  Low temperature photoluminescence spectra of a color center in hexagonal
boron nitride are analyzed. The acoustic phonon sideband can be described by a
deformation coupling proportional to strain to a phonon bath that is
effectively two dimensional. The optical phonon band is described by Frohlich
coupling to the LO-branches, and a deformation coupling proportional to lattice
displacement for the TO-branch. The resonances expressed in the optical band
vary from defect to defect, in some emitters, coupling to out-of-plane
polarized phonons is reported.
