-
SF2A-2012: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics; 12/2012
-
[show abstract]
[hide abstract]
ABSTRACT: In most microquasars, low-frequency quasi-periodic oscillations (LFQPO) have
been classified into three types (A, B and C depending on the peak distribution
in the PDS and the shape of the noise) but no explanation has been proposed
yet. The accretion-ejection instability (AEI) was presented in 1999 as a
possible explanation for the fast varying LFQPO that occur most often. Here we
look at a possible generalization to explain the characteristics of the other
two LFQPO types. It was recently shown that when the disk approaches its last
stable orbit, the AEI is markedly affected by relativistic effects. We focus on
the characteristics of the LFQPO that would result from the relativistic AEI
and compare them with the different LFQPO types. The effects of relativity on
the AEI seem to be able to explain most of the characteristics of the three
types of LFQPO within one formalism.
09/2012;
-
P. Ferrando,
A. Goldwurm,
P. Laurent,
O. Limousin,
V. Beckmann,
M. Arnaud,
X. Barcons,
D. Bomans,
I. Caballero,
F. Carrera, [......],
J. L. Sauvageot,
S. Soldi, M. Tagger,
C. Tenzer,
R. Terrier,
G. Trap,
P. Varnière,
J. Wilms,
W Zhang,
J. Zurita Heras
[show abstract]
[hide abstract]
ABSTRACT: Tracing the formation and evolution of all supermassive black holes,
including the obscured ones, understanding how black holes influence their
surroundings and how matter behaves under extreme conditions, are recognized as
key science objectives to be addressed by the next generation of instruments.
These are the main goals of the COSPIX proposal, made to ESA in December 2010
in the context of its call for selection of the M3 mission. In addition,
COSPIX, will also provide key measurements on the non thermal Universe,
particularly in relation to the question of the acceleration of particles, as
well as on many other fundamental questions as for example the energetic
particle content of clusters of galaxies. COSPIX is proposed as an observatory
operating from 0.3 to more than 100 keV. The payload features a single long
focal length focusing telescope offering an effective area close to ten times
larger than any scheduled focusing mission at 30 keV, an angular resolution
better than 20 arcseconds in hard X-rays, and polarimetric capabilities within
the same focal plane instrumentation. In this paper, we describe the science
objectives of the mission, its baseline design, and its performances, as
proposed to ESA.
04/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: A study of the trapped ion instability in the real geometry of the large tokamaks leads to the consideration of a new branch of that instability, driven by a resonance with the magnetic drift of the particles, both in collisional and non-collisional regimes.
Nuclear Fusion 01/2011; 17(1):109. · 4.09 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The spectacular variability of microquasars has led to a long string of efforts in order to classify their observed behaviors in a few states. The progress made in the understanding of the Quasi-Periodic Oscillations observed in these objects now makes it possible to develop a new way to find order in their behavior, based on the theorized physical processes associated with these oscillations. This will also have the interest of reuniting microquasars in a single classification based on the physical processes at work and therefore independent of their specificities (mass, variation timescale, outburst history, etc.). This classification is aimed to be a tool to further our understanding of microquasars behavior and not to replace phenomenological states. We start by considering three instabilities that can cause accretion in the disk. We compare the conditions for their development, and the Quasi-Periodic Oscillations they can be expected to produce, with the spectral states in which these Quasi-Periodic Oscillations are observed and sometimes coexist. From the three instabilities that we proposed to explain the three states of GRS 1915+105 we actually found the theoretical existence of four states. We compared those four states with observations and also how those four states can be seen in a model-independent fashion. Those four state can be used to find an order in microquasar observations, based on the properties of the Quasi-Periodic Oscillations and the physics of the associated instabilities. Comment: accepted by A&A
09/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: 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 Astrophysics
04/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: The origin and stability of a thin sheet of plasma in the magnetosphere of an accreting neutron star is investigated. First the radial extension of such a magnetospheric disc is explored. Then a mechanism for magnetospheric accretion is proposed, reconsidering the bending wave explored by Agapitou, Papaloizou & Terquem (1997), that was found to be stable in ideal MHD. We show that this warping becomes unstable and can reach high amplitudes, in a variant of Pringle's radiation-driven model for the warping of AGN accretion discs (Pringle (1996)). Finally we discuss how this mechanism might give a clue to explain the observed X-ray kHz QPO of neutron star binaries. Comment: Accepted for publication in MNRAS
06/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: We present a scenario for the variability of the microquasar GRS 1915+105. This starts from previous works, leading to the tentative identification of the accretion-ejection instability as the source of the low-frequency quasi-periodic oscillation of microquasars and other accreting sources. We follow the physics of this instability: its conditions (the magnetic field and geometry adapted to MHD jet models), its instability criterion, and its consequences (cooling down of the disk, heating and excitation of the corona). Comparing them to the observed properties of the source, in particular the detailed properties of its spectral states, we first derive a model of the ~30 minute cycles often exhibited by GRS 1915+105. In our model this is a limit cycle determined by the advection of poloidal magnetic flux to the inner region of the disk and its destruction by reconnection (leading to relativistic ejections) with the magnetic flux trapped in the vicinity of the central source. We show how this leads to natural explanations of observed behaviors of GRS 1915+105, including the three basic states of Belloni and coworkers. We then discuss how this could be extrapolated further to understand the longer term variability of this microquasar and others.
