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Flashes Shed Light on Cosmic Clashes (Forwarded)



 
 
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Old October 7th 05, 03:51 AM
Andrew Yee
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Default Flashes Shed Light on Cosmic Clashes (Forwarded)

ESO Education and Public Relations Dept.

--------------------------------------------------------------
Text with all links and the photos are available on the ESO
Website at URL:

http://www.eso.org/outreach/press-re...-26-05-p2.html
--------------------------------------------------------------

Contacts:

Kristian Pedersen, Jens Hjorth, Jesper Sollerman
Dark Cosmology Centre
Niels Bohr Institute
University of Copenhagen
Phone: +45 3532 5932, +45 3532 5928, +45 3532 5968

Under embargo until 5 October 2005, 19:00 CEST (17:00 GMT)

ESO Press Release 26/05

Flashes Shed Light on Cosmic Clashes

ESO's Telescopes See Afterglows of Elusive Short Bursts

It all started like in a James Bond movie. During the 1960s,
in the midst of the Cold War, the United States launched a
series of satellites sensitive to gamma radiation [1] to
monitor compliance with the Nuclear Test Ban Treaty.

No explosions were detected in the Earth's atmosphere. Instead,
mysterious flashes of gamma-rays were seen, that appeared to
come from outside the Solar System!

This information was of course a military secret and it wasn't
until 1973 that the discovery of these enigmatic explosions
could be announced. Unfortunately, the early gamma-ray
detectors couldn't localise the source of gamma-rays on the
sky very accurately, leading to a long-standing controversy
on their origin.

Within the last few years, however, it has become possible
to locate the sites of some of these events (e.g. with the
Compton Gamma-Ray Observatory or the Beppo-Sax satellite) and
since the beginning of 1997, astronomers have identified tens
of optical sources in the sky that are associated with gamma-
ray bursts.

They have been found to be situated at extremely large (i.e.,
"cosmological") distances. The farthest has recently been
found at a redshift of 6.3 [2], indicating it is seen as when
the Universe was less than 900 million years old (ESO PR
22/05). This implies that the energy release during a gamma-
ray burst within a few seconds is larger than that of the
Sun during its entire life time (about 10,000 million years).
Apart from the Big Bang itself, "Gamma-ray bursts" or GRBs are
in fact by far the most powerful events that are known in the
Universe.

The long and the short

The wealth of observations on GRBs has revealed that they
come in two different flavours: the long (lasting more than
2 seconds) and the short ones. The difference between the two
is not only in the duration: short bursts also consist of
higher energy photons than the long ones. One may thus infer
that the physical origins of the two are different.

Over the past few years, a large international effort has
convincingly shown that long gamma-ray bursts are linked with
the ultimate explosion of massive stars ("hypernovae"), in
particular, with the first detection of an otpical afterglow
of a gamma-ray burst by Jan van Paradijs. A key proof was
provided with the help of ESO telescopes in 2003 (ESO PR
16/03). On March 29, 2003, NASA's High Energy Transient
Explorer (HETE-II) satellite detected a very bright gamma-ray
burst. Following identification of the "optical afterglow"
by a 40-inch telescope at the Siding Spring Observatory
(Australia), a high-dispersion spectrum obtained with the
UVES spectrograph at the 8.2-m VLT KUEYEN telescope at the ESO
Paranal Observatory (Chile) allowed to measure its distance to
about 2,650 million light-years. This was the nearest normal
GRB ever detected and, using two other powerful instruments at
the ESO Very Large Telescope (VLT), the FORS1 and FORS2 multi-
mode instruments, astronomers obtained, over a period of one
month, spectra of the fading object.

The astronomers observed the gradual "emergence" with time of
a supernova-type spectrum, revealing the extremely violent
explosion of a star. With velocities well in excess of 30,000
km/sec (i.e., over 10% of the velocity of light), the ejected
material was moving at record speed, testifying to the
enormous power of the explosion. This set of data provided
irrefutable evidence of a direct connection between the GRB
and the "hypernova" explosion of a very massive, highly
evolved star.

