Electromagnetic and high-energy neutrino signals from catastrophic explosion events in active galactic nucleus accretion disks

Abstract

Active galactic nuclei (AGN) are a type of extremely bright astronomical objects in the universe, located at the centers of some galaxies. They are driven by the gravitational energy released as matter from the surrounding accretion disk falls into the central supermassive black hole. Numerous studies have shown that stars and compact objects (including white dwarfs, neutron stars, and black holes) may commonly exist within the AGN accretion disk around supermassive black holes. These stars and compact objects might originate from star clusters around the disk and be captured by it, or they might form in situ within the disk. These stars and compact objects evolve, accrete, and migrate within the AGN disk, increasing the probability of celestial encounters. Additionally, the friction from the disk material facilitates the formation of numerous binary systems. Therefore, AGN accretion disks serve as cradles for merger or collision events of stars and compact objects. Meanwhile, due to the accretion of disk material, massive stars in AGN disks are also prone to catastrophic explosions. These catastrophic mergers and explosion events occurring in AGN disks will exhibit significant differences from those occurring in the ordinary interstellar medium due to the relatively dense environment, possessing unique observational characteristics. Consequently, AGN accretion disks are the birthplaces of mergers or collision events of stars and compact objects. Moreover, due to the accretion of disk material, massive stars in AGN disks are highly susceptible to catastrophic explosions. These catastrophic mergers and explosion events occurring within AGN disks could have unique differences from those occurring in the ordinary interstellar medium due to the relatively dense environment, and will exhibit unique observational features. The discovery of the binary black hole merger, GW190521, by the LIGO-Virgo Collaboration in the third gravitational-wave observing run and its plausible associated optical electromagnetic counterpart, ZTF19abanrhr, located within the accretion disk of an AGN ignited interest in catastrophic explosion events within AGN disks in recent. Unlike typical catastrophic explosion events in the interstellar medium, those occurring within AGN disks often generate powerful shocks due to their dense surrounding environment. Consequently, these explosion events always produce bright shock breakout and shock cooling emissions, which could significantly affect their observational signatures and can be observed by present optical survey projects. Furthermore, the shocks can accelerate charged particles to generate cosmic rays, leading to non-thermal emissions by accelerated electrons, as well as the production of TeV-PeV neutrinos by accelerated protons. These high-energy astrophysical neutrinos can be triggered by IceCube Neutrino Observatory and next generation high-energy neutrino facility. This paper aims to provide a comprehensive review of current research on multi-wavelength radiation and high-energy neutrino emission from disk explosion events, while also summarizing the challenges and potential future directions in this field.