238 research outputs found
Warping and Precession of Accretion Disks Around Magnetic Stars: Nonlinear Evolution
The inner region of the accretion disk around a magnetized star (T Tauri
star, white dwarf or neutron star) is subjected to magnetic torques that induce
warping and precession of the disk. These torques arise from the interaction
between the stellar field and the induced electric currents in the disk. We
carry out numerical simulations of the nonlinear evolution of warped, viscous
accretion disks driven by the magnetic torques. We show that the disk can
develop into a highly warped steady state in which the disk attains a fixed
(warped) shape and precesses rigidly. The warp is most pronounced at the disk
inner radius (near the magnetosphere boundary). As the system parameters (such
as accretion rate) change, the disk can switch between a completely flat state
(warping stable) and a highly warped state. The precession of warped disks may
be responsible for a variety of quasi-periodic oscillations or radiation flux
variabilities observed in many different systems, including young stellar
objects and X-ray binaries.Comment: 16 pages, 7 figures; extended parameter searches, changes in
discussion; accepted for publication in Ap
Wave Excitation in Disks Around Rotating Magnetic Stars
The accretion disk around a rotating magnetic star (neutron star, white dwarf
or T Tauri star) is subjected to periodic vertical magnetic forces from the
star, with the forcing frequency equal to the stellar spin frequency or twice
the spin frequency. This gives rise bending waves in the disk that may
influence the variabilities of the system. We study the excitation, propagation
and dissipation of these waves using a hydrodynamical model coupled with a
generic model description of the magnetic forces. The bending waves are
excited at the Lindblad/vertical resonance, and propagate either to larger
radii or inward toward the corotation resonance where dissipation takes place.
While the resonant torque is negligible compared to the accretion torque, the
wave nevertheless may reach appreciable amplitude and can cause or modulate
flux variabilities from the system. We discuss applications of our result to
the observed quasi-periodic oscillations from various systems, in particular
neutron star low-mass X-ray binaries.Comment: Small changes/clarifications. To be published in ApJ, Aug.20,2008
issu
Discovery of a High-Latitude Accreting Millisecond Pulsar in an Ultracompact Binary
We have identified the third known accretion-powered millisecond pulsar, XTE
J0929-314, with the Rossi X-Ray Timing Explorer. The source is a faint,
high-Galactic-latitude X-ray transient (d >~ 5 kpc) that was in outburst during
2002 April-June. The 185 Hz (5.4 ms) pulsation had a fractional rms amplitude
of 3-7% and was generally broad and sinusoidal, although occasionally
double-peaked. The hard X-ray pulses arrived up to 770 microseconds earlier
than the soft X-ray pulses. The pulsar was spinning down at an average rate of
-(9.2 +/- 0.4) * 10^-14 Hz/s; the spin-down torque may arise from magnetic
coupling to the accretion disk, a magnetohydrodynamic wind, or gravitational
radiation from the rapidly spinning pulsar. The pulsations were modulated by a
43.6 min ultracompact binary orbit, yielding the smallest measured mass
function (2.7 * 10^-7 M_sun) of any stellar binary. The binary parameters imply
an approximately 0.01 M_sun white dwarf donor and a moderately high
inclination. We note that all three known accreting millisecond pulsars are
X-ray transients in very close binaries with extremely low mass transfer rates.
