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Generalities | Low-Frenquency QPO

 

 

Microquasars - Presentation / Low-Frenquency QPO

The significance of the LFQPOs can be summarized as follows (Remillard & McClintock 2006) :

 

(1) LFQPOs are always present during the Low Hard state, but are also almost always seen during the "steep power-law state", which is the non-thermal state that accreting black holes exhibit when they reach their highest luminosities (i.e. L_X > 0.4 L_Edd). They can be exceedingly strong with rms amplitudes (expressed as a fraction of the mean count rate) as high as r > 0.15 for sources such as GRS 1915+105 (Morgan et al. 1997) and XTE J1550–564 (Sobczak et al. 2000). More generally, they are seen with 0.03 < r < 0.15 whenever the steep power law contributes more than 20% of the flux at 2–20 keV (Sobczak et al. 2000). LFQPOs have been observed up to energies above 60 keV (Tomsick et al. 2001).

 

(2) In several sources, the LFQPO frequency is correlated with the total disk flux (but not with temperature or inner disk radius; Sobczak et al. 2000; Muno et al. 1999; Trudolyubov et al. 1999). This behavior, in combination with the the role of the steep power law mentioned above, suggests that LFQPOs may provide a vital clue on the coupling between the thermal and steep power-law components.

(3) In the case of GRS 1915+105, LFQPOs always appear while the source exhibits a quasi-steady radio jet, which has been imaged (VLBI) on several occasions (Dhawan et al. 2000). Steady radio jets are associated with the "low-hard" state of black hole binaries, where the X-ray spectrum appears as a power-law with a photon index ( 1.7) that is flatter than the steep power-law state ( > 2.4).

(4) LFQPOs can be quasi–stable features that persist for days or weeks. In GRS 1915+105, QPOs at 2.0–4.5 Hz persisted for 6 months during late 1996 and early 1997 (Muno et al. 2001).

(5) In a general sense, it can be argued that oscillations as distinct as these QPOs (often with Q > 10), represent global requirements for an organized emitting region. For example, in the context of models in which thermal radiation originates from MHD instabilities, one cannot accept the common picture of numerous and independent magnetic cells, which are distributed throughout the inner disk, unless they are also combined with some large scale coherent structures.