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

• 来源期刊: 天文和天体物理学研究
• 年，卷（期）: 2018,（7）
• 页码范围: 81-92
• 页数: 12页
• 语种: 英文
• DOI: 10.1088/1674-4527/18/7/82

摘要

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 ～ 100-200 eV with radiation radii/R∞opt ～ 5-14km.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 ～ 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.