A ~4.6 h quasi-periodic oscillation in the BL Lacertae PKS 2155-304?

We report a possible detection of an ~4.6-h quasi-periodic oscillation (QPO) in the 0.3-10 keV emission of the high-energy peaked blazar PKS 2155-304 from a 64 ks observation by the XMM-Newton EPIC/pn detector. We identify a total modulation of ~5% in the light curve and confirm that nominal period by periodogram, structure function, and wavelet analyses. The limited light curve duration allows the capture of only 3.8 cycles of this oscillation and thus precludes a very strong claim for this QPO, despite a nominally high (≳3σ) statistical significance. We briefly discuss models capable of producing an X-ray QPO of such a period in a blazar.