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

Understand

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.