  We study the effect of a missing spin in a one dimensional $S=1/2$
antiferromagnet with nearest neighbour Heisenberg exchange $J$ and six-spin
coupling $Q=4qJ$ using Quantum Monte-Carlo (QMC) and bosonization techniques.
For $q< q_c \approx 0.04$, the system is in a quasi-long range ordered
power-law antiferromagnetic phase, which gives way to a valence-bond solid
state that spontaneously breaks lattice translation symmetry for $q> q_c$. We
study the ground state spin texture $\Phi(r) =
<G_{\uparrow}|S^z(r)|G_{\uparrow}>$ in the the $S^z_{tot}=1/2$ ground state
$|G_{\uparrow}>$ of the system with a missing spin, focusing on the alternating
part $N_z(r)$. We find that our QMC results for $N_z$ at $q =q_c$ take on the
scaling form expected from bosonization considerations, but violate scaling for
$q < q_c$. Within the bosonization approach, such violations of scaling arise
from the presence of a marginally irrelevant sine-Gordon interaction, whose
effects we calculate using renormalization group (RG) improved perturbation
theory. Our field-theoretical predictions are found to agree well with the QMC
data for $q < q_c$.