An historical breakthrough

"The breakthrough in our understanding of long-duration GRBs,
which ultimately linked them with the energetic explosion of
a massive star as it collapses into a black hole, came from
the discovery of their long-lived X-ray and optical
afterglows," says Jens Hjorth (Dark Cosmology Centre, Niels
Bohr Institute, University of Copenhagen, Denmark). "Short
duration GRBs have however evaded optical detection for
more than 30 years," he adds.

Unlike long GRBs, there was indeed no detection of an
afterglow neither in X-rays nor in the optical for short
GRBs. It was thus not possible to know in which environment
they formed nor to study their light-curve or spectrum to
characterise them. That is, until very recently.

On May 9, 2005, the NASA/ASI/PPARC Swift satellite detected a
40-millisecond duration gamma-ray burst. Further observations
with the X-ray detector on board the satellite detected an X-
ray afterglow of a short burst for the first time. Thanks to
this, its position could be determined with an accuracy better
than 10 arcseconds, allowing astronomers [3] to point ESO's
Very Large Telescope towards it and to take images with FORS2.

The burst, named GRB 050509B, was found to sit very close to
a luminous, non-star forming elliptical galaxy lying 2,700
million light-years away (redshift 0.225) and belonging to
a cluster of galaxies. Based on the unlikeness of a chance
alignment between GRB 050509B and such a galaxy, it is
argued that this is the host galaxy of the burst. This, the
astronomers explain, makes it difficult for the hypernova
model to be invoked. Indeed, it is highly improbable to find
a core-collapse supernova in this galaxy.

On the other hand, the other prevailing model, the merging of
two neutron stars [4] in a binary, seems more likely. Such a
galaxy indeed is known to host many tight binaries with compact
stars.

To be sure that the hypernova model could be ruled out, Jens
Hjorth and his team performed further observations -- until
three weeks after the burst -- with the FORS1 and FORS2
instruments. With these observations, the astronomers are
confident that even the faintest supernovae would have been
detected. But none were found.

Another "first"

ESO PR Photo 32a/05 ESO PR Photo 32b/05
First Visible Light From Variability of GRB 050709
A Short Burst

Caption: ESO PR Photo 32a/05 shows the first image in the
visible (more precisely here, in the so-called R-band) of
a short gamma-ray burst. The image was taken with the
Danish 1.5-m telescope and the DFOSC camera at La Silla
on 11 July 2005. It shows the gamma-ray burst to be
situated on the edge of a low-redshift galaxy.

ESO PR Photo 32b/05 shows two images taken with the 1.5-m
telescope at La Silla of the short gamma-ray burst afterglow.
The panel (a) is the image taken on July 11, while panel (b)
shows the same field one week later, on July 18. Only the
galaxy is now visible: the afterglow has faded away in the
time span of one week. Panel (c) shows the difference
between panels (a) and (b) clearly revealing the short
gamma-ray burst visible light.

ESO PR Photo 32c/05
The Merging Scenario

Caption: ESO PR Photo 32c/05 shows an animation of two
neutron stars orbiting each other and gradually being
dragged together. The end result is a gigantic explosion
where two jets are emitted. If one of the jets points
toward the Earth we observe a short gamma-ray burst.
(Credit: Dark Cosmology Centre, Niels Bohr Institue,
University of Copenhagen/Jan Rasmussen DRC)

And as sometime happens, a few months later, the astronomers
were given the chance to study another afterglow of a short
burst. And this time, in the optical.

In the night of July 9 to 10, 2005, the NASA HETE-2 satellite
detected a 70-millisecond duration burst and was able, based on
the detection of X-rays, to precise its position. Thirty-three
hours after, Jens Hjorth and his team obtained images of this
region of the sky using the Danish 1.5m telescope at La Silla.
The images showed the presence of a fading source, sitting on
the edge of a galaxy, most probably the host galaxy of the
burst.

