SIMBAD references

The broad emission component of Hβ in the spectrum of the Seyfert 1.5 galaxy NGC 5548 recovered its intensity rather rapidly after the disappearance in the middle of 1992 and came back to the usual level (∼6x10^–13erg/cm2/s) before the end of 1992. Following the increase in strength of the broad component its redder part, which was faint before the disappearance, became dominant. The profile seems to have changed by two steps, namely when the broad component reappeared it showed a relatively symmetric profile, then a further increase of the intensity of the redder part occurred about one year later. The last profile is similar to those observed during the years from 1979 to 1984. The profile might have changed roughly every ten years. It seems, however, difficult to explain the rapid, almost discrete, changes of the profile as a result of an orbital motion of a binary system. There might exist an unknown mechanism to produce an asymmetric gas motion in the galactic nucleus. A fading of the narrow emission component of Hβ is noticed in 1994, which may be correlated with the unusually low luminosity of the nucleus in 1990.