2014MNRAS.445L..16A

The X-ray quasi-periodic oscillation (QPO) at ∼ 2.6x10^–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 ∼ 2.6x10^–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.