demonstrate conclusively, at the 4.6σ significance level, that the periodicity in this object is not due to random fluctuations in flux density. The observed period translates to 2.082 ± 0.003 years in the rest frame at the z = 1.285 redshift of PKS 2131−021. The periodic variation in PKS 2131−021 is remarkably sinusoidal.
We present a model in which orbital motion, combined with the strong Doppler boosting of the approaching relativistic jet, produces a sine-wave modulation in the flux density which easily fits the observations. Given the rapidly-developing field of gravitational wave experiments with pulsar timing arrays, closer counterparts to
PKS 2131−021 and searches using the techniques we have developed are strongly motivated. These resultsconstitute a compelling demonstration that the phenomenology, not the theory, must provide the lead in this field.
Keywords: galaxies: active, galaxies: jets, galaxies: BL Lacertae objects: individual (PKS 2131−021)