Re-observing the NLS1 Galaxy RE J1034+396. I. the Long-term, Recurrent
  X-ray QPO with a High Significance
  RE J1034+396 is a narrow-line Seyfert 1 galaxy (NLS1) in which the first
significant X-ray quasi-periodic oscillation (QPO) in an active galactic nuclei
(AGN) was observed in 2007. We report the detection of this QPO in a recent
XMM-Newton observation in 2018 with an even higher significance. The quality
factor of this QPO is 20, and its period is 3550 $\pm$ 80 s, which is 250 $\pm$
100 s shorter than in 2007. While the QPO's period has no significant energy
dependence, its fractional root-mean-square (rms) variability increases from 4%
in 0.3-1 keV to 12% in 1-4 keV bands. An interesting phenomenon is that the QPO
in 0.3-1 keV leads that in the 1-4 keV bands by 430 $\pm$ 50 s with a high
coherence, opposite to the soft X-ray lag reported for the observation in 2007.
We speculate that the QPO has an intrinsic hard lag, while the previous
reported soft lag is caused by the interference of stochastic variability. This
soft X-ray lead in the new data supports the idea that the QPO of RE J1034+396
is a possible AGN counterpart of the 67 Hz high-frequency QPO seen in the black
hole binary (BHB) GRS 1915+105. We also search for QPO harmonics, but do not
find any significant signals. Our new data reinforce previous results that the
QPO is seen in a specific spectral state, as the only 2 observations showing no
significant QPO signal exhibit an even stronger soft X-ray excess than the
other 6 observations which display the QPO. Therefore, our results imply that
the QPO in RE J1034+396 is physically linked to a soft X-ray component.
