Integral locates origin of high-energy emission from Crab Nebula (Forwarded)

ESA News
http://www.esa.int

29 August 2008

Integral locates origin of high-energy emission from Crab Nebula

Thanks to data from ESA's Integral gamma-ray observatory, scientists have
been able to locate where particles in the vicinity of the rotating
neutron-star in the Crab Nebula are accelerated to immense energies.

The discovery put in place another piece of the puzzle in understanding how
neutron stars work.

Rotating neutron-stars, or 'pulsars', are known to accelerate particles to
enormous energies, typically one hundred times more than the most powerful
accelerators on Earth, but scientists are still uncertain exactly how these
systems work and where the particles are accelerated.

A step forward in this understanding is now accomplished thanks to a team of
researchers from the UK and Italy, led by Professor Tony Dean of the
University of Southampton, who studied high-energy polarised light emitted
by the Crab Nebula -- one of the most dramatic sights in deep space.

The Crab Nebula is the result of a supernova explosion which was seen from
Earth on 4 July 1054. The explosion left behind a pulsar with a nebula of
radiating particles around it. The pulsar contains the mass of the Sun
squeezed into a volume of about 10 km radius, rotating very fast -- about 30
times a second -- thereby generating very powerful magnetic fields and
accelerating particles. A highly collimated jet, aligned with the spin axis
of the pulsar and a bright radiating donut' structure (or torus) around the
pulsar itself, are also seen.

So, the Crab is known to accelerate electrons -- and possibly other
particles -- to extremely high speed, and so produces high energy radiation.
But where exactly are these particles accelerated?

Looking into the heart of the pulsar with Integral's spectrometer (SPI), the
researchers made a detailed study to assess the polarisation -- or the
alignment -- of the waves of high-energy radiation originating from the
Crab.

They used data from more than 600 individual observations of the nebula, and
saw that this polarised radiation is highly aligned with the rotation axis
of the pulsar. So they concluded 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 discovery allows the researchers to discard other
theories that locate the origin of this radiation further away from the
pulsar.

Professor Tony Dean of the University's School of Physics and Astronomy
commented that the discovery of such alignment -- also matching with the
polarisation observed in the visible band -- is truly remarkable. "The
findings have clear implications on many aspects of high energy accelerators
such as the Crab," he added.

"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", said 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."

Notes for editors:

The results are published in the 29 August issue of the scientific journal
Science, in a paper titled Polarized gamma-ray emission from the Crab', by:
A. J. Dean, D.J. Clark, V.A.McBride, A.J.Bird, A.B.Hill and S.E.Shaw
(University of Southampton's School of Physics and Astronomy); J.B. Stephen
and L. Bassani (INAF-IASF, Bologna); and A. Bazzano and P. Ubertini
(INAF-IASF, Roma).

For more information:

Tony Dean, School of Physics and Astronomy, University of Southampton
Email: a.j.dean @ soton.ac.uk

John Stephen, Space Astrophysics Institute, INAF-IASF, Bologna
Email: stephen @ iasfbo.inaf.it

Pietro Ubertini, Space Astrophysics Institute, INAF-IASF, Roma
Email: pietro.ubertini @ iasf-roma.inaf.it

Christoph Winkler, ESA Integral Project Scientist
christoph.winkler @ esa.int

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