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The Case of GRS 1915+105 | Definitions of lag | LAG : a simple derivation | Application

 

 

Microquasars - LAG Model / Application to microquasar observations

Using the above argument it appears that the use of an FT can lead to an incorrect interpretation of the lag in the presence of an absorption which depends on the energy band. To use this idea for the observations of GRS 1915+105 , we need to find what may produce the "absorbed'' part of the QPO modulation. This has to be related to the jet, either having the same origin, or being a consequence of it. In the following we will assume that the QPO modulation is created by a hot spiral/point, for example in Varnire et al. (2002, 2005), and we are just interested in further absorption/modulation of this already existing modulation.

 

As mentioned before, we choose to keep the same mechanism for the QPO above and below 2Hz. Another possibility is that the QPO above 2Hz comes from the disk while the one below 2Hz is coming from the jet. This however, seems improbable because the passage through 2Hz is smooth in all variables (see Fig. 6 of Muno et al. 2001).

 

Suppose that the basis of the jet/corona gets "between'' the observer and the spiral during one orbit of the spiral in the disk. This is enough to "absorb'' a part of the flux modulation, especially if it happens when the spiral is "behind'' the black hole and therefore near the maximum of the modulation. This simple model is able to explain both the occurrence of changing sign lag and its relation with the jet. In the same way it can also explain the fact that absorption is energy dependant, which makes the coherence drop.

 

In fact, anything located inside the inner radius of the disk that can absorb a small part of the flux coming from the hot spiral could explain the changing sign of the lag and the complex behavior of the harmonics. But this needs to be related to the radio flux and therefore to the jet mechanism.

The first way to check this idea is to look at the QPO profile and see if there is an energy dependant departure from a sinusoidal signal. Morgan et al. (1997) show for the low-frequency QPO that there is indeed a departure from a sinusoid, which seems compatible with an absorption feature. This kind of analysis is difficult and rarely done for QPOs because of the lack of photons at these timescales. Another way to check the same properties is to see how the value of the lag depends on the energy band chosen.

 

Using the idea of an energy dependant absorption, we see that the negative lag will be more important between the lower energy band (say, 2-4 keV) and the highest possible band available, than between two high energy bands. It seems possible to have a change of the sign of the lag if we look to high enough energies (for example using INTEGRAL data).

This simple model can also be used with observational data to gain insight into the geometry near the black hole. The pulse shape of the QPO in different energy bands can allow us to constrain the relative geometry of the absorption region with respect to the emissive region (QPO origin), and also the column density of the absorber. We will test several mechanisms that could lead to this "absorbed-like'' profile and compare them with observational data.