  We study the out-of-equilibrium dynamics of a two-dimensional paraxial fluid
of light using a near-resonant laser propagating through a hot atomic vapor. We
observe a double shock-collapse instability: a shock (gradient catastrophe) for
the velocity, as well as an annular collapse singularity for the density. We
find experimental evidence that this instability results from the combined
effect of the nonlocal photon-photon interaction and the linear photon losses.
The theoretical analysis based on the method of characteristics reveals the
main counterintuitive result that dissipation (photon losses) is responsible
for an unexpected enhancement of the collapse instability. Detailed analytical
modeling makes it possible to evaluate the nonlocality range of the
interaction. The nonlocality is controlled by adjusting the atomic vapor
temperature and is seen to increase dramatically when the atomic density
becomes much larger than one atom per cubic wavelength. Interestingly, such a
large range of the nonlocal photon-photon interaction has not been observed in
an atomic vapor so far and its microscopic origin is currently unknown.
