribution), implying that X-ray reprocessing is not the main source of the optical/X-ray correlation. The correlated X-ray and optical variations in NGC 5548 may be caused by instabilities in the inner part of the accretion flow, where both the X-ray and the optical emission regions may be located.