  X-ray-dim isolated neutron stars (XDINSs), also known as the Magnificent
Seven, exhibits a Planck-like soft X-ray spectrum. In the
optical/ultraviolet(UV) band, there is an excess of radiation compared to the
extrapolation from the X-ray spectrum. A model of bremsstrahlung emission from
a nonuniform plasma atmosphere is proposed in the regime of a strangeon star to
explain the optical/UV excess and its spectral deviation. The nonuniform
atmosphere is formed by the accretion of interstellar matter which moves along
the magnetic field lines to polar caps, and these particles may spread out of
the pole regions. The modeled electron temperatures are $\sim100-200$ eV with
radiation radii $R_{\rm opt}^{\infty}\sim5-14$ km. The spectra of five sources
(RX J0720.4--3125, RX J0806.4--4123, RX J1308.6+2127, RX J1605.3+3249, RX
J1856.5--3754) from optical/UV to X-ray bands could be well fitted by the
radiative model, and exhibit gaussian absorption lines at $\sim 100-500$ eV as
would be expected. Furthermore, the surroundings (i.e., fallback disks or dusty
belts) of XDINSs could be tested by future observations with the Large Optical
Telescope (LOT).
