The thermal photon emissivity at the QCD chiral crossover is
investigated using imaginary momentum correlators. These have been
measured on a newly generated $20 \times 96^3$ lattice-QCD ensemble
with $\mathrm{O}(a)$-improved Wilson quarks and physical up, down and
strange quark masses at a temperature $T=154$\,MeV near the
pseudo-critical temperature. In order to realize the photon on-shell
condition, the spatially transverse Euclidean correlators have to be
evaluated at imaginary spatial momenta.
Employing a bounding method, we present a preliminary result on the
quantity $H_E(\omega_1)$, which corresponds to an energy-moment of the
photon spectral function $\sigma(\omega)/\omega$ defined by the weight function $1/(\omega^2+\omega_n^2)$, the $\omega_n$ being integer multiples of $2\pi T$.