White Dwarf--White Dwarf collisions in AGN discs via close encounters

Abstract

White dwarfs (WDs) in active galactic nucleus (AGNs) discs might migrate to the inner radii of the discs and form restricted three-body systems with two WDs moving around the central supermassive black hole (SMBH) in close orbits. These systems could be dynamical unstable, which can lead to very close encounters or direct collisions. In this work, we use N-body simulations to study the evolution of such systems with the different initial orbital separation $p$, relative orbital inclination $\Delta{i}$ and SMBH mass $M$. It is found that the close encounters of WDs mainly occur at $1.1R_{\rm H} \lesssim p \lesssim 2\sqrt{3}R_{\rm H}$, where $R_{\rm H}$ is the mutual Hill radius. For $p<1.1R_{\rm H}$, the majority of WDs move in horseshoe or tadpole orbits, and only few of them with small initial orbital phase difference undergo close encounters. For $p=3.0R_{\rm H}$, WD-WD collisions occur in most of the samples within a time of $10^5P_1$, and considerable collisions occur within a time of $t<62P_1$ for small orbital radii, where $P_1$ is the orbital period. The peak of the closest separation distribution increase and the WD-WD collision fraction decreases with an increase of the relative inclination. The closest separation distribution is similar in cases with the different SMBH mass, but the WD-WD collision fraction decreases as the mass of SMBHs increases. According to our estimation, the event rate of the cosmic WD-WD collision in AGN discs is about $300{\rm Gpc^{-3}yr^{-1}}$, roughly $1\%$ of the one of the observed type Ia supernova. The corresponding electromagnetic emission signals can be observed by large surveys of AGNs.