Supernovae and their remnants are the main galactic nucleosynthesis
sites. Few radioactive isotopes are accessible to gamma-ray astronomy
for probing these stellar explosions. Among them, 44Ti is a key
isotope for the investigation of the inner regions of supernovae and
their young remnants. With a lifetime of 86 years, it emits three
gamma-ray lines at 67.9, 78.4 (from 44Sc) and 1157 keV (from 44Ca),
observable with SPI and IBIS onboard INTEGRAL.
Cassiopeia A is the youngest known Galactic supernova remnant and is
indeed the only one from which the line emission from 44Ti decay has
been unambiguously detected.
The images at the top presents IBIS/ISGRI images centered on
Cassiopeia A in six energy bands (indicated in the images) around the
two 44Sc lines which show that the source brightens at the line
energies. The bottom image shows the resulting spectrum of the
supernova remnant well fitted by a power law with an index of ~ -3.3
and two separated lines at 67.9 and 78.4 keV, each detected at 3 sigma
above the continuum emission.
Deep observations (3.2 Ms effective exposure) with INTEGRAL IBIS/ISGRI
of the Cassiopeia region allowed to detect the two-low energy 44Sc
lines at 67.9 and 78.4 keV in Cassiopeia A. Besides the robustness
provided by these IBIS/ISGRI spectroimaging observations, the main
improvements compared to previous measurements are the clear
separation of the two lines and the significant detection of the hard
X-ray continuum up to 100 keV, well fitted by a single power-law. The
estimate of the line flux is sensitive to that of this underlying
continuum whose nature is still unknown (synchrotron or nonthermal
bremsstrahlung?). The 44Ti yield deduced from these observations is ~
1.6 * 10^(-4) solar masses. This mass of ejected 44Ti is generally
thought to be unusually large in comparison with spherical explosion
models but could be explained by several effects such as asymmetries
during the explosion and a high explosion energy (see the paper by
Renaud, M. et al., ApJ Volume 647 (2006), pages L41 - L44).
Based on this firm detection of the 44Ti signature in Cassiopeia A
with IBIS/ISGRI, the expected results with SPI, thanks to its fine
spectral resolution should help us for the first time to constrain the
kinematics of the innermost layers of the explosion.
