Thermonuclear Explosions and Accretion-induced Collapses of White Dwarfs in Active Galactic Nucleus Accretion Disks

Abstract

White dwarfs (WDs) embedded in gaseous disks of active galactic nucleus (AGNs) can rapidly accrete materials from the disks and grow in mass to reach or even exceed the Chandrasekhar limit. Binary WD (BWD) mergers are also believed to occur in AGN accretion disks. We study observational signatures from these events. We suggest that mass-accreting WDs and BWD mergers in AGN disks can lead to thermonuclear explosions that drive an ejecta shock breakout from the disk surface and power a slow-rising, relatively dim Type Ia supernova (SN). Such SNe Ia may be always outshone by the emission of the AGN disk around the supermassive black hole (BH) with a mass of M SMBH 10 8 M . Besides, accretion-induced collapses (AICs) of WDs in AGN disks may occur sometimes, which may form highly-magnetized millisecond neutron stars (NSs). The subsequent spin-down process of this nascent magnetar can deposit its rotational energy into the disk materials, resulting in a magnetardriven shock breakout and a luminous magnetar-powered transient. We show that such an AIC event could power a rapidly evolving and luminous transient for a magnetic field of B ∼ 10 15 G. The rising time and peak luminosity of the transient, powered by a magnetar with B ∼ 10 14 G, are predicted to have similar properties with those of superluminous supernovae. AIC events taking place in the inner parts of the disk around a relatively less massive supermassive BHs (M SMBH 10 8 M ) are more likely to power the transients that are much brighter than the AGN disk emission and hence easily to be identified.

Micro Tidal Disruption Events in Active Galactic Nuclei

“…The unique environments in AGN disks are expected to give rise to a wealth of other interesting phenomena, such as the tidal disruption of a star by a neutron star (albeit at a smaller rate of 0.1 Gpc −3 yr −1 ), the formation of Thorne-Zytkow objects (Thorne & Zytkow 1975) when a neutron star in the disk is unable to tidally disrupt a stellar companion due to a large stellar radius, the tidal disruption of white dwarfs by stellar-mass BHs, with unique signatures (Maguire et al 2020), and the accretion induced collapse of neutron stars (Perna et al 2021b) and white dwarfs (Zhu et al 2021a).…”

Supernova explosions in active galactic nuclear discs

“…The highly relativistic electromagnetic signatures of 𝛾-ray bursts (GRBs) and the accretion induced collapse of neutron stars (NS) into BHs have been recently explored (Zhu et al 2020(Zhu et al , 2021aPerna et al 2021b,a). Recently, Zhu et al (2021b) suggested that type Ia supernovae may potentially be observable in AGN discs. However, a detailed model for the interaction of a non-relativistic SN explosion with the AGN disc and its observational signatures is still lacking.…”