Description
Supermassive black hole binaries (SMBHB) form in galactic nuclei after galaxy mergers. In this presentation, I will demonstrate that inspiraling SMBHBs detectable with LISA may act as a binary gravitational lens leading to a new class of electromagnetic (EM) counterparts. The caustic (i.e. the curve on which the magnification diverges) of the binary lens is more complex than that of a single point mass, and the orbit and inspiral of the binary leads to the rotation and decreasing size of the caustic curve. This opens the possibility that a background source such as a star can cross the caustic multiple times during the binary orbital period leading to strong quasi-periodic time-dependent magnification. I will show examples where this EM signal may be detected, where the recurring peak magnification due to the SMBHB lens reaches a factor of 10^5 for giant stars with particular features which makes it possible to extract the properties of the supermassive binary such as the orbital period and eccentricity. These electromagnetic events may be targets for surveys such as LSST to identify inspiraling supermassive black holes even without a coincident gravitational wave detection.
