I have over the past two years studied several aspects of black hole scatterings in dense environments to establish new understanding of what observational imprints such formation channels leave in the GW signals. Part of my work has been centered on how to observationally probe the formation origin of BBH mergers that are eccentric, i.e. non-circular, when they become visible in the LIGO/Virgo detector bands. In this regard I have established two new research areas during my MC Fellowship, each of which are introduced in the following:
1) Formation of black hole mergers in gas-disks (AGN) around super massive BHs.
Using novel analytical and numerical techniques, I have found that the probability for forming an eccentric merger in a planar disk-like environment can be up to two orders-of-magnitude higher compared to the isotropic case found in most clusters. The probability for a merger to be eccentric in LIGO/Virgo if formed in a planar AGN disk can therefore approach unity, which is highly surprising and extremely relevant for several of the new GW observations. For example, the source GW190521 observed a few months back is the first source with a non-zero eccentricity. On top of this, its masses exceeded the mass gap limit, and a possible electromagnetic counterpart was also seen. However, its non-zero eccentricity was a major mystery before my work in which I combined all the pieces and illustrated this is a natural outcome of BBH mergers forming in AGN disk environments. This finding is now published in Nature with me as first author. We are currently planning many follow up papers on this discovery, many of which have not been submitted yet.
2) Non-zero eccentricity is a clear indicator of a dynamical origin. However, recent progress has also shown that many different kinds of dynamical systems exist that all can give rise to eccentric mergers. For example, few-body scatterings in clusters will result in BBH mergers similar to the one found through single-single GW captures in galactic nuclei. So, how do we tell the difference between these channels using GWs alone? In my recent work, I have shown that modulations of the burst timing signal can be used to tell the difference between these channels. For example, if the eccentric source is formed through a binary-single interaction, which is the dominant mechanism in clusters, then the third object will in some cases be close enough to the inspiraling eccentric BBH that its tidal force generates modulations in the burst arrival times. Another example are eccentric BBHs forming in AGN disks, where the gas drag also can modulate the signal. What I now have shown in a series of papers, that are soon to be published, is that one can probe all these environmental effects through accurate timing of the bursts. This will finally help distinguish different eccentric formation channels apart using GWs alone. All of these ideas are new, and will over the next few years be explored in much more detail.
