Magnetic Flood in GRS

We sum up progress accomplished, since the last microquasar workshop, on the physics of the Accretion-Ejection Instability (AEI), and its ability to explain the properties of the low-frequency QPO of microquasars. These results concern the basic theory of the instability, its numerical simulation and the resulting modelisation of the QPO, as well as detailed observations of the QPO properties. They converge to reinforce the ‘magnetic flood’ scenario, extrapolated from the AEI to explain the ∼ 30 minutes cycles of GRS 1915+105. We then discuss directions in which this scenario might be extended toward a more global view of the evolution of this source. 1. Overview This contribution is first a status report, in which we summarize recent results on the theory and numerical simulation of the Accretion-Ejection Instability (AEI), and on detailed observations of the properties of the low-frequency Quasi-Periodic Oscillation of microquasars, for which we believe that it provides a convincing explanation. This progress has been made in complementary directions (see the contributions of Peggy Varnière and Jerôme Rodriguez, these proceedings): − Theory: We have [1] computed what was only shown approximately in the original description of the AEI [2], and justifies the ‘E’ in its name: its unique ability to send upward, as Alfvén waves propagating in the corona, the accretion energy and angular momentum it extracts from the disk. We find that this mechanism is highly efficient, so that a large fraction of the accretion energy can be converted into Alfvén waves. Future work should show how this energy can be deposited in the corona to energize a wind or jet. − Numerical simulation: We have improved the numerical simulations presented by S. Caunt [3] at the last workshop. A simple model of heating and thickening of the disk at the spiral shock formed by the instability allows us to start producing synthetic light curves, which can be compared with the observed ones. Preliminary results show in particular that the AEI can reproduce the high rms amplitude of the QPO. − Observation: We have now published in final form the results presented at the last workshop, comparing theory and observation of relativistic effects when the inner disk edge approaches the last stable orbit, in particular in GRO J1655-40 and GRS 1915+105 [4]-[5]. Although the observational eviin collaboration with M. Muno(MIT) 2 Michel Tagger dence is fragile and can only be taken as an indication, an additional hint is now provided by observations of XTE J1550-564 [6]. − Energy spectrum of the QPO: It is commonly observed that the QPO is best correlated with the properties of the disk, although it affects more strongly the coronal emission, i.e. the power-law tail. However, in the same observations of XTE J1550-564, we find that the modulated emission has a different energy spectrum than this power-law. This is consistent with the presence of a hot point in the disk i.e., in our interpretation, the spiral shock. Thus these results converge to confirm the expected properties of the AEI and its ability to explain the main characteristics of the QPO. These expectations were the basis of the ‘Magnetic Flood’ scenario[7], which starts from the identification of the QPO with the AEI, and extrapolates to give a tentative explanation for the ∼30 minutes cycles of GRS 1915+105. In this scenario the cycles are determined by the processing of the poloidal (vertical) magnetic flux advected with the gas in the disk. We will show below how we have found a positive indication in favor of this scenario, in the detailed analysis of one of these cycles. We will then turn to unpublished work, presented in the undergraduate thesis of Fitzgibbon (1999) with E. Morgan and R. Remillard at MIT: defining states analogous to the ones of Belloni et al.[8], they show a striking regularity in the succession of these states: the source does not err between them, but repeatedly follows a well-defined sequence between them. We will briefly discuss how the Magnetic Flood scenario could be extrapolated again to explain this regularity. 2. The Magnetic Flood Scenario Presented previously [7] and at the last workshop, this starts from the identification of the AEI as the origin of the QPO, and follows a simple line of inferences which could explain the 30 mn cycles of GRS 1915+105. Its first step is that the apparition of the QPO, when the disk transitions from the high-soft to the low-hard state, must correspond to the crossing of a stability threshold. In our scenario the QPO appears when the magnetic pressure in the inner region of the disk builds up, reaching equipartition with the gas pressure; this is known to suppress the Magneto-Rotational turbulence (causing accretion in weakly magnetized disks), and conversely it is the most unstable situation for the AEI. As a consequence the disk cools down, since the AEI transports the accretion energy outward and upward, in the form of wave flux, rather than depositing it locally to heat the gas (as assumed in the α-disk model). This explains that the disk transitions to the low-hard state, characterized by weak disk emission and dominant coronal one, and by the presence of a strong QPO. The disk moves outward and then back in, as it is viscously refilled from the outer regions; the low state ends when the inner disk edge reaches the Last Stable Orbit, allowing its magnetic flux to reconnect with that trapped in the vicinity of the black hole (this may be the same force-free magnetic structure associated with the BlandfordZnajek mechanism). The reconnection, seen as the ‘spike’ ending the low-hard state (see the ‘Magnetic Bomb’ model presented by S. Eikenberry, this workshop) causes the ejection of the relativistic plasmoids later seen in IR and radio, and destroys magnetic flux: this allows a return to a lower magnetization state where the magneto-rotational instability dominates again the accretion process. Our detailed analysis of two such cycles was mainly motivated by a comparison of observations with the theory of relativistic effects on the AEI, mentioned in the previous section. However we also found that the QPO appeared shortly before the transition to the low state. Thus, if there is a causal relation between the low Magnetic Flood in GRS 1915+105 3 state and the QPO, it is not the one usually assumed: the QPO is not a property of the low state, but might well be what causes it, as predicted by our scenario. This scenario is compatible with all the observational facts; we use it as a guide 1997 1998 1999 2000 0 100 200 Year 0 1 2 3 4 5 6 7 8 9 10