  Frequency analysis of the rf emission of oscillating Josephson supercurrent
is a powerful passive way of probing properties of topological Josephson
junctions. In particular, measurements of the Josephson emission enables to
detect the expected presence of topological gapless Andreev bound states that
give rise to emission at half the Josephson frequency $f_J$, rather than
conventional emission at $f_J$. Here we report direct measurement of rf
emission spectra on Josephson junctions made of HgTe-based gate-tunable
topological weak links. The emission spectra exhibit a clear signal at half the
Josephson frequency $f_{\rm J}/2$. The linewidths of emission lines indicate a
coherence time of $0.3-\SI{4}{ns}$ for the $f_{\rm J}/2$ line, much shorter
than for the $f_{\rm J}$ line ($3-\SI{4}{ns}$). These observations strongly
point towards the presence of topological gapless Andreev bound states, and
pave the way for a future HgTe-based platform for topological quantum
computation.
