Evolution and merger of dynamical assembly black holes in AGN disks

Project Information

AGNBlackHoles

Grant agreement ID: 101065374

DOI

10.3030/101065374

Project terminated on 18 July 2023

EC signature date 20 July 2022

Start date 1 August 2022

End date 31 July 2024

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 230 774,40

Coordinated by

KOBENHAVNS UNIVERSITET
Denmark

Periodic Reporting for period 1 - AGNBlackHoles (Evolution and merger of dynamical assembly black holes in AGN disks)

Reporting period: 2022-08-01 to 2024-07-31

Summary of the context and overall objectives of the project

Several binary black hole (BBH) mergers have now been observed in gravitational waves (GWs) by aLIGO/VIRGO/ KAGRA. With an estimated merger rate of 16–38 Gpc3/yr, many hundreds of mergers are expected to be detected in the coming years. However, a central outstanding question still remains: What astrophysical mechanisms bring these observed BBHs to merger? To date, the following three distinct merger channels have been greatly discussed:
i) Isolated Binary Channel: In this scenario, two stars are born and evolve together in an isolated binary until they collapse into black holes (BHs). During this evolution, their separation can vary by orders of magnitude through mass transfer at the end of stellar evolution. If the final BBH separation is small enough, they will be able to merge and appear as an observable GW source.
ii) Dynamical Channel: This scenario mainly has two flavors: a BBH is assembled and brought to merger through a scattering process, involving a BBH directly interacting with an incoming single BH; a BBH is brought to merger with the aid of a third distant body through Lidov-Kozai (LK) oscillations.
iii) AGN disk Channel: In this scenario BHs either form in the active galactic nuclei (AGN) accretion disks as a result of star formation, or captured by the disk from the surrounding nuclear star cluster through gas-friction. The resultant BH population then undergoes scattering processes, together with gas-friction, leading to the mergers of BBHs.

