Detection of a QPO in 5 XMM-Newton observations of RE J1034+396
  The X-ray quasi-periodic oscillation (QPO) at $\sim 2.6 \times 10^{-4}$ Hz in
RE J1034+396 has been robustly observed in the 0.2-10 keV band of one $\sim 90$
ks XMM-Newton observation, but was not detected in subsequent observations.
Here, we investigate the power spectral density (PSD) of 8 archival XMM-Newton
observations of RE J1034+396, and search for the presence of QPOs in three
energy bands: soft (0.3-0.8 keV); hard (1.0-4.0 keV) and total (0.2-10.0 keV).
We find a significant detection of a QPO feature in the hard-band PSD of 5 low
flux/spectrally-harder observations. The QPO frequency has remained persistent
at $\sim 2.6 \times 10^{-4}$ Hz over $\sim 4$ years of observations, though it
is no longer detected in the soft band, except in the original observation.
This result increases the duration where the QPO is present by a factor of
three (now $\sim 250$ ks), allowing for a better understanding of the QPO
phenomenon observed in both Active Galactic Nuclei (AGN) and black hole X-ray
binaries (BHBs).
