The Origins of Peak Light in Tidal Disruption Events

Tidal disruption events (TDEs) occur when stars are ripped apart by massive black holes (MBHs). The ensuing multi-wavelength flares are possibly the brightest thermal transients in the Universe. TDE emission encodes the mass and even the spin of the underlying MBH, creating tremendous potential to measure MBH demographics, to resolve open questions on MBH origins and evolution, and even to test fundamental physics. Unfortunately, the geometry and power source for TDE optical/UV photospheres remain unclear, as the dynamic range of the problem has so far prevented {\it ab initio} hydrodynamical simulations. Here we present the first ever 3D radiation-hydrodynamic simulation of a TDE from disruption to peak emission, with typical astrophysical parameters. The light curve is initially powered by shocks near pericenter, with inefficient circularization and outflow production. Early times feature a novel source of X-ray emission. Near peak light, stream-disk interactions efficiently circularize returning debris and power stronger outflows. The peak optical/UV luminosities we find are typical of TDE observations. Our results show that peak emission in "typical" TDEs is shock- rather than accretion-powered, but that circularization begins to run away near peak light. This simulation shows how deterministic predictions of TDE light curves and spectra can be calculated before the next generation of time-domain surveys, such as VRO and {\it ULTRASAT}. Realistic simulations are urgently needed, as the observational sample of TDEs will soon grow from dozens to thousands of observed flares.