  We report on measurements of the linewidth {\Delta}f of THz radiation emitted
from intrinsic Josephson junction stacks, using a Nb/AlN/NbN integrated
receiver for detection. Previous resolution limited measurements indicated that
{\Delta}f may be below 1 GHz - much smaller than expected from a purely
cavity-induced synchronization. While at low bias we found {\Delta}f to be not
smaller than ? 500 MHz, at high bias, where a hotspot coexists with regions
which are still superconducting, {\Delta}f turned out to be as narrow as 23
MHz. We attribute this to the hotspot acting as a synchronizing element.
{\Delta}f decreases with increasing bath temperature, a behavior reminiscent of
motional narrowing in NMR or ESR, but hard to explain in standard
electrodynamic models of Josephson junctions.
