X-ray transients are accreting low-mass X-ray binaries that
spend most of their time in a faint, quiescent state. They
undergo large amplitude outbursts with rise times of only a
few days or weeks (or months in the case of GX 339-4), with
typical recurrence periods of many years. It is commonly
accepted, that the outburst of an X-ray transient is the result
of an accretion disc instability. During outbursts, the optically
thick and geometrically thin accretion disc emits at typical
X-ray energies of ~1 keV. The inner regions of the disc and the
compact object itself are embedded in a hot and tenuous medium
often referred to as the corona, where soft X-ray photons
originating in the disc undergo inverse Comptonization. The exact
origin of the high-energy spectrum is, however, subject to debate,
and part of the high-energy emission could instead arise from
synchrotron emission from a relativistic jet, which is usually seen
in the radio domain.
The X-ray transient GX 339-4 shows regular and recurrent outbursts,
which are well-sampled in X-rays. The compact object in this system
is a ~7-solar-mass black hole. On January 3, 2010, GX 339-4 entered
a new outburst, and M. Cadolle Bel et al. triggered INTEGRAL ToO
observations during the initial hard X-ray phase, and during its
declining hard X-ray phase. A unique data set was obtained with the
most comprehensive collection of simultaneous multi-wavelength data
collected so far during an outburst of GX 339-4.
Shown in the top panel are the high-energy data from RXTE/ASM (red),
Swift/BAT (green) and INTEGRAL/ISGRI (blue) observations of GX 339-4.
Also shown are the observation timelines from ATCA, Faulkes South,
REM/ROSS, REMIR and ESO/ISAAC telescopes. The blue numbers near the
bottom of the figure are the INTEGRAL revolution numbers. The bottom
panel shows the spectral energy distribution from radio to soft gamma-rays
at around 2010 March 4-6 (~ MJD 55260). Clear evolutions in the disc and
hot medium components were seen; the relative flux and contribution of the
latter decreased on average. For the first time, a jet being quenched in
many wavebands has been observed, after it had been bright and powerful
before as observed in the radio, near-infrared and optical. Variations are
observed on various time scales, down to at least 11 sec. These variations
are either related to the varying size of the acceleration zone above the
jet base, or to the magnetic field.
