HYPERACCRETING DISKS AROUND MAGNETARS FOR GAMMA-RAY BURSTS: EFFECTS OF STRONG MAGNETIC FIELDS

Journal Article · Sun Aug 01 00:00:00 EDT 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/718/2/841· OSTI ID:21455097

Dong, Zhang ^[1]

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1. Department of Astronomy, Ohio State University, 140 W. 18th Avenue, Columbus, OH 43210 (United States)

Hyperaccreting neutron stars or magnetar disks cooled via neutrino emission can be candidates of gamma-ray burst (GRB) central engines. The strong field {>=}10{sup 15}-10{sup 16} 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 {sub {phi}/}B{sub z}| and {Omega}/{Omega}{sub K} with B {sub {phi}} and B{sub z} being the azimuthal and vertical components of the disk field, and {Omega} and {Omega} {sub 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 Alfven radius, but will be thermally unstable near the Alfven 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 Alfven 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.

• OSTI ID: 21455097
• Journal Information: Astrophysical Journal, Vol. 718, Issue 2; Other Information: DOI: 10.1088/0004-637X/718/2/841; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ACCRETION DISKS
ANNIHILATION
COSMIC GAMMA BURSTS
COSMIC NEUTRINOS
LUMINOSITY
NEUTRON STARS
PULSARS
RELATIVISTIC RANGE
THERMODYNAMICS
COSMIC RADIATION
COSMIC RADIO SOURCES
ELEMENTARY PARTICLES
ENERGY RANGE
FERMIONS
INTERACTIONS
IONIZING RADIATIONS
LEPTONS
MASSLESS PARTICLES
NEUTRINOS
OPTICAL PROPERTIES
PARTICLE INTERACTIONS
PHYSICAL PROPERTIES
PRIMARY COSMIC RADIATION
RADIATIONS
STARS