An X-Ray Periodicity of ∼1.8 hr in Narrow-line Seyfert 1 Galaxy Mrk 766

In the narrow-line Seyfert 1 galaxy Mrk 766, a quasi-periodic oscillation (QPO) signal with a period of ∼6450 s is detected in the XMM-Newton data collected on 2005 May 31. This QPO signal is highly statistically significant at the ∼5σ confidence level, with a quality factor of Q=f/{{Δ }}f> 13.6. The X-ray intensity changed by a factor of 3, with a root mean square fractional variability of 14.3%. Furthermore, this QPO signal is present in the data of all three EPIC detectors and two RGS cameras and its frequency follows the {f}_{QPO}-{M}_{BH} relation spanning from stellar-mass to supermassive black holes. Interestingly, a possible QPO signal with a period of ∼4200 s had been reported in the literature. The frequency ratio of these two QPO signals is ∼3:2. Our result is also in support of the hypothesis that the QPO signals can just be transient. The spectral analysis reveals that the contribution of the soft excess component below ∼1 keV is different between epochs with and without QPO. This property, as well as the former frequency ratio, are well-detected in X-ray BH binaries, which may shed some light on the physical origins of our event.