Accretion-modified Stars in Accretion Disks of Active Galactic Nuclei: Slowly Transient Appearance

Abstract

Compact objects are expected to exist in the accretion disks of supermassive black holes (SMBHs) in active galactic nuclei (AGNs), and in the presence of such a dense environment (∼1014 cm−3), they will form a new kind of stellar population denoted as accretion-modified stars (AMSs). This hypothesis is supported by recent LIGO/Virgo detection of the mergers of very high-mass stellar binary black holes (BHs). We show that the AMSs will be trapped by the SMBH disk within a typical AGN lifetime. In the context of SMBH disks, the rates of Bondi accretion onto BHs are ∼109 L Edd/c 2, where L Edd is the Eddington luminosity and c is the speed of light. Outflows developed from the hyper-Eddington accretion strongly impact the Bondi sphere and induce episodic accretion. We show that the hyper-Eddington accretion will be halted after an accretion interval of t a ∼ 105 m 1 s, where m 1 = m •/10M ⊙ is the BH mass. The kinetic energy of the outflows accumulated during t a is equivalent to 10 supernovae driving an explosion of the Bondi sphere and developing blast waves. We demonstrate that a synchrotron flare from relativistic electrons accelerated by the blast waves peaks in the soft X-ray band (∼0.1 keV), significantly contributing to the radio, optical, UV, and soft X-ray emission of typical radio-quiet quasars. External inverse Compton scattering of the electrons peaks around 40 GeV and is detectable through Fermi-LAT. The flare, decaying with t −6/5 with a few months, will appear as a slowly varying transient. The flares, occurring at a rate of a few per year in radio-quiet quasars, provide a new mechanism for explaining AGN variability.

High-energy Neutrinos from Stellar Explosions in Active Galactic Nuclei Accretion Disks

“…For the calculations of the diffuse neutrino background emission, we show that SNe Ia, kilonovae, CCSNe, GRB-SNe, and choked GRBs in AGN disks may contribute to 10% of the observed diffuse neutrino background. SNe (Grishin et al 2021), Bondi explosions of stellar BHs (Wang et al 2021a) and mTDEs (Yang et al 2021) are predicted to have very high event rate densities in AGN disks. Such energetic events can contribute more considerably to the diffuse neutrino background.…”

“…These AGN stars will end up with supernovae (SNe) to pollute the disk with heavy elements, which can offer a possible explanation for the observational features of high-metallicity environment in AGN disks (e.g., Hamann & Ferland 1999;Warner et al 2003), and hence leave behind some stellar remnants inside the disks. AGN disks provide a natural environment for embedded stars and compact objects to grow, to accrete materials, and to migrate within it (e.g., Bellovary et al 2016;Cantiello et al 2021;Dittmann et al 2021;Jermyn et al 2021;Wang et al 2021a;Kaaz et al 2021;Kimura et al 2021;Peng & Chen 2021;Pan & Yang 2021). There could be abundant stars and compact objects gathering in the inner part of the AGN disks, so that some stellar explosions can frequently occur there.…”

“…Very recently, the field of stellar explosions in AGN accretion disks is exploding. The event rates and possible observable signatures for several kinds of stellar explosions occurring in AGN disks, such as SNe (Zhu et al 2021b;Grishin et al 2021;Moranchel-Basurto et al 2021), gamma-ray bursts (GRBs; Cheng & Wang 1999;Perna et al 2021a;Zhu et al 2021c,a), NS mergers (McKernan et al 2020;Zhu et al 2021c), binary BH (BBH) mergers (e.g., McKernan et al 2012;Bartos et al 2017;Yang et al 2019Yang et al , 2020McKernan et al 2019;Graham et al 2020;Tagawa et al 2020Tagawa et al , 2021Wang et al 2021b), Bondi explosions (Wang et al 2021a), microtidal disruption events (mTDEs; Yang et al 2021) and accretion-induced collapses of WDs (Zhu et al 2021b) and NSs (Perna et al 2021b), have been investigated in detail. Different from the classical low-density environments in which these stellar explosions occur, the AGN environment allows the ejecta launched from AGN stellar explosions to interact with the AGN gaseous disk to drive an energetic shock.…”

Can stellar-mass black hole growth disrupt disks of active galactic nuclei? The role of mechanical feedback

“…In such highly accreting flows, wind/outflow feedback also pushes back the accreting gas. If the wind is isotropically emitted, it significantly regulates accretion (Wang et al 2021a) and gas dynamical friction (Gruzinov et al 2020). On the other hand, if it is anisotropic, as predicted by numerical simulations (Jiao et al 2015;Kitaki et al 2021), the feedback does not efficiently regulate either accretion (Takeo et al 2020) or dynamical friction (Li et al 2020).…”

Accretion-modified Stars in Accretion Disks of Active Galactic Nuclei: Gravitational Wave Bursts and Electromagnetic Counterparts from Merging Stellar Black Hole Binaries

“…The outer parts of the accretion disk of supermassive black holes (SMBHs) in active galactic nuclei (AGNs) host many poorly understood, complicated processes. Star formation is unavoidable in these regions because of self-gravity (Paczyński 1977;Kolykhalov & Sunyaev 1980;Shlosman & Begelman 1989;Collin & Zahn 1999;Goodman 2003;Goodman & Tan 2004;Collin & Zahn 2008), producing compact stellar remnants from the rapid evolution of massive stars (Artymowicz et al 1993;Cheng & Wang 1999;Cantiello et al 2021;Moranchel-Basurto et al 2021;Wang et al 2021;Grishin et al 2021). Stellar evolution rapidly releases metals into the outer parts of the self-gravitating (SG) disk (Wang et al 2010(Wang et al , 2011(Wang et al , 2012, offering an explanation for the super-solar metallicities observed in AGNs across cosmic time (Hamann & Ferland 1999;Warner et al 2003;Nagao et al 2006;Shin et al 2013;Du & Wang 2014).…”