Anisotropic wind in tidal disruption events

Abstract: Over the coming years, the number of tidal disruption events (TDEs) is expected to substantially increase with observations from the Vera Rubin Observatory (g and r band) and {\it ULTRASAT} (near UV) wide-field surveys. These future samples have great promise to characterize the bottom end of the massive black hole mass function, but existing detections of intermediate mass black hole TDEs are primarily in X-rays, leaving their optical/UV emission largely unexplored. We present a time- and angle-dependent analysis of the outflow produced by dissipation near pericentre in a three-dimensional end-to-end radiation-hydrodynamics simulation of a TDE by a $10⁴ M_\odot$ black hole with realistic parameters, run with the code RICH. We find that outflow anisotropy produces viewing-angle-dependent observables. Towards the poles and the pericentre region, mass-loss rates are low and bolometric luminosities reach $\sim2$--$3$ times the Eddington luminosity. Towards the stream, the properties show a stronger dependence on latitude: the mass-loss rate increases and the bolometric luminosity decreases as the line of sight approaches the orbital plane. These denser regions favour H$α$ and H$β$ emission. Despite these variations, all viewing directions show a common spectral evolution, with an initial soft X-ray flare followed (around $1.25t_{\rm fb}\approx3$~days) by the reprocessing of shock-powered emission into the UV and optical bands. Although the optical/UV luminosities we predict for this TDE are likely too dim for past surveys (e.g. ASAS-SN, ZTF), they are within the detection capabilities of LSST and ULTRASAT to horizons of $\sim 790$ and $\sim 340$ Mpc, respectively, for the brightest viewing directions.