Looking for observational signatures of early binary black hole systems

Context. Many recent studies have focused on the observables associated with near-merger binary black-holes (BBHs) embedded in a circumbinary disk, but we still lack knowledge of the observables of BBHs in their early stage. In this stage, the separation between the two black holes is so large that both black holes could potentially retain the individual accretion disks that existed before the BBH was created. For such early BBH systems, it is interesting to look for observables originating in these individual disks, as their structure is likely to differ from that of mini-disks that are often observed in simulations of later BBH stages. Aims. In a companion paper, we presented a set of hydrodynamical simulations of an individual disk surrounding a primary black hole while it was affected by a secondary black hole in an early BBH system. This created three well-known characteristic features in the disk structure. Here, we explore the imprints of these three features on the observables associated with the thermal emission of the preexisting black hole disk. Our aim was twofold: we first determined which observables were best suited for detecting these early systems, and second, we determined what we might extrapolate about these systems based on observations. Methods. We used general relativistic ray-tracing in order to produce synthetic observations of the thermal emission emitted by early BBHs with different mass ratio and separations in order to search for distinctive observational features of early systems. Results. We found that, in the case of early BBHs with preexisting disk(s), a necessary, although not unique, observational feature is the truncation of their disk(s). Conclusions. This observable might be used for an automated search of potential BBHs and to potentially rule out some existing candidates.