Understanding Broad Line Emission in Tidal Disruption Events

When a star passes too close to a supermassive black hole (BH), the BH’s tidal forces become stronger than the star’s own self gravity, ripping the star into a stream of debris in a so-called tidal disruption event (TDE). Some of this debris is gravitationally bound to the BH and accretes at rates far above the Eddington limit, resulting in a bright flare and the formation of a “debris disk” on a timescale of weeks to months. The lightcurves of TDEs AT 2018hyz and PTF09djl reveal double-peaked broad line (BL) features in their spectra, indicating that TDE BL emission is at least partially produced in the debris disk itself as opposed to the usual model of line-emitting clouds. Therefore, the BL profiles of TDE spectra can constrain the geometry of the debris disk and more generally the process of super-Eddington accretion. In this work, we present a toy model for generating broad line profiles given the density and velocity fields of the debris disk. The model combines the standard “lamppost” approach for the spectrum incident on the disk with a grid of photoionization calculations in the spectral synthesis code CLOUDY. We use the model to generate simulated BL profiles from a general relativistic hydrodynamical simulation of a TDE debris disk carried out using the GPU-accelerated code H-AMR for various orientation angles. Preliminary results show that the equivalent width of the BL features depend strongly on the observer’s inclination angle. Meanwhile, the prominence of the double-peaked features depend strongly on the observer’s azimuthal offset, reflecting the highly non-axisymmetric features of the debris disk. We also find that the Balmer line emission dominates over Helium line emission for all orientations, suggesting that the compactness of the debris alone cannot explain the TDE spectral class TDE-He, which exhibits He line features but not H line features.