  We show that solutions to the parabolic-elliptic Keller-Segel system on
${\mathbb S}^1$ with critical fractional diffusion $(-\Delta)^\frac{1}{2}$
remain smooth for any initial data and any positive time. This disproves, at
least in the periodic setting, the large-data-blowup conjecture by Bournaveas
and Calvez. As a tool, we show smoothness of solutions to a modified critical
Burgers equation via a generalization of the method of moduli of continuity by
Kiselev, Nazarov and Shterenberg. over a setting where the considered equation
has no scaling. This auxiliary result may be interesting by itself. Finally, we
study the asymptotic behavior of global solutions, improving the existing
results.
