Observation of $e^{+}{e}^{-}\to \gamma X(3872)$ at BESIII

With data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies from 4.009 to 4.420 GeV, the process $e^{+}{e}^{-}\to \gamma X(3872)$ is observed for the first time with a statistical significance of $6.3\sigma$. The measured mass of the $X(3872)$ is ($3871.9\pm {0.7}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.2}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{.}}$) MeV/$c^2$, in agreement with previous measurements. Measurements of the product of the cross section $\sigma [e^{+}{e}^{-}\to \gamma X(3872)]$ and the branching fraction $\mathcal{B}[X(3872)\to {\pi }^{+}{\pi }^{-}J\mathrm{/}\psi ]$ at center-of-mass energies 4.009, 4.229, 4.260, and 4.360 GeV are reported. Our measurements are consistent with expectations for the radiative transition process $Y(4260) \to \gamma X(3872)$.