Premerger localization of intermediate mass binary black holes with LISA and prospects of joint observations with Athena and LSST

The planned Laser Interferometric Space Antenna (LISA) will be able to detect gravitational waves (GWs) from intermediate mass binary black holes (IMBBHs) in the mass range $\sim 10^{2} \mbox{-} 10^{4}\,M_{\odot}$ up to a redshift $z\sim20$. Modulation effects due to orbital motion of LISA around the Sun facilitate precise premerger localization of the sources, which in turn would help in electromagnetic (EM) followups. In this work, we calculate the uncertainties in sky position, luminosity distance, and time of coalescence as a function of time to coalescence. For representative masses of the IMBBHs, we synthesize a population of binaries uniformly located and oriented on a sphere of radius 3 Gpc and compute the projected parameter measurement uncertainties using the Fisher information matrix. We find that for systems with a total mass of $10^3\,M_{\odot}$, the errors in the sky position and luminosity distance are $\sim 0.4\,\text{deg}^2$ and $\sim 6\%$, respectively, one day prior to coalescence. The coalescence time can be predicted with an uncertainty $\lesssim 10$ sec, one day before coalescence. We also find that for $10^3\,M_{\odot}$, around $40\%$ ($100\%$) of the population has a source localization that is smaller than the field of view of Athena (LSST) one day before the merger. These extremely precise measurements can be used to alert ground-based GW detectors and EM telescopes about the time and location of these mergers. We also discuss mechanisms that may produce EM emission from IMBBH mergers and study its detectability using the planned Legacy Survey of Space and Time (LSST) in the optical and Athena in the x-ray bands. Detection of an EM transient may provide us vital clues about the environments where these mergers occur and the distance estimation can pave the way for cosmography.