  The spin-1/2 Heisenberg chain exhibits a quantum critical regime
characterized by quasi long-range magnetic order at zero temperature. We
quantify the strength of quantum fluctuations in the ground state by
determining the probability distributions of the components of the (staggered)
subsystem magnetization. Some of these exhibit scaling and the corresponding
universal scaling functions can be determined by free fermion methods and by
exploiting a relation with the boundary sine-Gordon model.
