  Tidal disruption events (TDEs) occur when a star gets torn apart by the
strong tidal forces of a supermassive black hole, which results in the
formation of a debris stream that partly falls back towards the compact object.
This gas moves along inclined orbital planes that intersect near pericenter,
resulting in a so-called "nozzle shock". We perform the first dedicated study
of this interaction, making use of a two-dimensional simulation that follows
the transverse gas evolution inside a given section of stream. This numerical
approach circumvents the lack of resolution encountered near pericenter passage
in global three-dimensional simulations using particle-based methods. As it
moves inward, we find that the gas motion is purely ballistic, which near
pericenter causes strong vertical compression that squeezes the stream into a
thin sheet. Dissipation takes place at the resulting nozzle shock, inducing a
rise in pressure that causes the collapsing gas to bounce back, although
without imparting significant net expansion. As it recedes to larger distances,
this matter continues to expand while remaining thin despite the influence of
pressure forces. This gas evolution specifies the strength of the subsequent
self-crossing shock, which we find to be more affected by black hole spin than
previously estimated. We also evaluate the impact of general-relativistic
effects, viscous dissipation, magnetic fields and radiative processes on the
nozzle shock. This study represents an important step forward in the
theoretical understanding of TDEs, bridging the gap between our robust
knowledge of the fallback rate and the more complex following stages, during
which most of the emission occurs.
