The puzzling harmonic behavior of the Cathedral QPO in XTE J1859+226

Abridged: We present a spectral and temporal analysis of the Cathedral QPO detected in the power density spectra of the microquasar XTE J1859+226 obtained with RXTE. This type of QPO manifests as two peaks with similar amplitudes and harmonically related centroid frequencies (~3 and ~6 Hz). The amplitude of the ~3 Hz feature varies in anticorrelation with the count rate, by about ~50. The ~6 Hz feature shows a slight increase (~7%) of its amplitude with count rate. The RMS-spectra of the two peaks are quite different. The ~3 Hz feature is softer than the other one, and shows a cut-off at an energy of ~6 keV while the RMS of the 6 Hz increases up to at least 20 keV. We also study the bicoherence, $b^2(\mu,\nu)$. The values b^2(~3,~3) and b^2(~6,~6) are rather high and similar to the type C QPOs of GRS 1915+105. By comparison with the latter source the fact that the bicoherence of the ~3 Hz feature is higher than that of the other peak, would tend to indicate that the ~3 Hz is the fundamental QPO. The value of b^2(~3,~6)$ is, however, low and therefore indicates a behavior that is different than that seen in GRS 1915+105. We suggest that, rather than pure harmonics, we may see different modes of the same underlying phenomenon competing to produce QPOs at different frequencies.Comment: 7 pages, 5 figures, published in ApJ, vol 735, p79 Version corrected for small mistake

Accretion-ejection instability in magnetized disks: Feeding the corona with Alfven waves

We present a detailed calculation of the mechanism by which the Accretion-Ejection Instability can extract accretion energy and angular momentum from a magnetized disk, and redirect them to its corona. In a disk threaded by a poloidal magnetic field of the order of equipartition with the gas pressure, the instability is composed of a spiral wave (analogous to galactic ones) and a Rossby vortex. The mechanism detailed here describes how the vortex, twisting the footpoints of field lines threading the disk, generates Alfven waves propagating to the corona. We find that this is a very efficient mechanism, providing to the corona (where it could feed a jet or a wind) a substantial fraction of the accretion energy.Comment: accepted by A&

Pre-transitional disk nature of the AB Aur disk

The disk around AB Aur was imaged and resolved at 24.6\,$\mu$m using the Cooled Mid-Infrared Camera and Spectrometer on the 8.2m Subaru Telescope. The gaussian full-width at half-maximum of the source size is estimated to be 90 $\pm$ 6 AU, indicating that the disk extends further out at 24.6\,$\mu$m than at shorter wavelengths. In order to interpret the extended 24.6\,$\mu$m image, we consider a disk with a reduced surface density within a boundary radius $R_c$, which is motivated by radio observations that suggest a reduced inner region within about 100 AU from the star. Introducing the surface density reduction factor $f_c$ for the inner disk, we determine that the best match with the observed radial intensity profile at 24.6\,$\mu$m is achieved with $R_c$=88 AU and $f_c$=0.01. We suggest that the extended emission at 24.6\,$\mu$m is due to the enhanced emission from a wall-like structure at the boundary radius (the inner edge of the outer disk), which is caused by a jump in the surface density at $R_c$. Such reduced inner disk and geometrically thick outer disk structure can also explain the more point-like nature at shorter wavelengths. We also note that this disk geometry is qualitatively similar to a pre-transitional disk, suggesting that the AB Aur disk is in a pre-transitional disk phase.Comment: 10 pages, 4 figures, accepted for publication in Ap

A simple model for the complex lag structure of microquasars

The phase lag structure between the hard and soft X-ray photons observed in GRS 1915+105 and XTE J1550+564 has been said to be ``complex'' because the phase of the Quasi-Periodic Oscillation fundamental Fourier mode changes with time and because the even and odd harmonics signs behave differentely. From simultaneous X-ray and radio observations this seems to be related to the presence of a jet (level of radio emission). We propose a simple idea where a partial absorption of the signal can shift the phases of the Fourier modes and account for the phase lag reversal. We also briefly discuss a possible physical mechanism that could lead to such an absorption of the quasi-periodic oscillation modulation.Comment: accepted by A&A Letter

Rossby Wave Instability and three-dimensional vortices in accretion disks

Context. The formation of vortices in accretion disks is of high interest in various astrophysical contexts, in particular for planet formation or in the disks of compact objects. But despite numerous attempts it has thus far not been possible to produce strong vortices in fully three-dimensional simulations of disks. Aims. The aim of this paper is to present the first 3D simulation of a strong vortex, established across the vertically stratified structure of a disk by the Rossby Wave Instability. Methods. Using the Versatile Advection Code (VAC), we set up a fully 3D cylindrical stratified disk potentially prone to the Rossby Wave Instability. Results. The simulation confirms the basic expectations obtained from previous 2D analytic and numerical works. The simulation exhibits a strong vortex that grows rapidly and saturates at a finite amplitude. On the other hand the third dimension shows unexpected additional behaviours that could be of strong importance in the astrophysical roles that such vortices can play.Comment: Accepted by Astronomy and Astrophysic

