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Outflow Bubbles from Compact Binary Mergers Embedded in Active Galactic Nuclei: Cavity Formation and the Impact on Electromagnetic Counterparts

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 Added by Shigeo Kimura
 Publication date 2021
  fields Physics
and research's language is English




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We propose a novel scenario for possible electromagnetic (EM) emission by compact binary mergers in the accretion disks of active galactic nuclei (AGNs). Nuclear star clusters in AGNs are a plausible formation site of compact-stellar binaries (CSBs) whose coalescences can be detected through gravitational waves (GWs). We investigate the accretion onto and outflows from CSBs embedded in AGN disks. We show that these outflows are likely to create outflow cavities in the AGN disks before the binaries merge, which makes EM or neutrino counterparts much less common than would otherwise be expected. We discuss the necessary conditions for detectable EM counterparts to mergers inside the outflow cavities. If the merger remnant black hole experiences a high recoil velocity and can enter the AGN disk, it can accrete gas with a super-Eddington rate, newly forming a cavity-like structure. This bubble can break out of the disk within a day to a week after the merger. Such breakout emission can be bright enough to be detectable by current soft X-ray instruments, such as Swift-XRT and Chandra.



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The first detection of a binary neutron star merger through gravitational waves and photons marked the dawn of multi-messenger astronomy with gravitational waves, and it greatly increased our insight in different fields of astrophysics and fundamental physics. However, many open questions on the physical process involved in a compact binary merger still remain and many of these processes concern plasma physics. With the second generation of gravitational wave interferometers approaching their design sensitivity, the new generation under design study, and new X-ray detectors under development, the high energy Universe will become more and more a unique laboratory for our understanding of plasma in extreme conditions. In this review, we discuss the main electromagnetic signals expected to follow the merger of two compact objects highlighting the main physical processes involved and some of the most important open problems in the field.
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The next two decades are expected to open the door to the first coincident detections of electromagnetic (EM) and gravitational wave (GW) signatures associated with massive black hole (MBH) binaries heading for coalescence. These detections will launch a new era of multimessenger astrophysics by expanding this growing field to the low-frequency GW regime and will provide unprecedented understanding of the evolution of MBHs and galaxies. They will also constitute fundamentally new probes of cosmology and would enable unique tests of gravity. The aim of this Living Review is to provide an introduction to this research topic by presenting a summary of key findings, physical processes and ideas pertaining to EM counterparts to MBH mergers as they are known at the time of this writing. We review current observational evidence for close MBH binaries, discuss relevant physical processes and timescales, and summarize the possible EM counterparts to GWs in the precursor, coalescence, and afterglow stages of a MBH merger. We also describe open questions and discuss future prospects in this dynamic and quick paced research area.
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