Microquasars - Presentation / Generalities

Microquasars are low-mass X-ray binaries which exhibit powerful ultra-relativistic jets (see e.g. the review of Mirabel & Rodriguez, 1999) and are thought to host a black hole. Microquasars are observed in different spectral states : the high state where the disk thermal emission is dominant, the low state where a non-thermal power-law dominates the thermal emission, and an intermediate state where both are seen.

In the low state, and sometimes in the intermediate state, we observe low frequency (of the order of 1 to 30 Hertz) Quasi-Periodic Oscillations (LFQPO) of the X-ray flux (e.g. Fender, 2001). The right panel of figure 1 shows the low-frequency QPO of GRS 1915+105.

The left figure is a multi-wavelength observation of GRS 1915+105 with X-ray data in black, and infra-red in red, radio in green (Mirabel et al., 1998). It is the class of Belloni et al. (1999) with the LFQPO observed in the low state (before the X-ray peak) and with ejection occurring at the X-ray peak.

The right figure represents a Fourier transform of the X-ray light curve for the same object (G. Markwardt). The LFQPO is prominent at a frequency of two Hertz, and higher harmonics are also seen.