First constraints on the hard X-ray morphology of the Crab nebula
The Crab nebula is a very efficient particle accelerator, showing evidences
for electrons accelerated up to energies of ~ 10^15 eV. Throughout the
nebula, the maximal energy of the relativistic electrons changes, and
therefore only an instrument with sufficient resolving power can probe the
particle acceleration processes in the different regions of the nebula.
Thanks to an unprecedented angular resolution in the hard X-ray/soft gamma-ray
energy range and to a very accurate calibration, in the case of a very bright
source like the Crab nebula the IBIS telescope on-board INTEGRAL can measure
the source position with sub-arcsecond accuracy, which allowed for the first
time to give constraints on the morphology of the nebula above 20 keV.
The figure shows the measurements of the position of Crab in 3 energy bands
(20-40, 40-100, and 100-200, labeled as 1,2, and 3) with corresponding error
circles (red circles), superimposed on the Chandra image of the nebula at
lower energies. The yellow, resp. black circles show the measurements in the
on-pulse, resp. off-pulse phases, i.e. to time bins where the emission from
the central pulsar is maximal, resp. minimal.
Thanks to these measurements, it is now possible to show that the size of the
nebula is decreasing with increasing energy, and to calculate the maximal
energy of electrons in the outer regions of the X-ray nebula, which is found
to be slightly below 10^14 eV.
Reference:
Eckert, D., Savchenko, V., Produit, N., & Ferrigno, C., 2009,
Astronomy & Astrophysics, in press
http://arxiv.org/abs/0910.1698
