Searching for the trigger of the active galactic nucleus quasi-periodic oscillation: 8 years of RE J1034+396

RE J1034+396 is one of the most extreme narrow-line type 1 Seyfert galaxies detected thus far, showing the only quasi-periodic oscillation (QPO) reliably detected in an active galactic nucleus (AGN). Comparison with similar spectral and timing properties observed in the black hole X-ray binary (BHB) GRS 1915+105 suggests that RE J1034+396 is a super-Eddington accretor. A more complete understanding of the behaviour of RE J1034+396 can therefore lead to a unification of the accretion physics between such extreme AGN and super-Eddington BHBs. Here we report on our latest XMM-Newton observations of RE J1034+396, which no longer show the QPO, indicating that this source shows a non-stationary power spectrum. We use spectral and temporal analysis across all five XMM-Newton observations of the source to probe the evolution of the object. The combination of the shape of the fractional variability with energy and the inferred velocity of absorbing material in the line of sight rules out an absorption-only method of creating the QPO. Instead the periodically changing absorption may be produced by the QPO causing a change in ionization state.

We extend our analysis by including the covariance spectra which give much better signal-to-noise ratio than an rms spectrum. These reveal a new aspect of the QPO, which is that there is also a small contribution from a soft component which is hotter than the soft excess seen in the mean spectrum. Folding the light curve on the QPO period shows that this component lags behind the hard X-rays. If this is due to reprocessing, then the lag corresponds to a light traveltime across ∼30R_g. Some of the remaining observations have similar energy spectra and covariance spectra, but none of them shows a significant QPO, so we conclude that none of these features is the trigger for the appearance of the QPO in this object.