arXiv subjects

Yoshiaki Kato

Publications and source records attributed to Yoshiaki Kato.

21 records · Page 2Linked to original sources

arXiv2003

Formation of Semi-relativisitc Jets from Magnetospheres of Accreting Neutron Stars: Injection of Hot Bubbles into a Magnetic Tower

We present the results of 2.5-dimensional resistive magnetohydrodynamic (MHD) simulations of the magnetic interaction between a weakly magnetized neutron star and its accretion disk. General relativistic effects are simulated by using the pseudo-Newtonian potential. We find that well-collimated jets traveling along the rotation axis of the disk are formed by the following mechanism: (1) The magnetic loops connecting the neutron star and the disk are twisted due to the differential rotation between the neutron star and the disk. (2) Twist injection from the disk initiates expansion of the loop. (3) The expanding magnetic loops create a magnetic tower in which accelerated disk material travel as collimated bipolar jets. The propagation speed of the working surface of the jet is the order of 10% of the speed of light ($\sim 0.1c$). (4) Magnetic reconnection taking place inside the expanding magnetic loops injects hot bubbles intermittently into the magnetic tower. The ejection speed of the bubble is the order of the local Alfvén speed of the launching point and $\sim 0.2c$ in our simulations. (5) The hot bubbles moving inside the tower catch up with the working surface of the jet. High energy electrons created by the magnetic reconnection are a plausible source of radio emission. Our model can explain the formation process of a narrow jet from a weakly magnetized ($|{\boldmath$B_{*}$}|\le 10^{9}$ gauss) neutron star and the correlation between radio flares of the core and of the lobe observed in Sco X-1.

astro-ph↗

arXiv2003

Magnetohydrodynamical Accretion Flows: Formation of Magnetic Tower Jet and Subsequent Quasi-Steady State

We present three-dimensional (3-D) magnetohydrodynamical (MHD) simulations of radiatively inefficient accretion flow around black holes. General relativistic effects are simulated by using the pseudo-Newtonian potential. We start calculations with a rotating torus threaded by localized poloidal magnetic fields with plasma beta, a ratio of the gas pressure to the magnetic pressure, $β=10$ and 100. When the bulk of torus material reaches the innermost region close to a central black hole, a magnetically driven jet emerges. This magnetic jet is derived by vertically inflating toroidal fields (`magnetic tower') and has a two-component structure: low-$β$ ($\lsim 1$) plasmas threaded with poloidal (vertical) fields are surrounded by that with toroidal fields. The collimation width of the jet depends on external pressure, pressure of ambient medium; the weaker the external pressure is, the wider and the longer-lasting becomes the jet. Unless the external pressure is negligible, the bipolar jet phase ceases after several dynamical timescales at the original torus position and a subsequent quasi-steady state starts. The black hole is surrounded by quasi-spherical zone with highly inhomogeneous structure in which toroidal fields are dominant except near the rotation axis. Mass accretion takes place mainly along the equatorial plane. Comparisons with other MHD simulation results and observational implications are discussed.

astro-ph↗

arXiv2000

On t-structures and Torsion Theories Induced by Compact Objects

First, we show that a compact object $C$ in a triangulated category, which satisfies suitable conditions, induces a $t$-structure. Second, in an abelian category we show that a complex $P^{\centerdot}$ of small projective objects of term length two, which satisfies suitable conditions, induces a torsion theory. In the case of module categories, using a torsion theory, we give equivalent conditions for $P^{\centerdot}$ to be a tilting complex. Finally, in the case of artin algebras, we give one to one correspondence between tilting complexes of term length two and torsion theories with certain conditions.

math.RA↗