Seminar: Taj Jankovič - Radiation-hydrodynamics of star-disc collisions

Speaker: Taj Jankovič, Institute of Physics of the Czech Academy of Sciences, Czech Republic

Title: Radiation-hydrodynamics of star-disc collisions

Abstract

Quasi-periodic eruptions (QPEs) are newly discovered transients of unknown
nature occurring near supermassive black holes, which feature bright X-ray bursts
separated by approximately 10 hours. A promising model for QPEs is the star-disc
model, where a star interacts periodically with a black hole’s pre-existing accretion
disc, creating shocks that expel dense gas clouds from which radiation emerges.
We performed the first 3D radiation-hydrodynamics simulations to investigate the
dynamics of the star-disc collisions, the properties of the ejected gas clouds, and
the resulting radiation signatures. The star was modeled as a solid, spherical body,
and the interaction was simulated for a small, local section of the accretion disc.
We found that star-disc collisions generate a nearly paraboloidal bow shock. The
heating of gas is not confined to the column of gas directly ahead of the star but
also extends laterally as the shock front expands sideways while traveling with the
star. The collision drives an outflow of gas both in the forward and backward
directions relative to the star’s motion. These outflows are asymmetric, with the
forward outflow carrying more mass and producing a brighter luminosity than the
backward component. We found that variations in stellar size, velocity, and disc
density systematically influence this asymmetry, suggesting that specific parameter
combinations could naturally reproduce the strong-weak brightness pattern
observed in some QPEs.