Accretion-Ejection Instability and a "Magnetic Flood" scenario for GRS 1915+105

We present an instability, occurring in the inner region of magnetized accretion disks, which seems to be a good candidate to explain the low-frequency QPO observed in many X-ray binaries. We then briefly show how, in the remarkable case of the microquasar GRS 1915+105, identifying this QPO with our instability leads to a scenario for the $\sim$ 30 mn cycles of this source. In this scenario the cycles are controlled by the build-up of magnetic flux in the disk.Comment: Proceedingd of the 5th Compton Symposium, Portsmouth, Sept. 199

Global MHD instabilities: from Low Frequency to High Frequency QPOs, and to Sgr A*

I review recent work that goes beyond our model for the Low-Frequency Quasi-Periodic Oscillation of microquasars, based on the Accretion-Ejection Instability. I show that similar instabilities, which can be viewed as strongly unstable versions of the diskoseismologic modes, provide explanations for both the High-Frequency QPO and for the quasi-periodicity observed durng the flares of Sgr A*, the supermassive black hole at the Galactic Center.Comment: 11 pages, 8 figures, in the proceedings of the VI Microquasar Workshop "Microquasars and beyond", Como, 2006 Sep 18-22 (Italy), ed: T. Belloni (2006), PoS(MQW6)03

A Possible Rossby Wave Instability Origin for the Flares in Sagittarius A*

In recent years, near-IR and X-ray flares have been detected from the Galaxy's central radio point source, Sagittarius A* (Sgr A*), believed to be a \~3.10^6 solar masses supermassive black hole. In some cases, the transient emission appears to be modulated with a (quasi-)periodic oscillation (QPO) of ~ 17-20 minutes. The implied ~ 3 r_S size of the emitter (where r_S = 2GM/c^2 is the Schwarzschild radius) points to an instability - possibly induced by accretion - near the Marginally Stable Orbit (MSO) of a slowly spinning object. But Sgr A* is not accreting via a large, `standard' disk; instead, the low density environment surrounding it apparently feeds the black hole with low angular momentum clumps of plasma that circularize within ~ 10-300 r_S and merge onto a compact, hot disk. In this Letter, we follow the evolution of the disk following such an event, and show that a Rossby wave instability, particularly in its magnetohydrodynamic (MHD) form, grows rapidly and produces a period of enhanced accretion lasting several hours. Both the amplitude of this response, and its duration, match the observed flare characteristics rather well.Comment: Accepted for publication in ApJ Letter

A possible interpretation for the apparent differences in LFQPO types in microquasars

International audienceIn 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 power density spectra 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. Methods. 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. Results. 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

General Relativistic Flux Modulations from Disk Instabilities in Sagittarius A*

Near-IR and X-ray flares have been detected from the supermassive black hole Sgr A* at the center of our Galaxy with a (quasi)-period of ~17-20 minutes, suggesting an emission region only a few Schwarzschild radii above the event horizon. The latest X-ray flare, detected with XMM-Newton, is notable for its detailed lightcurve, yielding not only the highest quality period thus far, but also important structure reflecting the geometry of the emitting region. Recent MHD simulations of Sgr A*'s disk have demonstrated the growth of a Rossby wave instability, that enhances the accretion rate for several hours, possibly accounting for the observed flares. In this Letter, we carry out ray-tracing calculations in a Schwarzschild metric to determine as accurately as possible the lightcurve produced by general relativistic effects during such a disruption. We find that the Rossby wave induced spiral pattern in the disk is an excellent fit to the data, implying a disk inclination angle of ~77 deg. Note, however, that if this association is correct, the observed period is not due to the underlying Keplerian motion but, rather, to the pattern speed. The favorable comparison between the observed and simulated lightcurves provides important additional evidence that the flares are produced in Sgr A*'s inner disk.Comment: 5 Pages, 3 Figures, accepted for publication in ApJ Lette

A General Relativistic Magnetohydrodynamics Simulation of Jet Formation

We have performed a fully three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulation of jet formation from a thin accretion disk around a Schwarzschild black hole with a free-falling corona. The initial simulation results show that a bipolar jet (velocity $\sim 0.3c$) is created as shown by previous two-dimensional axisymmetric simulations with mirror symmetry at the equator. The 3-D simulation ran over one hundred light-crossing time units ($\tau_{\rm S} = r_{\rm S}/c$ where $r_{\rm S} \equiv 2GM/c^2$) which is considerably longer than the previous simulations. We show that the jet is initially formed as predicted due in part to magnetic pressure from the twisting the initially uniform magnetic field and from gas pressure associated with shock formation in the region around $r = 3 r_{\rm S}$. At later times, the accretion disk becomes thick and the jet fades resulting in a wind that is ejected from the surface of the thickened (torus-like) disk. It should be noted that no streaming matter from a donor is included at the outer boundary in the simulation (an isolated black hole not binary black hole). The wind flows outwards with a wider angle than the initial jet. The widening of the jet is consistent with the outward moving torsional Alfv\'{e}n waves (TAWs). This evolution of disk-jet coupling suggests that the jet fades with a thickened accretion disk due to the lack of streaming material from an accompanying star.Comment: 27 pages, 8 figures, revised and accepted to ApJ (figures with better resolution: http://gammaray.nsstc.nasa.gov/~nishikawa/schb1.pdf

Accretion-Ejection Instability, MHD Rossby Wave Instability, diskoseismology, and the high-frequency QPO of microquasars

We present a possible explanation for the high-frequency Quasi-Periodic Oscillations of microquasars by an MHD instability that combines the physics developed, in different contexts, for the Accretion-Ejection Instability, the Rossby-Wave Instability, and the normal modes of diskoseismic models (which rely on the properties of the relativistic rotation curve in the vicinity of the Marginally Stable Orbit). This instability can appear as modes of azimuthal wavenumbers m=2, 3,... that have very similar pattern speeds \omega/m, while the m=1 mode, which would appear as the fundamental of this discrete spectrum, is less unstable. This would readily explain the 2:3 (and sometimes higher) frequency ratio observed between these QPO. These instabilites form eigenmodes, i.e. standing wave patterns at a constant frequency in the disk; they are strongly unstable, and thus do not need an external excitation mechanism to reach high amplitudes. Furthermore, they have the property that a fraction of the accretion energy can be emitted toward the corona: this would explain that these QPO are seen in a spectral state where Comptonized emission from the corona is always present. Their existence depends critically on the existence of a magnetic structure, formed by poloidal flux advected in the accretion process, in the central region between the disk and the black hole.Comment: To be published in Ap.