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$a Supermassive black hole binaries (SMBHBs) are natural by-products of galaxy mergers and are expected to be powerful multi-messenger sources. They can be powered by the accretion of matter and shine throughout the electromagnetic spectrum similarly to normal active galactic nuclei (AGNs). Current electromagnetic observatories have good chances to detect and identify these systems in the near future, but we need precise observational indicators to distinguish single AGNs from SMBHBs. In this work, we propose a novel electromagnetic signature from SMBHBs: periodic flares caused by the interaction between the jets launched by the black holes. We investigate close black hole binaries accreting matter from a circumbinary disc and the mini-discs formed around each hole, and launching magnetically-dominated jets in the direction of their spin through the Blandfor--Znajeck mechanism. If the spins are slightly inclined, the two jets will encounter each other once per orbit. We argue that this interaction may trigger strong magnetic reconnection events where particles are accelerated and form plasmoids that emit non-thermal radiation. We model the evolution of these particles and calculate the radiative output obtaining spectra and light curves at different wavelengths. We show that these flares can significantly shine in radio, soft X-rays, and \gamma rays, providing a periodic multi-wavelength electromagnetic signature for SMBHBs |
