Posted: September 03, 2008
A team of scientists lead by Professor Tony Dean of the University of Southampton has detected polarized gamma-ray emission from the vicinity of the Crab Nebula, providing insight into the processes that bring a dead star to life.
The Crab Nebula is the remnant of a supernova explosion that was first observed from Earth on 4 July 1054. The explosion left behind a pulsar – a rotating neutron star – that contains the mass of the Sun squeezed into a volume of about 10 kilometre radius, rotating 30 times a second, thereby generating very powerful magnetic fields and accelerating particles. But even 40 years after the discovery of the Crab pulsar, key questions remain surrounding how the pulsar’s rotation power is converted into the electromagnetic and kinetic power of electromagnetic fields and relativistic particles, and how these particles are accelerated to emit such high energy radiation.
This image shows the direction of polarization, or alignment, of the high-energy radiation emitted by the Crab Nebula, as detected by ESA’s Integral gamma-ray observatory. The shaded part represents the error in the determination of this direction, which appears remarkably aligned with the inner jets of the Crab, which are aligned with the rotation axis of the pulsar located at the centre of the system. Image: NASA/CXC/ASU/HST/J. Hester et al.
Using 600 individual observations from the gamma-ray telescope flying on the European Space Agency's INTEGRAL spacecraft, a team of researchers from the UK and Italy have helped to answer some of these questions. They detected polarized gamma-ray emission jetting from the vicinity of the Crab Nebula in the same alignment as the rotation axis of the pulsar, implying that a significant portion of the electrons generating the high energy radiation must originate from a highly organised structure located very close to the pulsar, very likely directly from the jets themselves.
"The remarkable alignment of the electric vector with the rotational axis of the pulsar, together with its similarity to the optical polarization angle, suggests that both fluxes originate at the same site close to the neutron star,” says Professor Tony Dean of the University's School of Physics and Astronomy. “The findings have clear implications on many aspects of high energy accelerators such as the Crab."
The discovery allows the researchers to discard other theories that locate the origin of this radiation further away from the pulsar. Finally understanding how the Crab system works could eventually provide the tools to diagnose the origin of powerful gamma-ray bursts, the most luminous events known, caused by the explosion of stars even more massive than those leading to Crab-like remnants.
"The detection of polarised radiation in space is very complicated and rare, as it requires dedicated instrumentation and an in-depth analysis of very complex data", adds Chris Winkler, INTEGRAL Project Scientist at ESA. "INTEGRAL's ability to detect polarised gamma-radiation and, as a consequence, to obtain important results like this one, confirms it once more as a world-class observatory."
INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) has been in Earth orbit for almost six years.