  Since 2016 October, the active galaxy PKS 2247-131 has undergone a gamma-ray
outburst, which we studied using data obtained with the Fermi Gamma-ray Space
Telescope. The emission arises from a relativistic jet in PKS 2247-131, as an
optical spectrum only shows a few weak absorption lines, typical of the BL
Lacertae sub-class of the blazar class of active galactic nuclei. Here we
report a ~34.5 day quasi-periodic oscillation (QPO) in the emission after the
initial flux peak of the outburst. Compared to one-year time-scale QPOs,
previously identified in blazars in Fermi energies, PKS 2247-131 exhibits the
first clear case of a relatively short, month-like oscillation. We show that
this QPO can be explained in terms of a helical structure in the jet, where the
viewing angle to the dominant emission region in the jet undergoes periodic
changes. The time scale of the QPO suggests the presence of binary supermassive
black holes in PKS 2247-131.
