Searching for the trigger of the AGN QPO: 8 years of RE J1034+396
  RE J1034+396 is one of the most extreme Narrow-line Seyfert 1s 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 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 lightcurve
on the QPO period shows that this component lags behind the hard X-rays. If
this is due to re-processing then the lag corresponding to a light travel time
across 30 R_g. Some of the remaining observations have similar energy spectra
and covariance spectra, but none of them show a significant QPO, so we conclude
that none of these features are the trigger for the appearance of the QPO in
this object.
