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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 $M_{rm SMBH}gtrsim 10^8,M_odot$. 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 magnetar-driven 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 $Bsim10^{15},{rm G}$. The rising time and peak luminosity of the transient, powered by a magnetar with $Bsim10^{14},{rm G}$, 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 ($M_{rm SMBH}lesssim10^8,M_odot$) are more likely to power the transients that are much brighter than the AGN disk emission and hence easily to be identified.
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The disks of active galactic nuclei (AGNs) have emerged as a rich environment for the evolution of stars and their compact remnants. The very dense medium favors rapid accretion, while torques and migration traps enhance binary formation and mergers. Both long and short gamma-ray bursts (GRBs) are hence expected. We show that AGN disks constitute an ideal environment for another interesting phenomenon: the accretion induced collapse (AIC) of neutron stars (NSs) to black holes (BHs). Rapid accretion in the dense disks can cause NSs to grow to the point of exceeding the maximum mass allowed by their equation of state. General relativistic magnetohydrodynamical simulations have shown that electromagnetic signatures are expected if the NS is surrounded by a mini-disk prior to collapse, which then rapidly accretes onto the BH, and/or if the NS is highly magnetized, from reconnection of the magnetosphere during collapse. Here we compute the rates of AICs and their locations within the disks for both isolated NSs, and for (initially stable) NSs formed from NS-NS mergers. We find that the global AIC rates are $sim 0.07-20$~Gpc$^{-3}$~yr$^{-1}$, and we discuss their observable prospects and signatures as they emerge from the dense disk environments.
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 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 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 $m_{1}=m_{bullet}/10sunm$ 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.
Supernova (SN) explosions can potentially affect the structure and evolution of circumnuclear disks in active galactic nuclei (AGN). Some previous studies have suggested that a relatively low rate of SN explosions can provide an effective value of alpha viscosity between 0.1 and 1 in AGN accretion disks within 1 pc scale. In order to test this possibility, we provide some analytic scalings of the evolution of a SN remnant embedded in a differentially rotating smooth disk. We calibrate our estimates using three-dimensional hydrodynamical simulations where the gas is modeled as adiabatic with index $gamma$. Our simulations are suited to include the fact that a fraction of the momentum injected by the SN escapes from the disk into the corona. Based on these results, we calculate the contribution of SN explosions to the effective alpha viscosity, denoted by $alpha_{SNe}$, in a model AGN accretion disk, where accretion is driven by the local viscosity $alpha$. We find that for AGN galaxies with a central black hole of $~ 10^8M_{cdot}$ and a disk with viscosity $alpha=0.1$, the contribution of SN explosions may be as large as $alpha_{SNe} simeq 0.02$, provided that $alpha gtrsim 1.1$. On the other hand, in the momentum conservation limit, which is valid when the push by the internal pressure of the SN remnant is negligible, we find $alpha_SNe lesssim 6times10^{-4}$.
Disks of gas accreting onto supermassive black holes are thought to power active galactic nuclei (AGN). Stars may form in gravitationally unstable regions of these disks, or may be captured from nuclear star clusters. Because of the dense gas environment, the evolution of such embedded stars can diverge dramatically from those in the interstellar medium. This work extends previous studies of stellar evolution in AGN disks by exploring a variety of ways that accretion onto stars in AGN disks may differ from Bondi accretion. We find that tidal effects from the supermassive black hole significantly alter the evolution of stars in AGN disks, and that our results do not depend critically on assumptions about radiative feedback on the accretion stream. Thus, in addition to depending on $rho/c_s^3$, the fate of stars in AGN disks depends sensitively on the distance to and mass of the supermassive black hole. This affects where in the disk stellar explosions occur, where compact remnants form and potentially merge to produce gravitational waves, and where different types of chemical enrichment take place.
Active galactic nuclei (AGN) are prominent environments for stellar capture, growth and formation. These environments may catalyze stellar mergers and explosive transients, such as thermonuclear and core-collapse supernovae (SNe). SN explosions in AGN discs generate strong shocks, leading to unique observable signatures. We develop an analytical model which follows the evolution of the shock propagating in the disc until it eventually breaks out. We derive the peak luminosity, bolometric lightcurve, and breakout time. The peak luminosities may exceed $10^{45}$ erg s$^{-1}$ and last from hours to days. The brightest explosions occur in regions of reduced density; either off-plane, or in discs around low-mass central black holes ($sim 10^6 M_odot$), or in starved subluminous AGNs. Explosions in the latter two sites are easier to observe due to a reduced AGN background luminosity. We perform suites of 1D Lagrangian radiative hydrodynamics SNEC code simulations to validate our results and obtain the luminosity in different bands, and 2D axisymmetric Eulerian hydrodynamics code HORMONE simulations to study the morphology of the ejecta and its deviation from spherical symmetry. The observed signature is expected to be a bright blue, UV, or X-ray flare on top of the AGN luminosity from the initial shock breakout, while the subsequent red part of the lightcurve will largely be unobservable. We estimate the upper limit for the total event rate to be $mathcal{R}lesssim 100 rm yr^{-1} Gpc^{-3}$ for optimal conditions and discuss the large uncertainties in this estimate. Future high-cadence transient searches may reveal these events. Some existing tidal disruption event candidates may originate from AGN supernovae.
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