Thorne-\.Zytkow Objects 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 (\(\sim 10^{14}\,{\rm cm^{-3}}\)), they will form Thorne-\.Zytkow objects (TZOs). 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 TZOs 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 \(\sim 10^{9}L_{\rm Edd}/c^{2}\), where \(L_{\rm Edd}\) 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 \(t_{\rm a}\sim 10^{5}m_{1}\,\)s, where \(m_{1}=\bhm/10\sunm\) is the BH mass. The kinetic energy of the outflows accumulated during \(t_{\rm a}\) 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 (\(\sim 0.1\,\)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 {\it 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.