INTEGRAL/IBIS sees Gamma Ray Burst outside Field of View
The Field of View (FOV) of the IBIS instrument is about 8 x 8 degrees
fully coded and about 29 x 29 degrees partially coded. Yet GRB030406
was observed outside this FOV (off-axis angle of 36.9 degrees). This
has been possible by using the Compton mode data of the IBIS
instrument. How does this work? IBIS consists of two detectors, ISGRI
and PICsIT. These two detectors can act as a Compton telescope: a
photon is detected and Compton scattered by the first detector and
then detected (and absorbed) by the second detector. The difference in
energy of the photon measured by the two detectors gives a so-called
'scattering angle' which, together with the positions in the two
detectors, can be used to construct circles on the sky from where the
radiation has to originate. However there is a 3 meter long collimator
on top of the detector units with lead walls, which acts as a shield
for photons with energies less than about 200 keV. One therefore needs
a very bright source with many photons above 200 keV to see the
source. GRB030406 was such a bright and hard source.
Marcinkowski et al. were able using this technique to derive a
position and the spectrum for GRB030406 (see
Astronomy and Astrophysics 452, 113). INTEGRAL typically observes about 1 GRB per
month in the FOV. Marcinkowski et al. estimate that with this
technique they can observe between 2 and 5 additional GRBs during a
year.
In the figure the likelihood of the the source position is plotted,
with the significance indicated on the right. The inset shows a
zoom-in centered on the position of the GRB (Right Ascension=285.4,
declination=-68.1).
