We consider the isentropic
compressible Euler equations with polytropic gamma law P(ρ) = ργ in dimensions
d ≤ 3. We address the borderline case when adiabatic index γ = 1 and establish
local theory in the Sobolev space Ct0Lxp∩ Ct0Ḣxk for d < p ≤ 4. This
covers a class of physical solutions which can decay to vacuum at spatial
infinity and are not compact perturbations of steady states. We construct a
blowup scenario where initially the fluid is quiet in a neighborhood of the
origin but is supersonic near the spatial infinity. For this special class of
noncompact initial data, we prove the formation of singularities in finite
time.