Universitat de - Departament d'Astronomia i Meteorologia ...

22 Chapter 1. Introduction and background
1.2.5 Black hole states and radio emission
Black holes mainly display two different kinds of behavior at X-ray energies. On one
hand, when the total X-ray luminosity is found to be low, they display a spectrum
significantly hard up to a few hundreds of keV. This is the so-called low/hard state.
On the other hand, when they exhibit a high X-ray luminosity, their spectrum is
substantially softer. This is the so-called high/soft state. The lower energy photons,
responsible for the enhancement of X-ray luminosity and steepening of the spectrum
in the high/soft state, are produced by a multicolor blackbody in the inner accretion
disk, while the hard X-ray tail in the low/hard state is thought to be the result of a
(inverse) Comptonizing corona above the disk. Apart from these two states, there
is also the off state, when X-ray emission is almost or totally suppressed, and the
hybrids very high and intermediate states, where both spectral characteristics (soft
disk photons and hard tail) coexist.
The accepted idea was that the accretion rate increased from the off state,
through the low/hard state, the intermediate state up to the high/soft state (and
eventually the very high state). However, this picture is not so clear now, because
the same characteristics of a given spectral state (X-ray spectral and timing proper-
ties) have been observed at very different X-ray luminosities (a traditional indicator
of accretion rate).
An important insight into the possible scenario related to the black hole states
has been provided by radio observations. In this sense, long-term multiwavelength
campaigns such as the one shown in Fig. 1.8 are very useful. There we can see two
years of monitoring of GX 339−4 at radio, hard X-rays and soft X-rays. In the
upper part of the figure it is marked the X-ray spectral state of the source along
time. Starting in the low/hard state, a transition is observed into the high/soft
state, which lasted around one year, followed by a transition back to the low/hard
state and ending with an off state. The radio flux is clearly correlated with the
hard X-rays in all spectral states. The correlation between the radio flux and the
soft X-rays is more complicated, but can be shown to behave as follows: during
the low/hard state radio emission is correlated with the soft X-ray photons, while
during the high/soft state radio emission is suppressed. Faint radio emission is
detected during the off state, compatible with it being a lower luminosity low/hard
state. It is interesting to note that the spectral index of the radio emission is flat
all the time, indicative of a compact and partially self-absorbed jet, except when