Simulating planet migration in globally evolving disks

Numerical simulations of planet-disk interactions are usually performed with hydro-codes that -- because they consider only an annulus of the disk, over a 2D grid -- can not take into account the global evolution of the disk. However, the latter governs planetary migration of type II, so that the accuracy of the planetary evolution can be questioned. To develop an algorithm that models the local planet-disk interactions together with the global viscous evolution of the disk, we surround the usual 2D grid with a 1D grid ranging over the real extension of the disk. The 1D and 2D grids are coupled at their common boundaries via ghost rings, paying particular attention to the fluxes at the interface, especially the flux of angular momentum carried by waves. The computation is done in the frame centered on the center of mass to ensure angular momentum conservation. The global evolution of the disk and the local planet-disk interactions are both well described and the feedback of one on the other can be studied with this algorithm, for a negligible additional computing cost with respect to usual algorithms.Comment: 12 pages, 11 figures, accepted for publication in A&

Gaps in Protoplanetary Disks as Signatures of Planets: I. Methodology and Validation

We examine the observational consequences of partial gaps being opened by planets in protoplanetary disks. We model the disk using a static alpha-disk model with detailed radiative transfer, parametrizing the shape and size of the partially cleared gaps based on the results of hydrodynamic simulations. Shadowing and illumination by stellar irradiation at the surface of the gap leads to increased contrast as the gap trough is deepened by shadowing and cooling and the far gap wall is puffed up by illumination and heating. In calculating observables, we find that multiple scattering is important and derive an approximation to include these effects. A gap produced by a 200 M_Earth (70 M_Earth) planet at 10 AU can lower/raise the midplane temperature of the disk by up to ~-25/+29% (~-11/+19%) by shadowing in the gap trough and illumination on the far shoulder of the gap. At the distance of Taurus, this gap would be resolvable with ~0.01" angular resolution. The gap contrast is most significant in scattered light and at thermal continuum wavelengths characteristic of the surface temperature, reducing or raising the surface brightness by up to order of magnitude. Since gaps sizes are correlated to planet mass, this is a promising way of finding and determining the masses of planets embedded in protoplanetary disks.Comment: 11 pages, 9 figures. Accepted to Ap

Accretion-ejection instability and QPO in black hole binaries. I. Observations

This is the first of two papers in which we address the physics of the low-frequency Quasi-Periodic Oscillation (QPO) of X-ray binaries, in particular those hosting a black hole. We discuss and repeat the recent analysis and spectral modelling of the micro-quasar GRO J1655 by Sobczak et al (2000, hereafter SMR), and compare it with GRS 1915; this leads us to confirm and analyze in more detail the different behavior noted by SMR, between GRO J1655 and other sources, when comparing the correlation between the QPO frequency and the disk inner radius. In a companion paper (Varniere et al., 2002, hereafter Paper II) we will show that these opposite behaviors can be explained in the context of the Accretion-Ejection Instability recently presented by Tagger and Pellat (1999). We thus propose that the difference between GRO J1655 and other sources comes from the fact that in the former, observed in a very high state, the disk inner radius always stays close to the Last Stable Orbit. In the course of this analysis, we also indicate interesting differences between the source properties, when the spectral fits give an anomalously low inner disk radius. This might indicate the presence of a spiral shock or a hot point in the disk.Comment: accepted by A&

A Closer Look at the LkCa 15 Protoplanetary Disk

We present 870 micron observations of dust continuum emission from the LkCa 15 protoplanetary disk at high angular resolution (with a characteristic scale of 0.25" = 35 AU), obtained with the IRAM Plateau de Bure interferometer and supplemented by slightly lower resolution observations from the Submillimeter Array. We fit these data with simple morphological models to characterize the spectacular ring-like emission structure of this disk. Our analysis indicates that a small amount of 870 micron dust emission (~5 mJy) originates inside a large (40-50 AU radius) low optical depth cavity. This result can be interpreted either in the context of an abrupt decrease by a factor of ~5 in the radial distribution of millimeter-sized dust grains or as indirect evidence for a gap in the disk, in agreement with previous inferences from the unresolved infrared spectrum and scattered light images. A preliminary model focused on the latter possibility suggests the presence of a low-mass (planetary) companion, having properties commensurate with those inferred from the recent discovery of LkCa 15b.Comment: in press at ApJ Letter