The Astrophysical Journal 12/2008; 607(1):410. · 6.02 Impact Factor
-
J. Rodriguez,
D. C. Hannikainen,
S. E. Shaw,
G. Pooley,
S. Corbel, M. Tagger,
I. F. Mirabel,
T. Belloni,
C. Cabanac,
M. Cadolle Bel,
J. Chenevez,
P. Kretschmar,
H. J. Lehto,
A. Paizis,
P. Varnière,
and O. Vilhu
[show abstract]
[hide abstract]
ABSTRACT: We report the results of simultaneous monitoring observations of the Galactic microquasar GRS 1915+105 with INTEGRAL and RXTE from 3 up to ~300 keV, and the Ryle Telescope at 15 GHz. We first identify the classes of variability in which GRS 1915+105 is found, and report some direct transitions between them. 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 (class χ) we observe a steady radio flux interpreted as the signature of a compact jet. We then turn to three particular observations during which we observe several types of soft X-ray dip and spike cycles, followed by radio flares, corresponding to classes ν, λ, and β types of variability. This is the first time ejections are reported during a class λ observation. We generalize the fact that a (nonmajor) 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 emission above 18 keV. We identify the trigger of the ejection as the X-ray spike. A possible correlation between the amplitude of the radio flare and the duration of the X-ray dip is found. The X-ray dips prior to ejections could thus represent the time during which the source accumulates energy and material that is ejected later. The fact that these results do not rely on any spectral modelling enhances their robustness.
The Astrophysical Journal 12/2008; 675(2):1436. · 6.02 Impact Factor
-
J. Rodriguez,
S. E. Shaw,
D. C. Hannikainen,
T. Belloni,
S. Corbel,
M. Cadolle Bel,
J. Chenevez,
L. Prat,
P. Kretschmar,
H. J. Lehto,
I. F. Mirabel,
A. Paizis,
G. Pooley, M. Tagger,
P. Varnière,
C. Cabanac,
and O. Vilhu
[show abstract]
[hide abstract]
ABSTRACT: This is the second paper presenting the results of 2 yr of monitoring of GRS 1915+105 with INTEGRAL, RXTE, and the Ryle Telescope. We present the X-ray spectral and temporal analysis of four observations showing strong radio to X-ray correlations. During one observation GRS 1915+105 was in a steady state, while during the three others it showed cycles of X-ray dips and spikes (followed by radio flares). Through time-resolved spectroscopy of these cycles, we suggest that the soft X-ray spike is the trigger of the ejection. The ejected medium is then the coronal material responsible for the hard X-ray emission. In the steady state observation, the X-ray spectrum is indicative of the hard-intermediate state, with the presence of a relatively strong emission at 15 GHz. The X-ray spectrum is the sum of a Comptonized component and an extra power law extending to energies >200 keV without any evidence for a cutoff. We observe a possible correlation of the radio flux with that of the power law component, which may indicate that we see direct emission from the jet at hard X-ray energies. We study the energy dependence of a ~4 Hz QPO during the hard-intermediate state observation. The QPO ``spectrum'' is well modeled by a power law with a cutoff at an energy of about 11 keV, and clearly differs from the relative contribution of the Comptonized component to the overall flux. This may rule out models of global oscillations of the Compton corona.
