The optical/UV excess of X-ray dim isolated neutron star: I. bremsstrahlung emission from a strangeon star plasma atmosphere

X-ray dim isolated neutron stars (XDINSs) are characterized by Planckian spectra in X-ray bands, but show optical/ultraviolet(UV) excesses which are the measured photometry exceeding that is extrapolated from X-ray spectra. To solve this problem, a radiative model of bremsstrahlung emission from a plasma atmosphere is established in the regime of strangeon star. A strangeon star atmosphere could simply be regarded as the upper layer of a normal neutron star. This plasma atmosphere, formed and maintained by the ISM-accreted matter due to the so-called strangeness barrier, is supposed to be of two-temperature. All the seven XDINS spectra could be well fitted by the radiative model, from optical/UV to X-ray bands. The fitted radiation radii of XDINSs are from 7 to 13km, while the modelled electron temperatures are between 50 and 250eV, except RX J0806.4-4123 with a radiation radius ~3.5km, indicating that this source could be a low-mass strangeon star candidate. This strangeon star model could further be tested by soft X-ray polarimetry, such as the Lightweight Asymmetry and Magnetism Probe which is expected to work on Chinese space station around 2020.