The optical/UV excess of X-ray-dim isolated neutron star II. Nonuniformity of plasma on a strangeon star surface. (July 2018)

• Record Type: Journal Article
• Authors: Wang, Wei-Yang; Feng, Yi; Lai, Xiao-Yu; Li, Yun-Yang; Lu, Ji-Guang; Chen, Xuelei; Xu, Ren-Xin
• Is Part Of: Research in astronomy and astrophysics. Volume 18:Number 7(2018:Jul.)
• Publication Date: 2018-07
• Subjects: X-rays: stars -- stars: neutron -- stars: individual; Astronomy -- Periodicals; Astrophysics -- Periodicals; 520.5
• DOI: 10.1088/1674-4527/18/7/82
• Languages: English
• ISSNs: 1674-4527

Abstract

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 ∼ 100 − 200 eV with radiation radii R opt ∞ ∼ 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 ∼ 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.