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 J1655-40 by Sobczak et al. (2000, hereafter SMR), and compare it with GRS 1915+105; this leads us to confirm and analyze in more detail the different behavior noted by SMR, between GRO J1655-40 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 & Pellat (1999). We thus propose that the difference between GRO J1655-40 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.