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

Abstract

EFFORTS to use the variability of X-ray emission from active galactic nuclei (AGNs) as a diagnostic of physical processes in the unresolvable cores have been hampered by the discovery1,2 that the variable emission has a 'red noise' character, which seems to indicate the absence of any preferred timescale. The only exception is the claim of a precise periodicity in emission from NGC68143. Here we analyse archival Exosat observations of the bright Seyfert galaxy NGC5548, and find in the power spectrum a broad peak corresponding to quasi-periodic oscillations (QPOs) with period of ∼500 s. Both the frequency and magnitude of the QPOs vary systematically with total X-ray luminosity. Similar behaviour has been seen in compact galactic X-ray binaries, and we suggest that intensity-correlated QPOs may be a generic feature of accretion onto a compact object. If the QPOs in NGC5548 derive from instability or variability in an accretion disc around a black hole, the black hole mass can be only a few hundred thousand solar masses, an uncomfortably small number for most AGN models.