Timing evidence in determining the accretion state of the Seyfert galaxy NGC 3783

Abstract

Previous observations with the Rossi X-ray Timing Explorer (RXTE) have suggested that the power spectral density (PSD) of NGC 3783 flattens to a slope near zero at low frequencies, in a similar manner to that of Galactic black hole X-ray binary systems (GBHs) in the `hard' state. The low radio flux emitted by this object, however, is inconsistent with a hard state interpretation. The accretion rate of NGC 3783 (~7 per cent of the Eddington rate) is similar to that of other active galactic nuclei (AGN) with `soft'-state PSDs and higher than that at which the GBH Cyg X-1, with which AGN are often compared, changes between `hard' and `soft' states (~2 per cent of the Eddington rate). If NGC 3783 really does have a `hard'-state PSD, it would be quite unusual and would indicate that AGN and GBHs are not quite as similar as we currently believe. Here we present an improved X-ray PSD of NGC 3783, spanning from ~10<sup>-8</sup> to ~10<sup>-3</sup> Hz, based on considerably extended (5.5 yr) RXTE observations combined with two orbits of continuous observation by XMM-Newton. We show that this PSD is, in fact, well fitted by a `soft' state model which has only one break, at high frequencies. Although a `hard'-state model can also fit the data, the improvement in fit by adding a second break at low frequency is not significant. Thus NGC 3783 is not unusual. These results leave Arakelian 564 as the only AGN which shows a second break at low frequencies, although in that case the very high accretion rate implies a `very high', rather than `hard' state PSD. The break frequency found in NGC 3783 is consistent with the expectation based on comparisons with other AGN and GBHs, given its black hole mass and accretion rate.