Origin of high energy emission from the Crab Nebula identified
Another piece of the jigsaw in understanding how neutron stars work has been
put in place following the discovery of the origin of the high energy emission
from rotation-powered pulsars.
Pulsar systems containing neutron stars accelerate particles to immense
energies, typically one hundred times more than the most powerful accelerators
on Earth, but it is still uncertain exactly how these systems work and where
the particles are accelerated.
However INTEGRAL observations have now revealed that these energetic photons
originate close to the pulsar. Imaging observations with IBIS were able to
locate the centroid of the Crab flux with an accuracy of 20 arcseconds between
18 and 60 keV to a zone encompassing the pulsar and the surrounding jet/torus
structure and the similarity in the detected spectral slope with that seen in
X-rays from the central zone indicates that the majority of the gamma-rays are
derived directly from the jet and/or the torus structure. Furthermore, by
analysing data from over 600 individual observations of the Crab with the
INTEGRAL spectrometer SPI, the polarization of the 100 keV- 1 MeV off-pulse
gamma-rays has been measured and compared to the output from a sophisticated
computer model. The results show polarization of 46 +/- 10% with an electric
vector of 123 +/- 11 degrees, closely aligned with the spin axis of the
neutron star, demonstrating that a significant fraction of the high energy
electrons responsible for the polarized photons are produced in a highly
ordered structure close to the pulsar.
Unpublished polarisation analysis of IBIS data taken in the same
observations support this finding (I. Grenier, private communication).
The figure shows the gamma-ray polarization vector superimposed on a composite
image of the Crab from Chandra (X-ray/blue) and HST (optical/red). The vector
is drawn so as to pass through the position of the pulsar. The limits on the
direction of the vector are indicated by the shading. The direction of the
polarization vector shows a remarkable alignment with the inner jet structure.
Image credits NASA/CXC/ASU/J. Hester et al. (2002) Ap. J 577, L49 (X-ray) and
NASA/HST/ASU/J. Hester et al. (2002) Ap. J 577, L49 (Optical)
