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星期五, 十月 22, 2010

Zhang 2009 磁星的超吸积

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Title:
Hyperaccreting Disks around Magnetars for Gamma-ray Bursts: Effects of Strong Magnetic Fields
Authors:
Zhang, Dong; Dai, Z. G.
Affiliation:
AA(Department of Astronomy, Ohio State University, 140 W. 18th Avenue, Columbus, OH 43210, USA ; Department of Astronomy, Nanjing University, Nanjing 210093, China; dzhang@astronomy.ohio-state.edu ), AB(Department of Astronomy, Nanjing University, Nanjing 210093, China dzg@nju.edu.cn)
Publication:
The Astrophysical Journal, Volume 718, Issue 2, pp. 841-866 (2010). (ApJ Homepage)
Publication Date:
08/2010
Origin:
IOP
ApJ Keywords:
accretion, accretion disks, gamma-ray burst: general, magnetic fields, neutrinos, stars: neutron
DOI:
10.1088/0004-637X/718/2/841
Bibliographic Code:
2010ApJ...718..841Z

Abstract

Hyperaccreting neutron stars or magnetar disks cooled via neutrino emission can be candidates of gamma-ray burst (GRB) central engines. The strong field >=1015-1016 G of a magnetar can play a significant role in affecting the disk properties and even lead to the funnel accretion process. In this paper, we investigate the effects of strong fields on the disks around magnetars, and discuss implications of such accreting magnetar systems for GRBs and GRB-like events. We discuss quantum effects of the strong fields on the disk thermodynamics and microphysics due to modifications of the electron distribution and energy in the strong field environment, and use the magnetohydrodynamical conservation equations to describe the behavior of the disk flow coupled with a large-scale field, which is generated by the star-disk interaction. If the disk field is open, the disk properties mainly depend on the ratio between |B phi/Bz | and Ω/Ω K with B phi and Bz being the azimuthal and vertical components of the disk field, and Ω and Ω K being the accretion flow angular velocity and Keplerian velocity, respectively. On the other hand, the disk properties also depend on the magnetar spin period if the disk field is closed. In general, stronger fields give higher disk densities, pressures, temperatures, and neutrino luminosity. Moreover, strong fields will change the electron fraction and degeneracy state significantly. A magnetized disk is always viscously stable outside the Alfvén radius, but will be thermally unstable near the Alfvén radius where the magnetic field plays a more important role in transferring the angular momentum and heating the disk than the viscous stress. The funnel accretion process will be important only for an extremely strong field, which creates a magnetosphere inside the Alfvén radius and truncates the plane disk. Because of higher temperature and more concentrated neutrino emission of a ring-like belt region on the magnetar surface covered by funnel accretion, the neutrino annihilation rate from the accreting magnetar can be much higher than that from an accreting neutron star without fields. Furthermore, the neutrino annihilation mechanism, which releases the gravitational energy of the surrounding disk, and the magnetically driven pulsar wind, which extracts the stellar rotational energy from the magnetar surface, can work together to generate and feed an ultrarelativistic jet along the stellar magnetic poles.
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