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

作　　者：Wei-Yang Wang Yi Feng Xiao-Yu Lai Yun-Yang Li Ji-Guang Lu Xuelei Chen Ren-Xin Xu 

机构地区：[1]Key Laboratory of Computational Astrophysics,National Astronomical Observatories,Chinese Academy ofSciences,Beijing 100101,China [2]School of Astronomy and Space Sciences,University of Chinese Academy of Sciences,Beijing 100049,China [3]School of Physics and State Key Laboratory of Nuclear Physics and Technology,Peking University,Beijing 100871,China [4]National Astronomical Observatories,Chinese Academy of Sciences,Beijing 100101,China [5]CAS Key Laboratory of FAST,National Astronomical Observatories,Chinese Academy of Sciences,Beijing 100101,China [6]Hubei University of Education,Wuhan 430205,China [7]Key Laboratory of Radio Astronomy,Chinese Academy of Sciences,Beijing 100101,China [8]Center for High Energy Physics,Peking University,Beijing 100871,China [9]Kavli Institute for Astronomy and Astrophysics,Peking University,Beijing 100871,China

出　　处：《Research in Astronomy and Astrophysics》2018年第7期81-92,共12页天文和天体物理学研究（英文版）

基　　金：support of MoST(2016YFE0100300);the National Natural Science Foundation of China(NSFC,Nos.11473044,11633004,11653003);CAS(QYZDJ-SSW-SLH017);supported by the Open Project Program of the Key Laboratory of FAST,Chinese Academy of Sciences(CAS);supported by the West Light Foundation(XBBS-2014-23); NSFC(No.11203018);supported by NSFC(No.11225314);the Open Project Program of the Key Laboratory of Radio Astronomy,CAS;supported by the National Key R&D Program of China(No.2017YFA0402602);NSFC(Nos.11673002 and U1531243);the Strategic Priority Program of CAS(No.XDB23010200);supported by Special Funding for Advanced Users,budgeted and administrated by the Center for Astronomical Mega-Science,Chinese Academy of Sciences(CAMS).

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 ＂spec- tral 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 devi- ation 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 radi- ation radii Ropt ~ 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.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 ＂spec- tral 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 devi- ation 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 radi- ation radii Ropt ~ 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.

关 键 词：X-rays stars - stars neutron - stars individual 

分 类 号：P145.6[天文地球—天体物理]