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
fuer 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 full text of this Press Release is available at
http://www.eso.org/outreach/press-rel/pr-2003/pr-30-03.html

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 fuer extraterrestrische Physik, Germany), Sylvio Klose,
Bringfried Stecklum (Thuringer Landessternwarte Tautenburg, Germany),
Klaus Reinsch (Universitatssternwarte Goettingen, Germany), Hans Martin
Schmid (Institut fuer Astronomie Zurich, 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 Koln,
Germany), Sergej Zharikov, Gaghik Tovmassian (Instituto de Astronomia
Ensenada, Mexico), Otto Baernbantner, Christop Ries (Wendelstein-
Observatorium Muenchen, 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, Norway), 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
Universitat 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.

Contacts
Jochen Greiner
Max-Planck-Institut fuer extraterrestrische Physik
Garching, Germany
Phone: +49 89 30000-3847
Fax: +49 89 30000-3404
email: jcg@mpe.mpg.de

Sylvio Klose
Thuringer Landessternwarte Tautenburg
Tautenburg, Germany
Phone: +49 36427 86353
Fax: +49 36427 86329
email: klose@tls-tautenburg.de

Chryssa Kouveliotou
National Space Science and Technology Center
Huntsville, USA
Phone: +001 256 961 7604
Fax: +001 256 961 7604
email: chryssa.kouveliotou@msfc.nasa.gov

Dieter Hartmann
Department of Physics and Astronomy, Clemson University
Clemson, USA
Phone: +001 864 656 5298
Fax: +001 864 656 0805
email: HDIETER@clemson.edu