Controlled by Distant Explosions (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 17 Apr 2007 19:01:19 GMT
ESO Education and Public Relations Dept.
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Contact
Paul Vreeswijk, Cédric Ledoux, Alain Smette
ESO, Chile
Phone: +56 2 463 3000
Andreas Jaunsen
Institute of Astrophysics, University of Oslo, Norway
Phone: +47 22 85 70 22
Sara L. Ellison
University of Victoria, Canada
Phone: +1 250 721 7737
For Immediate Release: 28 March 2007
ESO Science Release 17/07
Controlled by Distant Explosions
VLT Automatically Takes Detailed Spectra of Gamma-Ray Burst Afterglows
Only Minutes After Discovery
A time-series of high-resolution spectra in the optical and ultraviolet
has twice been obtained just a few minutes after the detection of a
gamma-ray bust explosion in a distant galaxy. The international team of
astronomers responsible for these observations derived new conclusive
evidence about the nature of the surroundings of these powerful explosions
linked to the death of massive stars.
At 11:08 pm on 17 April 2006, an alarm rang in the Control Room of ESO's
Very Large Telescope on Paranal, Chile. Fortunately, it did not announce
any catastrophe on the mountain, nor with one of the world's largest
telescopes. Instead, it signalled the doom of a massive star, 9.3 billion
light-years away, whose final scream of agony -- a powerful burst of gamma
rays -- had been recorded by the Swift satellite only two minutes earlier.
The alarm was triggered by the activation of the VLT Rapid Response Mode,
a novel system that allows for robotic observations without any human
intervention, except for the alignment of the spectrograph slit.
Starting less than 10 minutes after the Swift detection, a series of
spectra of increasing integration times (3, 5, 10, 20, 40 and 80 minutes)
were taken with the Ultraviolet and Visual Echelle Spectrograph (UVES),
mounted on Kueyen, the second Unit Telescope of the VLT.
"With the Rapid Response Mode, the VLT is directly controlled by a distant
explosion," said ESO astronomer Paul Vreeswijk, who requested the
observations and is lead-author of the paper reporting the results. "All I
really had to do, once I was informed of the gamma-ray burst detection,
was to phone the staff astronomers at the Paranal Observatory, Stefano
Bagnulo and Stan Stefl, to check that everything was fine."
The first spectrum of this time series was the quickest ever taken of a
gamma-ray burst afterglow, let alone with an instrument such as UVES,
which is capable of splitting the afterglow light with uttermost
precision. What is more, this amazing record was broken less than two
months later by the same team. On 7 June 2006, the Rapid-Response Mode
triggered UVES observations of the afterglow of an even more distant
gamma-ray source a mere 7.5 minutes after its detection by the Swift
satellite.
Gamma-ray bursts are the most intense explosions in the Universe. They are
also very brief. They randomly occur in galaxies in the distant Universe
and, after the energetic gamma-ray emission has ceased, they radiate an
afterglow flux at longer wavelengths (i.e. lower energies). They are
classified as long and short bursts according to their duration and burst
energetics, but hybrid bursts have also been discovered (see ESO PR
49/06). The scientific community agrees that gamma-ray bursts are
associated with the formation of black holes, but the exact nature of the
bursts remains enigmatic.
Because a gamma-ray burst typically occurs at very large distances, its
optical afterglow is faint. In addition, it fades very rapidly: in only a
few hours the optical afterglow brightness can fade by as much as a factor
of 500. This makes detailed spectral analysis possible only for a few
hours after the gamma-ray detection, even with large telescopes. During
the first minutes and hours after the explosion, there is also the
important opportunity to observe time-dependent phenomena related to the
influence of the explosion on its surroundings. The technical challenge
therefore consists of obtaining high-resolution spectroscopy with 8-10 m
class telescopes as quickly as possible.
"The afterglow spectra provide a wealth of information about the
composition of the interstellar medium of the galaxy in which the star
exploded. Some of us even hoped to characterize the gas in the vicinity of
the explosion," said team member Cédric Ledoux (ESO).
The Rapid Response Mode UVES observations of 17 April 2006 allowed the
astronomers to discover variable spectral features associated with a huge
gas cloud in the host galaxy of the gamma-ray burst. The cloud was found
to be neutral but excited by the radiation from the UV afterglow light.
From detailed modelling of these observations, the astronomers were able-- for the first time -- to not only pinpoint the physical mechanism
responsible for the excitation of the atoms, but also determine the
distance of the cloud to the GRB. This distance was found to be 5,500
light-years, which is much further out than was previously thought. Either
this is a special case, or the common picture that the features seen in
optical spectra originate very close to the explosion has to be revised.
As a comparison, this distance of 5,500 light-years is more than one fifth
of that between the Sun and the centre of our Galaxy.
"All the material in this region of space must have been ionised, that is,
the atoms have been stripped of most if not all of their electrons," said
co-author Alain Smette (ESO). "Were there any life in this region of the
Universe, it would most probably have been eradicated."
"With the Rapid-Response Mode of the VLT, we are really looking at
gamma-ray bursts as quickly as possible," said team member Andreas Jaunsen
from the University of Oslo (Norway). "This is crucial if we are to
unravel the mysteries of these gigantic explosions and their links with
black holes!"
More Information
The two gamma-ray bursts were discovered with the NASA/ASI/PPARC Swift
satellite, which is dedicated to the discovery of these powerful cosmic
explosions.
Preliminary reports on these observations have been presented in GCN GRB
Observation Reports 4974 and 5237. A paper is also in press in the journal
Astronomy & Astrophysics ("Rapid-Response Mode VLT/UVES spectroscopy of
GRB 060418 - Conclusive evidence for UV pumping from the time evolution of
Fe II and Ni II excited- and metastable-level populations" by P. M.
Vreeswijk et al.). DOI: 10.1051/0004-6361:20066780
The team is composed of Paul Vreeswijk, Cédric Ledoux, Alain Smette,
Andreas Kaufer and Palle Møller (ESO), Sara Ellison (University of
Victoria, Canada), Andreas Jaunsen (University of Oslo, Norway), Morten
Andersen (AIP, Potsdam, Germany), Andrew Fruchter (STScI, Baltimore, USA),
Johan Fynbo and Jens Hjorth (Dark Cosmology Centre, Copenhagen, Denmark),
Patrick Petitjean (IAP, Paris, France), Sandra Savaglio (MPE, Garching,
Germany), and Ralph Wijers (Astronomical Institute, University of
Amsterdam, The Netherlands). Paul Vreeswijk was at the time of this study
also associated with the Universidad de Chile, Santiago.
National contacts for the media:
Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Finland: Ms. Tiina Raivo, +358 9 7748 8369
Denmark: Dr. Michael Linden-Vørnle, +45-33-18 19 97
France: Dr. Daniel Kunth, +33-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Dr. Leopoldo Benacchio, +39-347-230 26 51
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25
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