This is an important clue to the physics governing whether or not persistent
millisecond pulsations are detected in low-mass X-ray binaries.Comment: 5 pages, 3 figures; accepted by ApJ Letters. Revised distance lower
limit and added a figure showing pulse profile
Double-Peaked X-Ray Lines from the Oxygen/Neon-Rich Accretion Disk in 4U1626-67
We report on a 39 ks observation of the 7.7-s low-mass X-ray binary pulsar
4U1626-67 with the High Energy Transmission Grating Spectrometer (HETGS) on the
Chandra X-Ray Observatory. This ultracompact system consists of a
disk-accreting magnetic neutron star and a very low mass, hydrogen-depleted
companion in a 42-min binary. We have resolved the previously reported Ne/O
emission line complex near 1 keV into Doppler pairs of broadened (2500 km/s
FWHM) lines from highly ionized Ne and O. In most cases, the blue and red line
components are of comparable strength, with blueshifts of 1550-2610 km/s and
redshifts of 770-1900 km/s. The lines appear to originate in hot (10^6 K),
dense material just below the X-ray-heated skin of the outer Keplerian
accretion disk, or else possibly in a disk wind driven from the pulsar's
magnetopause. The observed photoelectric absorption edges of Ne and O appear
nearly an order of magnitude stronger than expected from interstellar material
and are likely formed in cool, metal-rich material local to the source. Based
on the inferred local abundance ratios, we argue that the mass donor in this
binary is probably the 0.02 M_sun chemically fractionated core of a C-O-Ne or
O-Ne-Mg white dwarf which has previously crystallized.Comment: 9 pages. Accepted for publication in ApJ. Table 2 correcte
Constraints on Torque-Reversing Accretion-Powered X-ray Pulsars
The observed abrupt torque reversals in X-ray pulsars, 4U 1626-67, GX 1+4,
and OAO 1657-415, can be explained by transition in accretion flow rotation
from Keplerian to sub-Keplerian, which takes place at a critical accretion
rate, . When a pulsar system spins up near equilibrium
spin before the transition, the system goes into spin-down after transition to
sub-Keplerian. If a system is well into the spin-up regime, the transition can
cause a sharp decrease in spin-up rate but not a sudden spin-down. These
observable types of abrupt torque change are distinguished from the smooth
torque variation caused by change of in the Keplerian flow. The
observed abrupt torque reversal is expected when the pulsar magnetic field
where the
magnetic pitch parameter a few, is the X-ray luminosity in
, and is the pulsar spin period in 10s. Observed
quasi-periodic oscillation (QPO) periods tightly constrain the model. For 4U
1626-67, with . We estimate and
for GX 1+4, and and for OAO 1657-415. Reliable
detection of QPOs before and after torque reversal could directly test the
model.Comment: 11 pages, 2 figures, Ap
Magnetically Driven Warping, Precession and Resonances in Accretion Disks
The inner region of the accretion disk onto a rotating magnetized central
star (neutron star, white dwarf or T Tauri star) is subjected to magnetic
torques which induce warping and precession of the disk. The origin of these
torques lies in the interaction between the (induced) surface current on the
disk and the horizontal magnetic field (parallel to the disk) produced by the
inclined magnetic dipole. Under quite general conditions, there exists a
magnetic warping instability in which the magnetic torque drives the disk plane
away from the equatorial plane of the star toward a state where the disk normal
vector is perpendicular to the spin axis. Viscous stress tends to suppress the
warping instability at large radii, but the magnetic torque always dominates as
the disk approaches the magnetosphere boundary. The magnetic torque also drives
the tilted inner disk into retrograde precession around the stellar spin axis.
Moreover, resonant magnetic forcing on the disk can occur which may affect the
dynamics of the disk. The magnetically driven warping instability and
precession may be related to a number observational puzzles, including: (1)
Spin evolution (torque reversal) of accreting X-ray pulsars; (2) Quasi-periodic
oscillations in low-mass X-ray binaries; (3) Super-orbital periods in X-ray
binaries; (4) Photometric period variations of T Tauri stars.Comment: 39 pages including 1 ps figure; Published version; ApJ, 524,
1030-1047 (1999
Recent X-ray measurements of the accretion-powered pulsar 4U 1907+09
X-ray observations of the accreting X-ray pulsar 4U~1907+09, obtained during
February 1996 with the Proportional Counter Array on the Rossi X-ray Timing
Experiment (RXTE), have enabled the first measurement of the intrinsic pulse
period Ppulse since 1984: Ppulse=440.341[+0.012,-0.017] s. 4U 1907+09 is in a
binary system with a blue supergiant. The orbital parameters were solved and
this enabled the correction for orbital delay effects of a measurement of
Ppulse obtained in 1990 with Ginga. Thus, three spin down rates could be
extracted from four pulse periods obtained in 1983, 1984, 1990, and 1996. These
are within 8% equal to a value of dPpulse/dt=+0.225 s/yr. This suggest that the
pulsar is perhaps in a monotonous spin down mode since its discovery in 1983.
