X-ray outbursts of AXPs and SGRs

We show that the X‐ray enhancement light curves of transient AXP∕SGRs can be reproduced by the active fallback disk model. We solve the diffusion equation for the relaxation of a disk that has been pushed back by a soft gamma‐ray burst. Our preliminary results indicate that a critical temperature around 1500 K leads to a thermal‐viscous instability in the fallback disks of all AXP∕SGRs. The effect of the instability on the light curves are different for transient and persistent sources due to different pre‐burst disk conditions in these systems

Dead Zones and the Diversity of Exoplanetary Systems

Planetary migration provides a theoretical basis for the observed diversity of exoplanetary systems. We demonstrate that dust settling - an inescapable feature of disk evolution - gives even more rapid type I migration by up to a factor of about 2 than occurs in disks with fully mixed dust. On the other hand, type II migration becomes slower by a factor of 2 due to dust settling. This even more problematic type I migration can be resolved by the presence of a dead zone; the inner, high density region of a disk which features a low level of turbulence. We show that enhanced dust settling in the dead zone leaves a dusty wall at its outer edge. Back-heating of the dead zone by this wall produces a positive radial gradient for the disk temperature, which acts as a barrier for type I migration.Comment: 2 pages, 1 figure, published in Proceedings IAU Symposium No. 27

The Galactic center X-ray transients AX J1745.6-2901 and GRS 1741-2853

AX J1745.6-2901 and GRS 1741-2853 are two transient neutron star low-mass X-ray binaries that are located within ~10' from the Galactic center. Multi-year monitoring observations with the Swift/XRT has exposed several accretion outbursts from these objects. We report on their updated X-ray light curves and renewed activity that occurred in 2010-2013.Comment: 2 pages, 1 figure, 1 table. To appear in conference proceedings of IAU symposium 303 "The Galactic Center: Feeding and Feedback in a Normal Galactic Nucleus

New Global 3D MHD Simulations of Black Hole Disk Accretion and Outflows

It is widely accepted that quasars and other active galactic nuclei (AGN) are powered by accretion of matter onto a central supermassive black hole. While numerical simulations have demonstrated the importance of magnetic fields in generating the turbulence believed necessary for accretion, so far they have not produced the high mass accretion rates required to explain the most powerful sources. We describe new global 3D simulations we are developing to assess the importance of radiation and non-ideal MHD in generating magnetized outflows that can enhance the overall rates of angular momentum transport and mass accretion.Comment: 2 pages, including 1 colour figure. To appear in proceedings of IAU Symposium 259: "Cosmic Magnetic Fields: From Planets, To Stars and Galaxies", Tenerife, Nov 200

Global Alfven Wave Heating of the Magnetosphere of Young Stars

Excitation of a Global Alfven wave (GAW) is proposed as a viable mechanism to explain plasma heating in the magnetosphere of young stars. The wave and basic plasma parameters are compatible with the requirement that the dissipation length of GAWs be comparable to the distance between the shocked region at the star's surface and the truncation region in the accretion disk. A two-fluid magnetohydrodynamic plasma model is used in the analysis. A current carrying filament along magnetic field lines acts as a waveguide for the GAW. The current in the filament is driven by plasma waves along the magnetic field lines and/or by plasma crossing magnetic field lines in the truncated region of the disk of the accreting plasma. The conversion of a small fraction of the kinetic energy into GAW energy is sufficient to heat the plasma filament to observed temperatures.Comment: Submitted to ApJ, aheatf.tex, 2 figure