65 research outputs found
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,
. 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
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
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&
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
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&
LkH 330: Evidence for dust clearing through resolved submillimeter imaging
Mid-infrared spectrophotometric observations have revealed a small sub-class
of circumstellar disks with spectral energy distributions (SEDs) suggestive of
large inner gaps with low dust content. However, such data provide only an
indirect and model dependent method of finding central holes. We present here
the direct characterization of a 40 AU radius inner gap in the disk around LkHa
330 through 340 GHz (880 micron) dust continuum imaging with the Submillimeter
Array (SMA). This large gap is fully resolved by the SMA observations and
mostly empty of dust with less than 1.3 x 10^-6 M_solar of solid particles
inside of 40 AU. Gas (as traced by accretion markers and CO M-band emission) is
still present in the inner disk and the outer edge of the gap rises steeply --
features in better agreement with the underlying cause being gravitational
perturbation than a more gradual process such as grain growth. Importantly, the
good agreement of the spatially resolved data and spectrophometry-based model
lends confidence to current interpretations of SEDs with significant dust
emission deficits as arising from disks with inner gaps or holes. Further
SED-based searches can therefore be expected to yield numerous additional
candidates that can be examined at high spatial resolution.Comment: 11 pages, 3 figures, accepted to ApJ
Empirical relation between angular momentum transport and thermal-to-magnetic pressure ratio in shearing box simulations
By combining data from different published 3-D simulations of Keplerian
shearing boxes unstable to the magnetorotational instability (MRI), we
highlight tight anti-correlations between the total effective inferred angular
momentum transport parameter, , its separate Maxwell and Reynolds
contributions and , and the kinetic to magnetic
pressure ratio , defined with the initial or saturated (when available)
thermal pressure.
Plots of , and
vs are well fit by straight lines even as ,
,and vary by four orders of magnitude over the
simulations included. The ratio and the product
are quite constant and largely independent of the presence
or absence of weak mean fields, the choice of initial and boundary conditions,
and the resolution. In short, simulations have more strongly constrained the
product than itself.Comment: 22 pages (includes 10 tables and 3 figs.), accepted by New Astronom
Two years of INTEGRAL monitoring of GRS 1915+105 Part 1: multiwavelength coverage with INTEGRAL, RXTE, and the Ryle radio Telescope
(Abridged) We report the results of monitoring observations of the Galactic
microquasar GRS 1915+105 performed simultaneously with INTEGRAL and RXTE Ryle .
We present the results of the whole \integral campaign, report the sources that
are detected and their fluxes and identify the classes of variability in which
GRS 1915+105 is found. The accretion ejection connections are studied in a
model independent manner through the source light curves, hardness ratio, and
color color diagrams. During a period of steady ``hard'' X-ray state (the
so-called class chi) we observe a steady radio flux. We then turn to 3
particular observations during which we observe several types of soft X-ray
dips and spikes cycles, followed by radio flares. During these observations GRS
1915+105 is in the so-called nu, lambda, and beta classes of variability. The
observation of ejections during class lambda are the first ever reported. We
generalize the fact that a (non-major) discrete ejection always occurs, in GRS
1915+105, as a response to an X-ray sequence composed of a spectrally hard
X-ray dip terminated by an X-ray spike marking the disappearance of the hard
X-ray emission above 18 keV. We also identify the trigger of the ejection as
this X-ray spike. A possible correlation between the amplitude of the radio
flare and the duration of the X-ray dip is found in our data. In this case the
X-ray dips prior to ejections could be seen as the time during which the source
accumulates energy and material that is ejected later.Comment: 17 pages, 14 figures. Accepted for publication in ApJ, scheduled for
the March 20, 2008, vol676 issue. Table 3 has been degrade
Gap Formation in the Dust Layer of 3D Protoplanetary Disks
We numerically model the evolution of dust in a protoplanetary disk using a
two-phase (gas+dust) Smoothed Particle Hydrodynamics (SPH) code, which is
non-self-gravitating and locally isothermal. The code follows the three
dimensional distribution of dust in a protoplanetary disk as it interacts with
the gas via aerodynamic drag. In this work, we present the evolution of a disk
comprising 1% dust by mass in the presence of an embedded planet for two
different disk configurations: a small, minimum mass solar nebular (MMSN) disk
and a larger, more massive Classical T Tauri star (CTTS) disk. We then vary the
grain size and planetary mass to see how they effect the resulting disk
structure. We find that gap formation is much more rapid and striking in the
dust layer than in the gaseous disk and that a system with a given stellar,
disk and planetary mass will have a different appearance depending on the grain
size and that such differences will be detectable in the millimetre domain with
ALMA. For low mass planets in our MMSN models, a gap can open in the dust disk
while not in the gas disk. We also note that dust accumulates at the external
edge of the planetary gap and speculate that the presence of a planet in the
disk may facilitate the growth of planetesimals in this high density region.Comment: 5 page, 4 figures. Accepted for publication in Astrophysics & Space
Scienc
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