  We consider a special case of the Patlak-Keller-Segel system in a disc, which
arises in the modelling of chemotaxis phenomena. For a critical value of the
total mass, the solutions are known to be global in time but with density
becoming unbounded, leading to a phenomenon of mass-concentration in infinite
time. We establish the precise grow-up rate and obtain refined asymptotic
estimates of the solutions. Unlike in most of the similar, recently studied,
grow-up problems, the rate is neither polynomial nor exponential. In fact, the
maximum of the density behaves like $e^{\sqrt{2t}}$ for large time. In
particular, our study provides a rigorous proof of a behaviour suggested by
Sire and Chavanis [Phys. Rev. E, 2002] on the basis of formal arguments.
