Spinning-Down of Moving Magnetars in the Propeller Regime

We use axisymmetric magnetohydrodynamic simulations to investigate the spinning-down of magnetars rotating in the propeller regime and moving supersonically through the interstellar medium. The simulations indicate that magnetars spin-down rapidly due to this interaction, faster than for the case of a non-moving star. From many simulation runs we have derived an approximate scaling laws for the angular momentum loss rate, \dot{L} \propto \~\eta_m^{0.3}\mu^{0.6}\rho^{0.8}{\cal M}^{-0.4} \Omega_*^{1.5}, where \rho is the density of the interstellar medium, \cal M is Mach number, \mu is the star's magnetic moment, \Omega_* is its angular velocity, and \eta_m is magnetic diffusivity. A magnetar with a surface magnetic field of 10^{13} - 10^{15} G is found to spin-down to a period P > 10^5-10^6 s in \sim 10^4 - 10^5 years. There is however uncertainty about the value of the magnetic diffusivity so that the time-scale may be longer. We discuss this model in respect of Soft Gamma Repeaters (SGRs) and the isolated neutron star candidate RXJ1856.5-3754.Comment: 10 pages, 4 figures, accepted by MNRAS. See version with better resolution figures and animation at http://astrosun2.astro.cornell.edu/us-rus/propeller.ht

Gamma-ray Flares and VLBI Outbursts of Blazars

A model is developed for the time dependent electromagnetic - radio to gamma-ray - emission of active galactic nuclei, specifically, the blazars, based on the acceleration and creation of leptons at a propagating discontinuity or {\it front} of a Poynting flux jet. The front corresponds to a discrete relativistic jet component as observed with very-long-baseline-interferometry (VLBI). Equations are derived for the number, momentum, and energy of particles in the front taking into account synchrotron, synchrotron-self-Compton (SSC), and inverse-Compton processes as well as photon-photon pair production. The apparent synchrotron, SSC, and inverse-Compton luminosities as functions of time are determined. Predictions of the model are compared with observations in the gamma, optical and radio bands. The delay between the high-energy gamma-ray flare and the onset of the radio is explained by self-absorption and/or free-free absorption by external plasma. Two types of gamma-ray flares are predicted depending on pair creation in the front.Comment: 11 pages, submitted to ApJ. 10 figures can be obtained from R. Lovelace by sending postal address to [email protected]

Boundary Between Stable and Unstable Regimes of Accretion

We investigated the boundary between stable and unstable regimes of accretion and its dependence on different parameters. Simulations were performed using a "cubed sphere" code with high grid resolution (244 grid points in the azimuthal direction), which is twice as high as that used in our earlier studies. We chose a very low viscosity value, with alpha-parameter alpha=0.02. We observed from the simulations that the boundary strongly depends on the ratio between magnetospheric radius r_m (where the magnetic stress in the magnetosphere matches the matter stress in the disk) and corotation radius r_cor (where the Keplerian velocity in the disk is equal to the angular velocity of the star). For a small misalignment angle of the dipole field, Theta=5 degrees, accretion is unstable if r_cor/r_m>1.35, and is stable otherwise. In cases of a larger misalignment angle of the dipole, Theta=20 degrees, instability occurs at slightly larger values, r_cor/r_m>1.41.Comment: 4 pages, 4 figures, conference proceedings: "Physics at the Magnetospheric Boundary", Geneva, Switzerland, 25-28 June, 201

Accretion dynamics in the classical T Tauri star V2129 Oph

We analyze the photometric and spectroscopic variability of the classical T Tauri star V2129 Oph over several rotational cycles to test the dynamical predictions of magnetospheric accretion models. The photometric variability and the radial velocity variations in the photospheric lines can be explained by rotational modulation due to cold spots, while the radial velocity variations of the He I (5876 \AA) line and the veiling variability are due to hot spot rotational modulation. The hot and cold spots are located at high latitudes and about the same phase, but the hot spot is expected to sit at the chromospheric level, while the cold spot is at the photospheric level. Using the dipole+octupole magnetic-field configuration previously proposed in the literature for the system, we compute 3D MHD magnetospheric simulations of the star-disk system. We use the simulation's density, velocity and scaled temperature structures as input to a radiative transfer code, from which we calculate theoretical line profiles at all rotational phases. The theoretical profiles tend to be narrower than the observed ones, but the qualitative behavior and the observed rotational modulation of the H\alpha and H\beta emission lines are well reproduced by the theoretical profiles. The spectroscopic and photometric variability observed in V2129 Oph support the general predictions of complex magnetospheric accretion models with non-axisymmetric, multipolar fields.Comment: Accepted by Astronomy and Astrophysic

Three-dimensional simulations of rotationally-induced line variability from a Classical T Tauri star with a misaligned magnetic dipole

We present three-dimensional (3-D) simulations of rotationally induced line variability arising from complex circumstellar environment of classical T Tauri stars (CTTS) using the results of the 3-D magnetohydrodynamic (MHD) simulations of Romanova et al., who considered accretion onto a CTTS with a misaligned dipole magnetic axis with respect to the rotational axis. The density, velocity and temperature structures of the MHD simulations are mapped on to the radiative transfer grid, and corresponding line source function and the observed profiles of neutral hydrogen lines (H-beta, Pa-beta and Br-gamma) are computed using the Sobolev escape probability method. We study the dependency of line variability on inclination angles (i) and magnetic axis misalignment angles (Theta). By comparing our models with the Pa-beta profiles of 42 CTTS observed by Folha & Emerson, we find that models with a smaller misaligngment angle (Theta<~15 deg.) are more consistent with the observations which show that majority of Pa-beta are rather symmetric around the line centre. For a high inclination system with a small dipole misalignment angle (Theta ~ 15 deg.), only one accretion funnel (on the upper hemisphere) is visible to an observer at any given rotational phase. This can cause an anti-correlation of the line equivalent width in the blue wing (v0) over a half of a rotational period, and a positive correlation over other half. We find a good overall agreement of the line variability behaviour predicted by our model and those from observations. (Abridged)Comment: 15 pages, 13 figures. Accepted for publication in MNRAS. A version with full resolution figures can be downloaded from http://www.physics.unlv.edu/~rk/preprint/inclined_dipole.pd