The major question is: How do we observationally tell these channels apart? The observed BH mass spectrum, BBH orbital eccentricity, and BH spin directions will differ between the different channels. Therefore, resolving the observational signatures in the AGN channel plays an absolute key role in the blooming field of GW astrophysics. My proposed program aims at resolving how BBH evolve and merge in AGN disks under the influence of both gas, post-Newtonian (PN) effects and tertiary companion (such as the central supermassive BH). The main research objectives (RO) addressed by the planned research are briefly outlined below:
(RO1) Modeling Black Hole Interactions in an AGN Disk: I will here study the dynamical evolution of BBHs in the AGN disk with the inclusion of both gas-friction, and PN-corrections and the perturbation of the tertiary companion. For this, I need to at least perform the following two main tasks: (i) modeling the formation of hierarchical triple/multiple systems, including the prescriptions for weak dynamical encounters leading to ‘dissipative-capture-formation’; (ii) developing a module for evolving these hierarchical systems on secular timescales; A N-body code or a secular code including the non-secular orbital effect has to be developed. In all these interactions, it is expected that PN-corrections will lead to major effects that have not yet been discussed in this context. I will use my extended knowledge on few-body dynamics to study not only these effects in AGN disks for the first time, but also what effects to the global potential from the AGN disk and the perturbation of the tertiary companion like the supermassive BH (SMBH).
(RO2) Predicting Observables: I will here study the observable properties of the BBH evolution and mergers in the AGN disk taking into account the dynamics described in RO1. With the numerical framework mentioned above, I will be able to systematically study how the GW observables, i.e. mass spectrum, orbital eccentricity in LIGO/Virgo, and relative BH spin orientations, relate to the dynamical processes. A proper inclusion of post-Newtonian (PN) dynamics for describing the evolution of the BH systems in the AGN disk will give rise to an un-explored rich eccentricity distribution observable by LISA and LIGO/Virgo, similar to what is found in the cluster case. Similarly, a proper inclusion of secular LK-effects for all the hierarchical BH systems that are formed in the disk will also give rise to great changes in the current set of reported observables.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project has to be unfortunately terminated early since I accepted a tenure track position at Zhejiang University. The project has been carried out for 10 months and achieved most of its objectives and milestones for the period, with relatively minor deviations
For RO1, the BH interactions in the AGN disk can be broadly divided into two categories: Case I, BHB interacts with the central SMBH; Case II, BHB interacts with another stellar-mass object embedded in the disk. For both cases, the hierarchical triples can be formed due to dissipative-capture. I have studied the dynamics of the BHB under the perturbation of either the SMBH or the stellar-mass companion, considering the PN effect. I have developed a new approach to evolve the three-body system based on the orbital averaging approximation, in which I only averaged over the orbital period of the inner binary of the hierarchical triple. Using this technique, I was able to capture the non-secular effect during the evolution of the BHB. I have found that for coplanar triples how the PN corrections, especially the apsidal precessions of the inner and outer orbits, modify the orbital decay of the BHB around the SMBH for Case I (the paper is in preparation) and trigger the secular resonance, leading to the eccentricity excitation for Case II ( The results were published in Liu B., D'Orazio D. J., Vigna-Gómez A., Samsing J., 2022, PhRvD, 106, 123010).
For RO2, the observables of triples formed in the disk environment have been studied. For Case I, the BHBs can be formed due to GW-captured near a SMBH. These BHBs are highly eccentric such that they can merge quickly and move around the SMBH with part of the Keplerian orbits. I have studied the modulation of GW signal of a merging BHB due to the tertiary companion, i.e. the so-called Roemer delay. I have shown that the time delay can be very different respected to an unperturbed merging binary. This is because the tidal effect of the tertiary SMBH generates a distinct orbital decay rate. For Case II, the inner binary could be an inspiraling BHB (e.g. LISA source) and the outer binary consists of the inner binary and a stellar-mass object. These inspiraling BHBs are expected to be numerous in AGN disk and may remain stable as disc disperses. I have shown that the properties of the inspiraling BHBs can be probed via the secular variation of the orbital motion of the tertiary companion when the tertiary is a visible object and the system locates in the local universe. The result was published in Liu B., D'Orazio D. J., Vigna-Gómez A., Samsing J., 2022, PhRvD, 106, 123010.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The MSCA fellowship has had a significant impact on my career, particularly on the scientific front. One of the key achievements was the publication of my paper (Liu B., D'Orazio D. J., Vigna-Gómez A., Samsing J., 2022, PhRvD, 106, 123010) presenting a novel strategy for searching for inspiraling BHBs in addition to GW detections. This work has great potential to enhance the analysis of data obtained by Gaia, and I have made sure to effectively communicate this new knowledge through scientific publications and direct interactions with the target group. Moreover, during the fellowship, I proposed a new method for detecting the environment of GW sources by studying the modulation of the GW signal. While the paper presenting this research is still in preparation, the robust signature identified in the GW signal holds significant implications for understanding BHB formation. This, in turn, will enable astronomers working with GW detectors to consider seriously in the data analysis in the forthcoming observation events.
During my time at the NBI, I made a significant contribution by bringing in my expertise in the field of secular dynamics of few-body systems, which was not previously available within the astrophysics group. This transfer of knowledge was facilitated through regular group meetings with fellow members and ongoing discussions with my main supervisor, Prof. M. Pessah. These interactions allowed me to share my insights and methodologies with the group, enriching the collective understanding of few-body dynamics. Additionally, my time at the NBI provided me with an opportunity to expand my knowledge beyond my primary research focus. Interactions with researchers at the NBIA exposed me to diverse topics of study, particularly in other areas of few-body dynamics and high-energy astrophysics related to transient phenomena.
The work accomplished during the fellowship and the successful completion of the training objectives have significantly elevated my professional standing. This experience has had a profound impact on my career prospects, leading to a remarkable achievement of being offered a tenure track position at Zhejiang University in China.

A figure from Liu Bin et al, 2022, PhRvD, 106, 123010