An accretion event in the Seyfert galaxy NGC 5548

Abstract

Accretion on to a massive collapsed central object is now commonly thought to power active galactic nuclei (AGN)^1–3. If this is indeed so, then changes in luminosity of the system should correspond to changes in accretion rate, or, equivalently, the mass flux at regions a few Schwarzschild radii from the putative black hole. The observational consequences of such a change in accretion rate may not be pronounced, as gas near the central source of radiation is expected to be very highly ionized and difficult to observe outside the X-ray band. Here we draw attention to a remarkable change which occurred in the spectrum of the low-luminosity, optically variable, Seyfert galaxy NGC 5548. An increase in optical luminosity was accompanied by the appearance of a strong, abnormally broad He II 4,686-Å emission line. The appearance of this line indicates the presence of a component of highly ionized gas that did not contribute to the optical spectrum before the increase in luminosity. The greater line width suggests that the gas producing the He II emission is much closer to the central object than the broad region (BLR). This observation may be the first direct evidence of the accretion onto a central object.