Quasi-periodicities of PKS 2155-304

Abstract

We have searched for periodicities in our VRIJHK photometry of PKS 2155-304, which covers the years 2005-2012. A peak of the Fourier spectrum with high significance is found at T$\sim$315 days, confirming the recent findings by \cite{Zhang2014}. The examination of the gamma-ray light curves from the\textit{ Fermi} archives yields a significant signal at $\sim$ 2T, which, while nominally significant, involves data spanning only $\sim$ 6T. Assuming a black hole mass of $10^{9}\ M_{\odot}$ the Keplerian distance corresponding to the quasi-period T is $\sim10^{16}$ cm, about 50 Schwarzschild radii.