The optical/UV excess of X-ray-dim isolated neutron star II. Nonuniformity of plasma on a strangeon star surface

Abstract

Several X-ray-dim isolated neutron stars (XDINSs), also known as the Magnificent Seven, exhibit a Planck-like soft X-ray spectrum. In the optical/ultraviolet (UV) band, there is an excess of radiation compared to an extrapolation from the X-ray spectrum. However, the majority exhibits "spectral deviations": the fact that there is more flux at longer wavelengths makes spectra deviate from the Rayleigh-Jeans law. 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 strangeon matter, which has negligible emission, and is formed by the accretion of ISM-fed debris disk matter moving along the magnetic field lines to near the polar caps. These particles may spread out of the polar regions which makes the atmosphere non-uniform. The modeled electron temperatures are with radiation radii Ropt∞ ∼ 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 can be fitted well by the radiative model, and exhibit Gaussian absorption lines at as would be expected. Furthermore, the surroundings (i.e., fallback disks or dusty belts) of XDINSs could be tested by future infrared/submillimeter observations.