PREPRINT

Interacting Kilonovae: Long-lasting Electromagnetic Counterparts to Binary Mergers in the Accretion Disks of Active Galactic Nuclei

Jia Ren, Ken Chen, Yun Wang, Zi-Gao Dai

Submitted on 6 November 2022

Abstract

We investigate the dynamics and electromagnetic (EM) signatures of neutron star-neutron star (NS-NS) or neutron star-black hole (NS-BH) merger ejecta that occurs in the accretion disk of an active galactic nucleus (AGN). We find that the interaction between ejecta and disk gas leads to important effects on the dynamics and radiation. We show five stages of the ejecta dynamics: gravitational slowing down, coasting, Sedov-Taylor deceleration in the disk, re-acceleration after the breakout from the disk surface, and momentum-conserved snowplow phase. Meanwhile, the radiation from the ejecta is so bright that its typical peak luminosity reaches a few times 10431044 erg s1. Since most of the radiation energy has converted from the kinetic energy of merger ejecta, we call such an explosive phenomenon an interacting kilonova (IKN). It should be emphasized that IKNe are very promising, bright EM counterparts to NS-NS/BH-NS merger events in AGN disks. The bright peak luminosity and long rising time (i.e., ten to twenty days in UV bands, thirty to fifty days in optical bands, and one hundred days to hundreds of days in IR bands) allow most survey telescopes to have ample time to detect an IKN. However, the peak brightness, peak time, and evolution pattern of the light curve of an IKN are similar to a superluminous supernova in a galactic nucleus and a tidal disruption event making it difficult to distinguish between them. But it also suggests that IKNe might have been present in recorded AGN transients.

Preprint

Comment: ApJL accepted

Subjects: Astrophysics - High Energy Astrophysical Phenomena; Astrophysics - Cosmology and Nongalactic Astrophysics

URL: http://arxiv.org/abs/2211.03097

From top to bottom, the optical depth, the propagated distance of the ejecta, the disk density in the position of the ejecta, the accelerated velocities, and the dynamics of the ejecta evolve, respectively. Three panels from left to right-hand sides show the dynamics and other properties of ejecta at $0^{\circ}$, $45^{\circ}$, and $88^{\circ}$ directions, respectively. The labels (a) - (e) mark five stages of the ejecta dynamics that are separated by vertical dash-doted lines. The blue solid, red dash-doted, and green dash-doted lines are the accelerated velocities from the pressure, the swept gas, and the gravity of the central compact remnant, respectively. Note that the pressure accelerates the ejecta and the last two terms decelerate the ejecta. The merger occurs in the disk radius as $10^3~r_{\rm s}$ with the mass of SMBH being $10^7M_{\odot}$. To emphasize the main concern about dynamics, we have ignored the change of disk parameters, e.g., $\rho_0$ and $P_0$, during ejecta propagation.