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

## TL;DR

This paper proposes a bremsstrahlung emission model from a nonuniform plasma atmosphere on a strangeon star to explain the optical/UV excess observed in X-ray-dim isolated neutron stars, fitting multiple sources' spectra.

## Contribution

It introduces a novel plasma atmosphere model on strangeon stars to account for optical/UV excess in XDINSs, supported by spectral fits and predicted absorption features.

## Key findings

- Spectral fits for five XDINSs match the model well.
- Electron temperatures are estimated around 100-200 eV.
- Absorption lines at 100-500 eV are predicted.

## Abstract

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).