  Non-magnetic impurities break a quantum spin chain into finite segments and
induce Friedel-like oscillations in the local susceptibility near the edges.
The signature of these oscillations has been observed in Knight shift
experiments on the high-temperature superconductor YBa$_2$Cu$_3$O$_{6.5}$ and
on the spin-chain compound Sr$_2$CuO$_3$. Here we analytically calculate NMR
spectra, compare with the available experimental data for Sr$_2$CuO$_3$, and
show that the interchain coupling is responsible for the complicated and so far
unexplained lineshape. Our results are based on a parameter-free formula for
the local susceptibility of a finite spin chain obtained by bosonization which
is checked by comparing with quantum Monte Carlo and density-matrix
renormalization group calculations.
