Quasi-periodic oscillations in X-ray emission from the Seyfert galaxy NGC5548

Seyfert galaxy NGC5548 emits X-rays in quasi-periodic oscillations (QPOs). Analysis of data obtained by the Exosat satellite shows that NGC5548 is the first object with an active galactic nuclei (AGN) known to produce X-ray QPOs. Similar findings from compact galaxy X-ray binaries indicate that QPOs result from material being absorbed within a dense object, probably a black hole. However, the low estimated mass for the black hole that may exist within NGC5548's AGN may be hard to reconcile with current theories of AGN structure.