  IGR J17464-3213, already known as the HEAO-1 transient source H1743-322, has
been detected during a state transition by the SPI/INTEGRAL telescope. We
describe the spectral evolution and flaring activity of IGR
J17464-3213/H1743-322 from 2003 March 21 to 2003 April 22. During the first
part, the source followed a continuous spectral softening, with the peak of the
spectral energy distribution shifting from 100 keV down to a few keV. However
the thermal disk and the hard X-ray components had a similar intensity,
indicating that the source was in an intermediate state throughout our
observations and evolving toward the soft state. In the second part of our
observations, the ASM/RXTE and SPI/INTEGRAL light curve showed a strong flaring
activity. Two flare events lasting about 1 day each have been detected with SPI
and are probably due to instabilities in the accretion disk associated with the
state transition. During these flares, the low (1.5-12 keV) and high (20-200
keV) energy fluxes monitored with ASM/RXTE and SPI/INTEGRAL, are correlated and
the spectral shape (above 20 keV) remains unchanged while the luminosity
increases by a factor greater than 2.

Astrophysics