Accretion-modified Stars in Accretion Disks of Active Galactic Nuclei: The Low-luminosity Cases and an Application to Sgr A*

Abstract

In this paper, we investigate the astrophysical processes of stellar-mass black holes (sMBHs) embedded in advection-dominated accretion flows (ADAFs) of supermassive black holes (SMBHs) in low-luminosity active galactic nuclei. The sMBH is undergoing Bondi accretion at a rate lower than the SMBH. Outflows from the sMBH-ADAF dynamically interact with their surroundings and form a cavity inside the SMBH-ADAF, thereby quenching the accretion onto the sMBH. Rejuvenation of the Bondi accretion is rapidly done by turbulence. These processes give rise to quasi-periodic episodes of sMBH activities and create flickerings from relativistic jets developed by the Blandford–Znajek mechanism if the sMBH is maximally rotating. Accumulating successive sMBH-outflows trigger a viscous instability of the SMBH-ADAF, leading to a flare following a series of flickerings. Recently, the similarity of near-infrared flare’s orbits has been found by GRAVITY/VLTI astrometric observations of Sgr A∗: their loci during the last 4 yr consist of a ring in agreement with the well-determined SMBH mass. We apply the present model to Sgr A*, which shows quasi-periodic flickerings. An sMBH of ∼40M ⊙ is preferred orbiting around the central SMBH of Sgr A* from fitting radio to X-ray continuum. Such an extreme mass ratio inspiraling provides an excellent laboratory for LISA/Taiji/Tianqin detection of mHz gravitational waves with strains of ∼10−17, as well as their polarization.

Accretion-modified Stars in Accretion Disks of Active Galactic Nuclei: Observational Characteristics in Different Regions of the Disks

“…) discussed in Wang et al (2023a) and point out the difference that the role of external pressure of the SMBH disk is unimportant in our model. In Section 3, we demonstrate that only a small fraction of the kinetic energy is utilized to work against the external pressure of the SMBH disk.…”

“…As a consequence, outbursts from the BHs emerge as the Bondi explosion through the interaction between powerful outflows and dense AGN disk gas (Wang et al 2021a;Tagawa et al 2022;, resulting in thermal and nonthermal radiations (e.g., Wang et al 2021a;Tagawa et al 2023b). Wang et al (2023a) first applied the AMS model to Sgr A * with low accretion rates, naturally reproducing the orbit observed by GRAVITY/VLTI (GRAVITY Collaboration et al 2020;Gravity Collaboration et al 2023) and successfully explaining the period of the flare/ flicking in the light curve (Genzel et al 2003(Genzel et al , 2010 and spectral energy distribution (SED) from radio to X-ray bands (Witzel et al 2021;Boyce et al 2022). It is important to note that the above AMS is situated within advection-dominated accretion flows (ADAFs; .…”

“…Considering the environment of accretion BHs, we explore the AMSs located in the outer, middle, and inner regions of the standard disk (Shakura & Sunyaev 1973), leading to distinct physical processes and observational characteristics. This is the fourth paper in a series aimed at investigating the behavior of BHs within the AGN disks (Wang et al 2021a(Wang et al , 2021b(Wang et al , 2023a. This paper is structured as follows.…”

“…In fact, f adv is quite uncertain in consideration of advection, photon trapping, and outflows (e.g., see the simulations in Takeuchi et al 2009). These powerful outflows will significantly impact the local structures of the SMBH disk, generating a cavity (Wang et al 2021a(Wang et al , 2021bKimura et al 2021) with a radius denoted as R cav , provided that the kinetic energies of the outflows exceed the local dissipation rates of gravitational energy within the SMBH disk (Wang et al 2023a)…”

Stellar Black Holes Can “Stretch” Supermassive Black Hole Accretion Disks

“…Very recently, Gravity Collaboration et al (2023) found that the Sagittarius A * flare is located at about 9 gravitational radii, consistent with the Keplerian orbital motion of the innermost accretion zone hot spot. While the physical origin of the flare remains unclear, it is proposed that a star (Leibowitz 2020) or a ∼40 M e sBH (Wang et al 2023a) orbiting periodically around the central SMBH drives this Sagittarius A * flare through interaction with the SMBH accretion disk. The star or the sBH is embedded in the advection-dominated accretion flow around the SMBH in the Galactic center.…”