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 $m=1$ 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

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

New Torque Reversal and Spin-Up of 4u 1626- 67 Observed by Fermi/GBM and Swift/BAT

After about 18 years of steadily spinning down, the accretion-powered pulsar 4U 1626-67, experienced a torque reversal at the beginning of 2008. For the present study we have used all available Fermi/GBM data since its launch in 2008 June 11 and over 5 yr of hard X-ray Swift/BAT observations (starting from 2004 October up to the present time). This second detected torque reversal is centered near MJD 54500 (2008 Feb 4) and it lasts approximately 150 days. From 2004 up to the end of 2007 4U 1626-67 the spin-down rate decreased at a mean rate of ~ -5.5E-13 Hz s-1 until the source reversed torque again. Since then it has been following a steady spin-up at a mean rate of ~ 5E-13 Hz s-1. In addition, 4U 1626-67 increased its flux simultaneously (a ~2.5 factor). We present detailed long-term timing analysis of this source and a long term spectral hardness ratio study in order to see whether there are spectral changes around this new observed torque reversal.Comment: 12 pages, 9 figure

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

Membrane and synaptic defects leading to neurodegeneration in Adar mutant Drosophila are rescued by increased autophagy

BackgroundIn fly brains, the Drosophila Adar (adenosine deaminase acting on RNA) enzyme edits hundreds of transcripts to generate edited isoforms of encoded proteins. Nearly all editing events are absent or less efficient in larvae but increase at metamorphosis; the larger number and higher levels of editing suggest editing is most required when the brain is most complex. This idea is consistent with the fact that Adar mutations affect the adult brain most dramatically. However, it is unknown whether Drosophila Adar RNA editing events mediate some coherent physiological effect. To address this question, we performed a genetic screen for suppressors of Adar mutant defects. Adar5G1 null mutant flies are partially viable, severely locomotion defective, aberrantly accumulate axonal neurotransmitter pre-synaptic vesicles and associated proteins, and develop an age-dependent vacuolar brain neurodegeneration.ResultsA genetic screen revealed suppression of all Adar5G1 mutant phenotypes tested by reduced dosage of the Tor gene, which encodes a pro-growth kinase that increases translation and reduces autophagy in well-fed conditions. Suppression of Adar5G1 phenotypes by reduced Tor is due to increased autophagy; overexpression of Atg5, which increases canonical autophagy initiation, reduces aberrant accumulation of synaptic vesicle proteins and suppresses all Adar mutant phenotypes tested. Endosomal microautophagy (eMI) is another Tor-inhibited autophagy pathway involved in synaptic homeostasis in Drosophila. Increased expression of the key eMI protein Hsc70-4 also reduces aberrant accumulation of synaptic vesicle proteins and suppresses all Adar5G1 mutant phenotypes tested.ConclusionsThese findings link Drosophila Adar mutant synaptic and neurotransmission defects to more general cellular defects in autophagy; presumably, edited isoforms of CNS proteins are required for optimum synaptic response capabilities in the brain during the behaviorally complex adult life stage

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

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, $\sim 10^{16}-10^{17}g/s$. 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 ${\dot M}$ in the Keplerian flow. The observed abrupt torque reversal is expected when the pulsar magnetic field $B_*\sim 5\times 10^{11}b_p^{-1/2}L_{x,36}^{1/2}P_{*,10}^{1/2}G$ where the magnetic pitch parameter $b_p\sim$ a few, $L_{x,36}$ is the X-ray luminosity in $10^{36} erg/s$, and $P_{*,10}$ is the pulsar spin period in 10s. Observed quasi-periodic oscillation (QPO) periods tightly constrain the model. For 4U 1626-67, ${\dot M}\approx 2.7\times 10^{16}g/s$ with $b_p^{1/2} B_*\approx 2\times 10^{12}G$. We estimate ${\dot M}\sim 6\times 10^{16} g/s$ and $b_p^{1/2}B_*\sim 5\times 10^{13}G$ for GX 1+4, and ${\dot M} \sim 1\times 10^{17} g/s$ and $b_p^{1/2}B_*\sim 2\times 10^{13}G$ for OAO 1657-415. Reliable detection of QPOs before and after torque reversal could directly test the model.Comment: 11 pages, 2 figures, Ap

Siphon flow in a cool magnetic loop

We investigate the properties of a structure in the solar chromosphere in an active region to find out whether the feature is consistent with a siphon flow in a magnetic loop filled with chromospheric material