"We have thus discovered the first optical afterglow of a short
gamma-ray burst", says co-author Kristian Pedersen, also from
the Dark Cosmology Centre of the University of Copenhagen.

The burst, named GRB 050709, resides 11,000 light-years from
the centre of a star-forming dwarf galaxy that is about 2,000
million light-years away and is quite young -- about 400
million years old. From the observations conducted until 20
days after the burst, the astronomers can rule out the
occurrence of an energetic hypernova as found in most long
GRBs. This gives further credit to the hypothesis that short
GRBs are the consequence of the merging of two very compact
stars.

"It is striking that the two short bursts that could finally be
localised appear in quite different environments", says Jesper
Sollerman, a member of the team from Stockholm Observatory
(Sweden).

"But with a sample of only two events," stresses Jens Hjorth,
"it would be prudent not to draw definitive conclusions at
this stage about the progenitors of short gamma-ray bursts."

Hjorth and his colleagues may be cautious, yet astronomers
cannot but marvel at the new chapter in astronomy that has
just been opened.

More information

Some of the results described in this ESO Press Release will
appear in the October 6, 2005 issue of the journal Nature ("The
optical afterglow of the short gamma-ray burst GRB 050709", by
J. Hjorth et al. and "A short gamma-ray burst apparently
associated with an elliptical galaxy" by N. Gehrels et al.).
Other results are either in press or published: "GRB 050059B:
Constraints on short gamma-ray burst models" by J. Hjorth et
al. (Astrophysical Journal Letters vol. 630, p. 117) and "The
host galaxy cluster of the short gamma-ray burst GBR 050509B"
by K. Pedersen et al., to appear in Astrophysical Journal
Letters. The press release issued by the Dark Cosmology Centre
is available at
http://www.astro.ku.dk/dark
The NASA Swift homepage -- from which the historical
information was taken -- is at
http://swift.sonoma.edu/about_swift/grbs.html.

Note

[1]: Gamma radiation and X-rays are electromagnetic radiation
like visible light, radio waves, and ultraviolet light. These
electromagnetic radiations differ only in the amount of energy
they have. Gamma rays and X-rays are the most energetic of
these.

[2]: In astronomy, the redshift denotes the fraction by which
the lines in the spectrum of an object are shifted towards
longer wavelengths. The observed redshift of a remote galaxy
provides an estimate of its distance.

[3]: The team behind the discovery of the optical afterglow of
GRB050709 is led by Jens Hjorth (Dark Cosmology Centre - DARK,
Niels Bohr Institute, University of Copenhagen), and includes
Darach Watson (DARK), Johan P.U. Fynbo (DARK), Paul A. Price
(Institute for Astronomy, University of Hawaii), Brian L.
Jensen (DARK), Uffe G. Joergensen (DARK), Daniel Kubas (ESO,
Santiago), Javier Gorosabel (Instituto de Astrofisica de
Andalucia), Pàll Jakobsson (DARK), Jesper Sollerman (DARK and
Department of Astronomy, Stockholm University), Kristian
Pedersen (DARK), and Chryssa Kouveliotou (NASA/Marshall Space
Flight Center). The team is part of the Gamma-Ray burst
Afterglow Collaboration at ESO (GRACE) carrying out gamma-ray
burst afterglow studies.

[4]: A neutron star is like one big atom with a diameter of
10-20 kilometres, and weighing about as much as the Sun. Thus,
a pinhead of neutron star material (1 millimetre across) weighs
almost 1 million tons, or about as much as the largest oil
carrier ever built, fully loaded.

National contacts for the media:

Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Finland: Ms. Terhi Loukiainen, +358 9 7748 8385
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: Prof. Massimo Capaccioli, +39-081-55 75 511
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

--------------------------------------------------------------
ESO Press Information is available on the WWW at
http://www.eso.org/outreach/press-rel/
--------------------------------------------------------------
(c) ESO Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
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