Observing a Burst with Sunglasses: Unique Five-Week VLT Study ofthe Polarisation of a Gamma-Ray Burst Afterglow (Forwarded)

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Max-Planck-Institut für extraterrestrische Physik
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Sylvio Klose
Thüringer Landessternwarte Tautenburg
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Chryssa Kouveliotou
National Space Science and Technology Center
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FAX: +001 256 961 7604
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Dieter Hartmann
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ESO Press Release 30/03

For immediate release: 12 November 2003

Observing a Burst with Sunglasses

Unique Five-Week VLT Study of the Polarisation of a Gamma-Ray
Burst Afterglow

"Gamma-ray bursts (GRBs)" are certainly amongst the most dramatic
events known in astrophysics. These short flashes of energetic
gamma-rays, first detected in the late 1960's by military
satellites, last from less than one second to several minutes.

GRBs have been found to be situated at extremely large
("cosmological") distances. The energy released in a few seconds
during such an event is larger than that of the Sun during its
entire lifetime of more than 10,000 million years. The GRBs are
indeed the most powerful events since the Big Bang known in the
Universe, cf. ESO PR 08/99 and ESO PR 20/00.

During the past years circumstantial evidence has mounted that
GRBs signal the collapse of extremely massive stars, the
so-called hypernovae. This was finally demonstrated some months
ago when astronomers, using the FORS instrument on ESO's Very
Large Telescope (VLT), documented in unprecedented detail the
changes in the spectrum of the light source ("the optical
afterglow") of the gamma-ray burst GRB 030329 (cf. ESO PR 16/03).
A conclusive and direct link between cosmological gamma-ray
bursts and explosions of very massive stars was provided on
this occasion.

Gamma-Ray Burst GRB 030329 was discovered on March 29, 2003 by
NASA's High Energy Transient Explorer spacecraft. Follow-up
observations with the UVES spectrograph at the 8.2-m VLT KUEYEN
telescope at the Paranal Observatory (Chile) showed the burst
to have a redshift of 0.1685 [1]. This corresponds to a
distance of about 2,650 million light-years, making GRB 030329
the second-nearest long-duration GRB ever detected. The
proximity of GRB 030329 resulted in very bright afterglow
emission, permitting the most extensive follow-up observations
of any afterglow to date.

A team of astronomers [2] led by Jochen Greiner of the
Max-Planck-Institut für extraterrestrische Physik (Germany)
decided to make use of this unique opportunity to study the
polarisation properties of the afterglow of GRB 030329 as it
developed after the explosion.

Hypernovae, the source of GRBs, are indeed so far away that
they can only be seen as unresolved points of light. To probe
their spatial structure, astronomers have thus to rely on a
trick: polarimetry (see ESO PR 23/03).

Polarimetry works as follows: light is composed of
electromagnetic waves which oscillate in certain directions
(planes). Reflection or scattering of light favours certain
orientations of the electric and magnetic fields over others.
This is why polarising sunglasses can filter out the glint of
sunlight reflecting off a pond.

The radiation in a gamma-ray burst is generated in an ordered
magnetic field, as so-called synchrotron radiation [3]. If the
hypernova is spherically symmetric, all orientations of the
electromagnetic waves will be present equally and will average
out, so there will be no net polarisation. If, however, the gas
is not ejected symmetrically, but into a jet, a slight net
polarisation will be imprinted on the light. This net
polarisation will change with time since the opening angle of
the jet widens with time, and we see a different fraction of
the emission cone.

Studying the polarisation properties of the afterglow of a
gamma-ray burst thus allows to gain knowledge about the
underlying spatial structures and the strength and orientation
of the magnetic field in the region where the radiation is
generated. "And doing this over a long period of time, as the
afterglow fades and evolves, provides us with a unique
diagnostic tool for gamma-ray burst studies", says Jochen
Greiner.

Although previous single measurements of the polarisation of
GRB's optical afterglow exist, no detailed study has ever been
done of the evolution of polarisation with time. This is indeed
a very demanding task, only possible with an extremely stable
instrument on the largest telescope ... and a sufficient bright
optical afterglow.

