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The upcoming Laser Interferometer Space Antenna (LISA) is expected to detectgravitational waves (GWs) from massive black hole binaries (MBHB). Finding theelectromagnetic (EM) counterparts for these GW events will be crucial forunderstanding how and where MBHBs merge, measuring their redshifts,constraining the Hubble constant and the graviton mass, and for other novelscience applications. However, due to poor GW sky localisation,multi-wavelength, time-dependent electromagnetic (EM) models are needed toidentify the right host galaxy among many candidates. We studied merging MBHBsembedded in a circumbinary disc using high-resolution two-dimensionalsimulations, with a $\Gamma$-law equation of state, incorporating viscousheating, shock heating, and radiative cooling. We simulate the binary fromlarge separation until after merger, allowing us to model the decoupling of thebinary from the circumbinary disc (CBD). We compute the EM signatures andidentify distinct features before, during, and after the merger. Our mainresult is a multi-band EM signature: we find that the MBHB produces strongthermal X-ray emission until 1-2 days prior to the merger. However, as thebinary decouples from the CBD, the X-ray-bright minidiscs rapidly shrink insize, become disrupted, and the accretion rate drops precipitously. As aresult, the thermal X-ray luminosity drops by orders of magnitude, and thesource remains X-ray dark for several days after the merger, regardless of anypost-merger effects such as GW recoil or mass loss. Looking for the abruptspectral change where the thermal X-ray disappears is a tell-tale EM signatureof LISA mergers that does not require extensive pre-merger monitoring.

Disappearing thermal X-ray emission as a tell-tale signature of merging massive black hole binaries