Quasi-periodic modulation observed in the gamma-ray blazar PG 1553+113 and the MAGIC campaign 2015-2017

A gamma-ray nearly-periodic oscillation was observed from the well-known GeV/TeV BL Lac object PG 1553+113 by the Fermi Large Area Telescope (LAT). The quasi-periodicity in the gamma-ray flux (E>100 MeV and E>1 GeV), reported for the first time in an active galactic nucleus, is significant with a < 1% probability over red-noise fluctuations. The period of the gamma-ray modulation is 2.18 ± 0.08 observed with 3.5 oscillation maxima. It is supported by significant cross-correlated variations observed in radio and optical flux light curves. Interestingly, one of the physical scenarios that can account for such variability pattern is the presence of a supermassive black hole binary in the nucleus of PG 1553+113. The MAGIC telescopes have observed PG 1553+113 at VHE since 2005. An intense multi-wavelength (MWL) campaign started in 2015 aimed at revealing the physical scenarios that can account for such a variability pattern. The campaign will regularly monitor the source activity from radio to VHE (E>100 GeV) gamma rays and will cover the next maximum of activity, expected between the end of 2016 and beginning of 2017. The MWL data collected during this campaign, coupled with the gamma-ray ones form MAGIC, will be the key to determine the nature of the periodicity to disentangle the processes driving the periodic modulation from flaring activity typical in blazar objects.