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 .
However , the majority exhibits “ spectral deviations ” : the fact that there are more flux at longer wavelengths makes spectra deviating from Rayleigh-Jeans laws .
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 as well as X-ray pulsation .
The atmosphere is on the surface of emission-negligible strangeon matter , formed by the accretion of ISM-fed debris disk matter moveing along the magnetic field lines to near polar caps , and these particles may spread out of the pole regions that makes the atmosphere non-uniform .
The modeled electron temperatures are \sim 100 - 200 eV with radiation radii R _ { opt } ^ { \infty } \sim 5 - 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 infrared/submillimeter observations .