First constraints on the hard X-ray morphology of the Crab nebula
The Crab nebula is a very efficient particle accelerator, showing evidence for electrons accelerated up to energies of ~ 1015 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. This has allowed for the first time to give constraints on the morphology of the nebula above 20 keV.
The figure shows the position of the centroid of the emission from the Crab nebula as measured in 3 energy bands (20-40 keV, 40-100 keV, and 100-200 keV) labelled 1, 2, and 3. The measured positions are indicated in red by their corresponding error circles, superimposed on a Chandra image of the nebula at lower energies. The yellow and black circles show the measurements in respectively the on-pulse and off-pulse phases, corresponding to time bins where the emission from the Crab pulsar at the heart of the nebula is maximal, respectively 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 1014 eV.
"INTEGRAL probes the morphology of the Crab nebula in hard X-rays/soft gamma-rays", Eckert, D., Savchenko, V., Produit, N., & Ferrigno, C., 2009, Astronomy & Astrophysics, in press