Time-domain and Multi-messenger Science with Supermassive Black Hole Binaries (Tingting Liu)

A supermassive black hole binary (SMBHB) is thought to form at the final stage of a galaxy merger when the two SMBHs are sufficiently close to become gravitationally bound, before gravitational radiation brings the SMBHs to coalescence. Last year, the pulsar timing array (PTA) collaborations announced evidence for a stochastic gravitational-wave (GW) background that is consistent with expectations from a population of SMBHBs, cracking open the window onto the low-frequency GW universe. The Laser Interferometer Space Antenna (LISA), which was recently adopted by ESA with a planned launch date in 2035, will probe, among many other sources, merging massive black hole binaries (MBHBs). In this talk, I will briefly review the wide range of time-domain and multi-messenger science opportunities with SMBHBs/MBHBs in the era of LISA and the next-generation PTA experiments. I will argue that the spatial resolution, sensitivity, field of view, and survey and rapid response capabilities of AXIS will make it a powerful observatory for SMBHB/MBHB science.