The Astrophysical Journal 12/2008; 675(2):1449. · 6.02 Impact Factor
-
SF2A-2008; 11/2008
-
[show abstract]
[hide abstract]
ABSTRACT: Similarly to the larger Galactic ridge, the Galactic center region presents a hard diffuse emission whose origin has been strongly debated for the past two decades: does this emission result from the contribution of numerous, yet unresolved, discrete point sources ? Or does it originate in a truly diffuse, hot plasma ? The Galactic center region (GC) is however different on many respects from the outer parts of the Galaxy, which makes the diffuse emission issue at the Galactic center unique. Although recent observations seem to favour a point sources origin in the far Galactic ridge, the situation is still unclear at the GC and new observations are required. Here we present results on the modeling of the truly diffuse plasma. Interestingly, such a plasma would strongly affect the dynamics of orbiting molecular clouds and thus the central engine activity. Discriminating between the two hypothesis has thus become a crucial issue in the understanding of this central region that makes the link between the inner small accretion disk and the large scale Galactic dynamics. We investigate the new inputs we can expect from Simbol-X on this matter.
03/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: X-ray binaries and AGN show observational evidence for magnetized hot plasmas. Despite years of data, very little is known on these {\it coronae} especially on the mechanisms responsible for their heating, and most models simply assume their existence. However, understanding its properties has now become a key issue of the AGN and microquasars modelling. Here we consider the effect of a strong vertical magnetic field on the corona AGN and X-ray binaries and show that its modeling (structure, heating) must be reconsidered. As a first step, we present one mechanism that could extract energy from the accretion disks and deposits it in the coronae: the {\it magnetic pumping
03/2008;
-
J. Rodriguez,
S. E. Shaw,
D. C. Hannikainen,
T. Belloni,
S. Corbel,
M. Cadolle Bel,
J. Chenevez,
L. Prat,
P. Kretschmar,
H. J. Lehto,
I. F. Mirabel,
A. Paizis,
G. Pooley, M. Tagger,
P. Varniere,
C. Cabanac,
O. Vilhu
[show abstract]
[hide abstract]
ABSTRACT: (abridged) This is the second paper presenting the results of two years of monitoring of GRS 1915+105 with \integral and \rxte and the Ryle Telescope. We present the X-ray spectral and temporal analysis of four observations which showed strong radio to X-ray correlations. During one observation GRS 1915+105 was in a steady state, while during the three others it showed cycles of X-ray dips and spikes (followed by radio flares). We present the time-resolved spectroscopy of these cyclesand show that in all cases the hard X-ray component (the Comptonized emission from a coronal medium) is suppressed in coincidence with a soft X-ray spike that ends the cycle. We interpret these results as evidence that the soft X-ray spike is the trigger of the ejection, and that the ejected medium is the coronal material. In the steady state observation, the X-ray spectrum is indicative of the hard-intermediate state, with the presence of a relatively strong emission at 15 GHz. The X-ray spectra are the sum of a Comptonized component and an extra power law extending to energies >200 keV without any evidence for a cut-off. We observe a possible correlation of the radio flux with that of the power law component, which may indicate that we see direct emission from the jet at hard X-ray energies. We study the energy dependence of a ~4 Hz QPO during the hard-intermediate state observation. The QPO-``spectrum'' is well modeled by a power law with a cut-off at an energy about 11 keV that clearly differs from the relative contribution of the Comptonized component to the overall flux. This may rule out models of global oscillations of the Compton corona. Comment: 13 pages, 8 figures (3 color), accepted for publication in ApJ, scheduled for the March 20, 2008, vol676 issue. Paper 1 is the next astro-ph number
12/2007;
-
J. Rodriguez,
D. C. Hannikainen,
S. E. Shaw,
G. Pooley,
S. Corbel, M. Tagger,
I. F. Mirabel,
T. Belloni,
C. Cabanac,
M. Cadolle Bel,
J. Chenevez,
P. Kretschmar,
H. J. Lehto,
A. Paizis,
P. Varniere,
O. Vilhu
[show abstract]
[hide abstract]
ABSTRACT: (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 degraded
12/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: In addition to lines originating in a soft phase at ~0.8 keV and to cold molecular clouds, the X-ray spectra from the Galactic center region also exhibit properties similar to those of a diffuse, thin, very hot plasma at 8 keV on a scale of hundreds of parsecs. This phase is surprising for more than one reason. First, such a hot plasma should not be bound to the Galactic plane and the power needed to sustain the escaping matter would be higher then any known source. Second, there is no known mechanism able to heat the plasma to more than a few keV. Recently we have suggested that, hydrogen having escaped, the hot plasma could be a helium plasma, heavy enough to be gravitationally confined. In this case, the required power is much more reasonable. We present here a possible heating mechanism which taps the gravitational energy of the molecular