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), and
compare it with GRS 1915+105; this leads us to confirm and
analyze in more detail the different behavior noted by Sobczak et al., between GRO J1655-40 and other sources, when comparing
the correlation between the QPO frequency and the disk inner
radius. In a companion paper (Varni\\`ere 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 and 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. |
