Observation of e+e- → γX(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-→ γX(3872) is observed for the first time with a statistical significance of 6.3σ. The measured mass of the X(3872) is (3871.9 ± 0.7s tat ± 0.2 syst)  MeV/c(2), in agreement with previous measurements. Measurements of the product of the cross section σ[e+e- → γX(3872)] and the branching fraction B[X(3872)→π+π-J/ψ] 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) → γX(3872).