There is mounting evidence that type transitions are a common property of AGN: the broad lines in at least eleven Seyfert galaxies have appeared or disappeared, leading to the reclassification of their nuclei from type 1–1.5 to type 1.8–2 or vice versa. We show that these phenomena find a natural explanation in the starburst model for AGN as transient phases without supernova activity in a 10–60 Myr old metal-rich massive stellar cluster with a low supernova rate (νSN ≲ 3 yr−1).

Type transient AGN: casuistry

Spectroscopic observations of Seyfert galaxies established early on that the broad permitted lines can experience strong variations in time scales of a few weeks–months. There is a growing number of extreme cases in which the broad components have temporarilly disappeared or become so weak that a reclassification of the objects has been allowed: from Seyfert nuclei of type 1–1.5 to type 1.8–2/LINER. Among them we can find the prototypes NGC 4151 and NGC 5548, along with NGC 1566, NGC 3516, NGC 6814, NGC 7603, Mrk 372 and 3C 390.3. In most of these cases, we know that the transitions took place while the nuclei were in deep photometric minima. Conversely, there are some narrow-line objects that have developed prominent broad components while brightening. Among them we can find Mrk 6, Mrk 993 and Mrk 1018.