Detection of a QPO in five XMM-Newton observations of RE J1034+396.

The X-ray quasi-periodic oscillation (QPO) at ∼2.6 × 10^-4 Hz in RE J1034+396 has been robustly observed in the 0.2-10 keV band of one ∼90 ks XMM-Newton observation, but was not detected in subsequent observations. Here, we investigate the power spectral density (PSD) of eight 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 five low-flux/spectrally harder observations. The QPO frequency has remained persistent at ∼2.6 × 10^-4 Hz over ∼4 yr 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 3 (now ∼250 ks), allowing for a better understanding of the QPO phenomenon observed in both active galactic nuclei and black hole X-ray Binaries.