Black-hole transients are binaries containing a late-type star
and a black hole. Although they spent most of the time in a
low-accretion quiescent state, they undergo very bright outbursts
which can last months to years, where their luminosity reaches a
sizable fraction of the Eddington luminosity. The evolution of
spectral and timing characteristics of their X-ray emission is
very complex. At the start and at the end of the outburst the
energy spectrum is rather hard, with a strong thermal tail
extending above 100 keV. In the middle of the outburst a soft
state is often reached, where the emission is very soft, with an
additional hard tail, possibly of non-thermal origin, extending
up to 1 MeV.
The transitions between these two states are particularly interesting
as the emission changes rapidly on a time scale of a day to a few
weeks. They hold the key to the understanding of the physical nature
of the observed components. Unfortunately, these transitions are
usually either too short to be caught or too slow to be followed with
the required continuity of coverage.
In September 2012, a new transient of this class was discovered by the
Swift satellite and was given the name Swift J174510.8-2624. Located
close to the center of our galaxy, it was observed serendipitously by
INTEGRAL and was seen to brighten rapidly. A dense observing campaign
with INTEGRAL was triggered (PI: T. Belloni, INAF-OAB, Italy), featuring
a near-continuous pointing for two full weeks. This dataset, together
with subsequent serendipitous pointings which had to stop by the end of
October due to satellite constraints, constitutes the best high-energy
X-ray coverage of a transition. The source was also monitored by Swift
and with a number of ground-based telescopes from the optical to the
radio band. XMM-Newton, Chandra and Suzaku observations were also made.
The figure shows the time evolution of Swift J174510.8-2624 as observed
by INTEGRAL and Swift. The top panel shows the light curve from the
Swift X-Ray Telescope (XRT; in blue) and that from the INTEGRAL JEM-X
(in red). The middle panel shows the INTEGRAL IBIS rate (in red),
showing that the source flux exceeded the Crab level. The peak of the
outburst was not observed, but the blue points from the Swift Burst
Alert Telescope (BAT), scaled to match the IBIS ones, fill the gap.
The bottom panel shows the evolution of the ratio between the hard
(40-80 keV) and soft (20-40 keV) bands in IBIS, a crude measure of the
hardness of the spectrum. The image background is the IBIS image of the
field.
The full analysis of all multi-wavelength data is in progress. Unfortunately,
this source did not undergo a full transition, but after leaving the hard
state lingered in the intermediate domain. However, the existing spectral
evolution is remarkable. One can notice that the IBIS spectrum started
softening first (bottom panel), then the hard flux reached its peak (middle
panel), then the soft flux (top panel). This dataset will provide
unprecedented information on the physics of the different components in the
spectrum.