As soon as GRB 030329 was detected, the team of astronomers
therefore turned to the powerful multi-mode FORS1 instrument
on the VLT ANTU telescope. They obtained 31 polarimetric
observations over a period of 38 days, enabling them to measure,
for the first time, the changes of the polarisation of an
optical gamma-ray burst afterglow with time. This unique set of
observational data documents the physical changes in the remote
object in unsurpassed detail.

Their data show the presence of polarisation at the level of
0.3 to 2.5% throughout the 38-day period with significant
variability in strength and orientation on timescales down to
hours. This particular behaviour has not been predicted by
any of the major theories.

Unfortunately, the very complex light curve of this GRB
afterglow, in itself not understood, prevents a straightforward
application of existing polarisation models. "It turns out that
deriving the direction of the jet and the magnetic field
structure is not as simple as we thought originally", notes
Olaf Reimer, another member of the team. "But the rapid changes
of the polarisation properties, even during smooth phases of
the afterglow light curve, provide a challenge to afterglow
theory".

"Possibly", adds Jochen Greiner, "the overall low level of
polarisation indicates that the strength of the magnetic field
in the parallel and perpendicular directions do not differ by
more than 10%, thus suggesting a field strongly coupled with
the moving material. This is different from the large-scale
field which is left-over from the exploding star and which is
thought to produce the high-level of polarisation in the
gamma-rays."

More Information

The research described in this Press Release will appear under
the title "The evolution of the polarisation of the afterglow
of GRB 030329" by Jochen Greiner et al. in the November 13,
2003 issue of the science journal "Nature".

A German translation of the information of this page can be
found at Astronomie.de .

Notes

[1]: In astronomy, the "redshift" denotes the factor by which
the lines in the spectrum of an object are shifted towards
longer wavelengths. Since the redshift of a cosmological object
increases with distance, the observed redshift of a remote
galaxy also provides an estimate of its distance.

[2]: Members of the team include Jochen Greiner, Arne Rau
(Max-Planck-Institut für extraterrestrische Physik, Germany),
Sylvio Klose, Bringfried Stecklum (Thüringer Landessternwarte
Tautenburg, Germany), Klaus Reinsch (Universitätssternwarte
Göttingen, Germany), Hans Martin Schmid (Institut für
Astronomie Zürich, Switzerland ), Re'em Sari (California
Institute of Technology, USA), Dieter H. Hartmann (Clemson
University, USA), Chryssa Kouveliotou (NSSTC, Huntsville,
Alabama, USA), Eliana Palazzi (Istituto di Astrofisica
Spaziale e Fisica Cosmica, Bologna, Italy), Christian
Straubmeier (Physikalisches Institut Köln, Germany), Sergej
Zharikov, Gaghik Tovmassian (Instituto de Astronomia Ensenada,
Mexico), Otto Bärnbantner, Christop Ries (Wendelstein-
Observatorium München, Germany), Emmanuel Jehin, Andreas
Kaufer (European Southern Observatory, Chile), Arne Henden
(USNO Flagstaff, USA), Anlaug A. Kaas (NOT, La Palma, Spain),
Tommy Grav (University of Oslo, N), Jens Hjorth, Holger
Pedersen (Astronomical Observatory Copenhagen, Denmark),
Ralph A.M.J. Wijers (Astronomical Institute Anton Pannekoek,
Amsterdam, The Netherlands), Hye-Sook Park (Lawrence Livermore
Nat. Laboratory, USA), Grant Williams (MMT Observatory,
Tucson, USA), Olaf Reimer (Theoretische Weltraum-und
Astrophysik Universität Bochum, Germany)

[3]: When electrons -- which are electrically charged -- move
through a magnetic field, they spiral around an axis defined
by the local magnetic field. Electrons of high energy spiral
very rapidly, at speeds near the speed of light. Under such
conditions, the electrons emit highly polarised electromagnetic
radiation. The intensity of this radiation is related to the
strength of the magnetic field and the number and energy
distribution of the electrons caught in this field. Many cosmic
radio sources have been found to emit synchrotron radiation --
one of the best examples is the famous Crab Nebula, depicted
in ESO PR Photo 40f/99.

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