IBIS Compton mode image of the Crab pulsar in the 200-500 keV energy range.
The IBIS Compton mode is based on events detected in coincidence by the two
Integral/IBIS main detectors, ISGRI and PICsIT. In this mode, we complete the
ISGRI excellent imaging capacities in the high energy range, above 200 keV,
where Compton mode detection efficiency become better than ISGRI alone ones.
We also increase the sensitivity by selecting events coming from a sky region
around the observed source.
Indeed, if a high energy photon coming from the sky makes a Compton scattering
in one detector, and loses its remaining energy in the second, we can determine
the sky position of the source, knowing the photon interaction point and energy
deposit in each detector. This "Compton imaging", also use for the localisation
of out the field of view GRB, has a precision of a few degrees. As we know in
advance the source position, this allows us anyway to select events consistent
with this position, thus rejecting a great part of background, before using
the coded mask deconvolution to get the best IBIS imaging capacities.
This technique is applied on the image shown, using observations of the Crab
pulsar between 200 keV and 500 keV, for a total observing time of 300 ks. The
pulsar is clearly visible at a 16 sigma level. Also, as the IBIS Compton mode
has the same time resolution as the ISGRI ones (i.e. around 100 microseconds),
it can provide light curves of the few high energy pulsars we know. Further
details of this work are described in M. Forot et al., ApJ 668, 1259, 2007.
