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2018RAA....18...82W - Research in Astron. and astroph., 18, 82-82 (2018/July-0)

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

WANG W.-Y., FENG Y., LAI X.-Y., LI Y.-Y., LU J.-G., CHEN X. and XU R.-X.

Abstract (from CDS):

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.

Abstract Copyright:

Journal keyword(s): X-rays: stars - stars: neutron - stars: individual

Simbad objects: 9

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2022.08.11-05:19:30

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