  Motivated by the ongoing effort to search for high-resolution signatures of
quantum spin liquids, we investigate the temperature dependence of the indirect
resonant inelastic x-ray scattering (RIXS) response for the Kitaev honeycomb
model. We find that, as a result of spin fractionalization, the RIXS response
changes qualitatively at two well-separated temperature scales, $T_L$ and
$T_H$, which correspond to the characteristic energies of the two kinds of
fractionalized excitations, $\mathbb{Z}_2$ gauge fluxes and Majorana fermions,
respectively. While thermally excited $\mathbb{Z}_2$ gauge fluxes at
temperature $T_L$ lead to a general broadening and softening of the response,
the thermal proliferation of Majorana fermions at temperature $T_H \sim 10 \,
T_L$ results in a significant shift of the spectral weight, both in terms of
energy and momentum. Due to its exclusively indirect nature, the RIXS process
we consider gives rise to a universal magnetic response and, from an
experimental perspective, it directly corresponds to the $K$-edge of Ru$^{3+}$
in the Kitaev candidate material $\alpha$-RuCl$_3$.