Furthermore, the RXTE observations show transient ~18 s oscillations during a
flare that lasted about 1 hour. The oscillations may be interpreted as
Keplerian motion of an accretion disk near the magnetospheric radius. This, and
the notion that the co-rotation radius is much larger than any conceivable
value for the magnetospheric radius (because of the long spin period), renders
it unlikely that this pulsar spins near equilibrium like is suspected for other
slowing accreting X-ray pulsars. We suggest as an alternative that perhaps the
frequent occurrence of a retrograde transient accretion disk may be
consistently slowing the pulsar down. Further observations of flares can
provide more evidence of this.Comment: 26 pages, 11 figures, to be published in Astrophysical Journal part I
on March 20, 199
Automatic detection of limb prominences in 304 A EUV images
A new algorithm for automatic detection of prominences on the solar limb in 304 A EUV images is presented, and results of its application to SOHO/EIT data discussed. The detection is based on the method of moments combined with a
classifier analysis aimed at discriminating between limb prominences, active regions, and the quiet corona. This classifier analysis is based on a Support Vector Machine (SVM). Using a set of 12 moments of the radial intensity profiles, the algorithm performs well in discriminating between the above three categories of limb structures, with a misclassification rate of 7%. Pixels detected as belonging to a prominence are then used as starting point to reconstruct the whole prominence by morphological image processing techniques. It is planned that a catalogue of limb prominences identified in SOHO and STEREO data using this method will be made publicly available to the scientific community
On the structure and evolution of a polar crown prominence/filament system
Polar crown prominences are made of chromospheric plasma partially circling
the Suns poles between 60 and 70 degree latitude. We aim to diagnose the 3D
dynamics of a polar crown prominence using high cadence EUV images from the
Solar Dynamics Observatory (SDO)/AIA at 304 and 171A and the Ahead spacecraft
of the Solar Terrestrial Relations Observatory (STEREO-A)/EUVI at 195A. Using
time series across specific structures we compare flows across the disk in 195A
with the prominence dynamics seen on the limb. The densest prominence material
forms vertical columns which are separated by many tens of Mm and connected by
dynamic bridges of plasma that are clearly visible in 304/171A two-color
images. We also observe intermittent but repetitious flows with velocity 15
km/s in the prominence that appear to be associated with EUV bright points on
the solar disk. The boundary between the prominence and the overlying cavity
appears as a sharp edge. We discuss the structure of the coronal cavity seen
both above and around the prominence. SDO/HMI and GONG magnetograms are used to
infer the underlying magnetic topology. The evolution and structure of the
prominence with respect to the magnetic field seems to agree with the filament
linkage model.Comment: 24 pages, 14 figures, Accepted for publication in Solar Physics
Journal, Movies can be found at http://www2.mps.mpg.de/data/outgoing/panesar
Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
This review paper outlines background information and covers recent advances
made via the analysis of spectra and images of prominence plasma and the
increased sophistication of non-LTE (ie when there is a departure from Local
Thermodynamic Equilibrium) radiative transfer models. We first describe the
spectral inversion techniques that have been used to infer the plasma
parameters important for the general properties of the prominence plasma in
both its cool core and the hotter prominence-corona transition region. We also
review studies devoted to the observation of bulk motions of the prominence
plasma and to the determination of prominence mass. However, a simple inversion
of spectroscopic data usually fails when the lines become optically thick at
certain wavelengths. Therefore, complex non-LTE models become necessary. We
thus present the basics of non-LTE radiative transfer theory and the associated
multi-level radiative transfer problems. The main results of one- and
two-dimensional models of the prominences and their fine-structures are
presented. We then discuss the energy balance in various prominence models.
Finally, we outline the outstanding observational and theoretical questions,
and the directions for future progress in our understanding of solar
prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a
better resolution in the published version. New version reflects minor
changes brought after proof editin
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