clouds. We note that the 8 keV plasma is highly viscous and we show how viscous friction of molecular clouds flowing within the hot phase can dissipate energy in the gas and heat it. We detail the MHD wake of a spherical cloud by considering the different MHD waves the cloud can excite. We find that most of the energy is dissipated by the damping of Alfvenic perturbations in two possible manners, namely by non-linear effects and by a large scale curvature of the field lines. Depending on the field strength, this heating can balance the radiative cooling. We note that the plasma parameters may be optimal to make the dissipation most efficient, suggesting a self-regulation mechanism. The loss of kinetic and gravitational energy also causes accretion of the clouds and may have significant action on the gas dynamics in this region between the large scale, bar dominated flow and the central accretion to the massive black hole. Comment: 17 pages, 6 figures, accepted for publication in A&A
03/2006;
-
[show abstract]
[hide abstract]
ABSTRACT: Recent X-ray observations by the space mission Chandra confirmed the astonishing evidence for a diffuse, hot, thermal plasma at a temperature of 9. $10^7$ K (8 keV) found by previous surveys to extend over a few hundred parsecs in the Galactic Centre region. This plasma coexists with the usual components of the interstellar medium such as cold molecular clouds and a soft (~0.8 keV) component produced by supernova remnants, and its origin remains uncertain. First, simple calculations using a mean sound speed for a hydrogen-dominated plasma have suggested that it should not be gravitationally bound, and thus requires a huge energy source to heat it in less than the escape time. Second, an astrophysical mechanism must be found to generate such a high temperature. No known source has been identified to fulfill both requirements. Here we address the energetics problem and show that the hot component could actually be a gravitationally confined helium plasma. We illustrate the new prospects this opens by discussing the origin of this gas, and by suggesting possible heating mechanisms. Comment: 9 pages, accepted for publication in APJL
08/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: Most microquasar models involve a hot plasma in a corona above the disk or at the base of the jet. The Accretion-Ejection Instability (AEI) occurring in magnetized disks leads to the growth of a spiral density wave which can explain low frequency QPOs. It has already been shown to be very efficient in extracting accretion energy from the disk and emitting it upward in the corona as Alfvén waves. Here we present a simple mechanism which also allows the AEI to excite coronal ions. This heating is due to magnetic pumping, i.e. a resonant process occurring as the magnetic field lines emerging from the disk are periodically com-pressed by the spiral wave. We show how it acts on a collisionless population of ions, trapped above the disk by the joint action of gravity and magnetic stresses. We discuss the efficiency of this mechanism in heating coronal particles and ex-plaining observational evidence.
Chin. J. Astron. Astrophys. 01/2005; 5:43-48.
-
[show abstract]
[hide abstract]
ABSTRACT: We report on RXTE observations of the microquasar XTE J1550 during a ~70 day outburst in April-June 2000. We focus here on the temporal properties of the source and study the behavior of low frequency (0.1-10 Hz) quasi periodic oscillations (LFQPO), which seem to be of different types. We focus on the so-called type C (according to the classification of Remillard and collaborators), which corresponds to a strong 0.1-6 Hz LFQPO, found to be present during at least 17 observations. We find that the frequency of the QPO is better correlated with the soft X-ray (< 7 keV) flux than with the hard flux (> 7 keV). If soft X-rays represent the behavior of an accretion disk, the relation shows that the disk may set the LFQPO frequency. In two cases, the identification of the type of QPO is not straightforward. If the QPOs in those two cases are type A (or B), then we may be seeing the QPO type alternate between type C and type A (or B), and this may represent some rapid changes in the physical properties of the accretion flow, before the system stabilizes and slowly decays toward the end of the outburst. On the other hand, if all the QPOs are of type C, we may be observing an inversion in the frequency vs. flux relation, similar to that seen in GRO J1655-40. We discuss the QPO behavior in the framework of theoretical models. Comment: 9 pages, 4 figures, accepted for publication in ApJ
05/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: The Accretion-Ejection Instability (AEI) has been proposed to explain the low frequency Quasi-Periodic Oscillation (QPO) observed in low-mass X-Ray Binaries. Its frequency, typically a fraction of the Keplerian frequency at the disk inner radius, is in the right range indicated by observations. With numerical simulation we will show how this instability is able to produce a modulation of the X-ray flux and what are the characteristic of this modulation. More simulations are required, especially 3D MHD simulations. We will briefly present a new code in development: AstroBear which will allow us to create synthetic spectra.
11/2003;
