16 research outputs found
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
Bondi-Hoyle Accretion onto Magnetized Neutron Star
Axisymmetric MHD simulations are used to investigate the Bondi-Hoyle
accretion onto an isolated magnetized neutron star moving supersonically (with
Mach number of 3) through the interstellar medium. The star is assumed to have
a dipole magnetic field aligned with its motion and a magnetospheric radius R_m
less then the accretion radius R_BH, so that the gravitational focusing is
important. We find that the accretion rate to a magnetized star is smaller than
that to a non-magnetized star for the parameters considered. Close to the star
the accreting matter falls to the star's surface along the magnetic poles with
a larger mass flow to the leeward pole of the star. In the case of a relatively
large stellar magnetic field, the star's magnetic field is stretched in the
direction of the matter flow outside of R_m (towards the windward side of the
star). For weaker magnetic fields we observed oscillations of the closed
magnetosphere frontward and backward. These are accompanied by strong
oscillations of the mass accretion rate which varies by factors ~ 3. Old slowly
rotating neutron stars with no radio emission may be visible in the X-ray band
due to accretion of interstellar matter. In general, the star's velocity,
magnetic moment, and angular velocity vectors may all be in different
directions so that the accretion luminosity will be modulated at the star's
rotation rate.Comment: 9 pages, 9 figures, accepted for publication in MNRA
Long duration radio transients lacking optical counterparts are possibly Galactic Neutron Stars
(abridged) Recently, a new class of radio transients in the 5-GHz band was
detected by Bower et al. We present new deep near-Infrared (IR) observations of
the field containing these transients, and find no counterparts down to a
limiting magnitude of K=20.4 mag. We argue that the bright (>1 Jy) radio
transients recently reported by Kida et al. are consistent with being
additional examples of the Bower et al. transients. We refer to these groups of
events as "long-duration radio transients". The main characteristics of this
population are: time scales longer than 30 minute but shorter than several
days; rate, ~10^3 deg^-2 yr^-1; progenitors sky surface density of >60 deg^-2
(95% C.L.) at Galactic latitude ~40 deg; 1.4-5 GHz spectral slopes, f_\nu ~
\nu^alpha, with alpha>0; and most notably the lack of any counterparts in
quiescence in any wavelength. We rule out an association with many types of
objects. Galactic brown-dwarfs or some sort of exotic explosions remain
plausible options. We argue that an attractive progenitor candidate for these
radio transients is the class of Galactic isolated old neutron stars (NS). We
confront this hypothesis with Monte-Carlo simulations of the space distribution
of old NSs, and find satisfactory agreement for the large areal density.
Furthermore, the lack of quiescent counterparts is explained quite naturally.
In this framework we find: the mean distance to events in the Bower et al.
sample is of order kpc; the typical distance to the Kida et al. transients are
constrained to be between 30 pc and 900 pc (95% C.L.); these events should
repeat with a time scale of order several months; and sub-mJy level bursts
should exhibit Galactic latitude dependence. We discuss possible mechanisms
giving rise to the observed radio emission.Comment: Submitted to ApJ, 17 pages, 10 figure
Spherical Bondi accretion onto a magnetic dipole
Quasi-spherical supersonic (Bondi-type) accretion to a star with a dipole
magnetic field is investigated using resistive magnetohydrodynamic simulations.
A systematic study is made of accretion to a non-rotating star, while sample
results for a rotating star are also presented. A new stationary subsonic
accretion flow is found with a steady rate of accretion to the magnetized star
smaller than the Bondi accretion rate. Dependences of the accretion rate and
the flow pattern on the magnetic momentum of the star and the magnetic
diffusivity are presented. For slow star's rotation the accretion flow is
similar to that in non-rotating case, but in the case of fast rotation the
structure of the subsonic accretion flow is fundamentally different and
includes a region of ``propeller'' outflow. The methods and results described
here are of general interest and can be applied to systems where matter
accretes with low angular momentum.Comment: 15 pages, 15 figures, used emulapj.st
ACCRETION ONTO A MAGNETIC DIPOLE: RESULTS OF 2D NUMERICAL SIMULATIONS
Different regimes of accretion to a star with a dipole magnetic field were investigated using 2D numerical axisymmetric resistive MHD simulations. Numerical technique was improved over our recently published results (Toropin et al., 1999, referred as T99 below). A new model for the gravitating star with a dipole magnetic field was adopted for presented simulation set. Spherical accretion to a non-rotating star with a dipole field was modeled. Existence of the stationary accretion flow with polar columns inside the Alfven surface was confirmed. The accretion rate to the dipole in the axially symmetric flow is always smaller than in the Bondi accretion to corresponding non-magnetized star. Investigations of the cylindrical accretion (parallel to the star's magnetic momentum) were started. If the value of the star's gravitational capture radius is close to its Alfven radius then the magnetic field surveys as an effective obstacle for the incoming flow deflecting it from the star. Simulated flow structure is discussed