X-ray transients in the numerical merger sample. Similar to the right panel of Fig. 5. Black dots and red crosses correspond to the X-ray post-merger BH fluxes for the pre-transient case (fiducial properties) and the brightening (which assumes f_Edd = 1). We only show mergers for which either of the two configurations is observable. Pre-transient and brightening fluxes are connected by orange dashed lines if the pre-transient source (total flux from the BH and the galaxy) is undetected and the brightening source is detected and the BH is observable, and orange solid lines if both fluxes are detectable and the flux change is larger than a factor of 2. GW-undetectable remnant BHs are shown in purple (pre-transient) and orange (brightening).