An Analytic, Fully Relativistic Framework for Tidal Disruption Event Streams in Schwarzschild Geometry

Abstract

We present an analytic and fully relativistic framework for studying the self-intersection of tidal disruption event (TDE) streams, restricting ourselves to the Schwarzschild spacetime. By taking advantage of the closed-form solution to the geodesic equations in the Schwarzschild metric, we calculate properties of the self-intersection without numerically evaluating the geodesic equations or making any post-Newtonian approximations. Our analytic treatment also facilitates geometric definitions of the orbital semi-major axis and eccentricity, as opposed to Newtonian formulas which lead to unphysical results for highly-relativistic orbits. Combined with assumptions about energy dissipation during the self-intersection shock, our framework enables the calculation of quantities such as the fraction of material unbound during the self-intersection shock, and the characteristic semi-major axes and eccentricities of the material which remains in orbit after the collision. As an example, we calculate grids of post-intersection properties in stellar and supermassive black hole (SMBH) masses for disruptions of main sequence stars, identifying regions where no material is ejected during self intersection (e.g. SMBH mass $\lesssim 5\times10^6\, {\rm M_\odot}$ for $1\,{\rm M_\odot}$ stars disrupted at the tidal radius), potentially explaining the TDEs observed by SGR/eROSITA which are visible in X-rays but not optical wavelengths. We also identify parameters for which the post-intersection accretion flow has low eccentricity ($e\lesssim0.6$), and find that the luminosity generated by self-intersection shocks only agrees with observed trends in the relationship between light curve decay timescales and peak luminosities over a narrow range of SMBH masses.