Accretion-Ejection Instability in magnetized disks,
and low-frequency QPOs in GRS 1915+105

Michel Tagger


I will show that an Accretion-Ejection Instability may exist in the inner regions of magnetized accretion disks. It appears as a large scale, coherent spiral structure in the disk, whose properties make it a very good candidate to explain the low frequency QPOs ( ~ 1 Hz) observed in X ray binaries. The instability proceeds by extracting angular momentum from the inner region of the disk; this momentum will end up emitted as Alfven waves toward the corona of the disk, where it might energize a jet or wind. The instability appears when the magnetic pressure becomes comparable with the thermal pressure of the disk.

I will then discuss how this instability compares with the observations of a low-frequency QPO in the low state of the micro-quasar GRS 1915+105. This leads to a consistent scenario for the ~ 30 mn cycles of this source, associated with relativistic ejections. In this ``Magnetic Flood'' scenario, the cycles are controlled by the accumulation of magnetic flux in the disk.

File translated from TEX by TTH, version 2.32.
On 16 Jul 1999, 09:19.