observational overview of state transitions in x-ray binaries

Power
type-B frequency up
1 10 100 1000
Power
2
2 5 10 20 50 100
Frequency (Hz)
0.01 0.1 1 10 100 1000
Frequency (Hz)
soft
Power
1 10 100 1000
rms-drop
0.01 0.1 1 10 100 1000
Frequency (Hz)
type-C frequency up
Power
1 10 100 1000
0.01 0.1 1 10 100 1000
Frequency (Hz)
hard
Power
1 10 100 1000
0.01 0.1 1 10 100 1000
Frequency (Hz)
Power
1 10 100 1000
0.01 0.1 1 10 100 1000
Frequency (Hz)
High Frequency QPOs
Let’s take a look at what happens during the transitions between the hard and soft state.
These transitions are host to various types of QPOs, with the two most common ones being
named type-B and type-C QPOs - not very imaginative. Let me start with the type-C. It
usually sits on top of a strong noise component in a power spectrum. And it actually evolves
from one of the broad noise components seen in the hard state. The evolution of the type-C
power spectra is shown at the top. From hard to soft, or right to left, the type-C QPO
increases in frequency by about a factor of a hundred, while it becomes weaker. Weak
remnants of this QPO can be found in the soft state, and we found that the maximum
frequency observed scales inversely with the mass of the black hole in the few systems in
which it was found. The type-C QPOs can change from 0.01 to a few tens of Hz at varying
luminosity levels.
It is still not clear what the changes mean, but if the QPO frequency is related to a radius, for
example the inner disk radius or the outer radius of the corona, this radius is getting smaller.
Of course, as Phil mentioned earlier, it doesn’t have to be a radius, it could also be scale
height of the disk for example.
This smooth evolution in type-C QPOs is almost always briefly interrupted by a short and
sudden change in the variability properties. At a more or less fixed value of the spectral
hardness, we see a drop in the overall strength of the variability. Just before that we actually
see a change in the type of QPO from type-C to type-B. The broad peaked noise component
disappears as well. The evolution of type-B QPO, which is shown on the left, is actually quite
different - rather than a frequency dependence on hardness, we see a dependence of
frequency on luminosity.
The zone of strange power spectral properties also gives rise to the so-called high frequency
QPOs, wich are typically observed at a few hundred Hz. They are observed as single peaks, or
as pairs. The single peaks can move around by up to 15-20%, but the pairs are only observed
at fixed frequencies, sometimes even years apart, suggesting they are set by GR.