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             Information from the European Southern Observatory

ESO Press Release 16/03

18 June 2003                                               [ESO Logo]

Embargoed until Wednesday, June 18, 2003, 20:00 hrs CEST (18 hrs UT)
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Cosmological Gamma-Ray Bursts and Hypernovae Conclusively Linked

Part 1 of 3

Clearest-Ever Evidence from VLT Spectra of Powerful Event

A very bright burst of gamma-rays was observed on March 29, 2003 by
NASA's High Energy Transient Explorer (HETE-II), in a sky region
within the constellation Leo.

Within 90 min, a new, very bright light source (the "optical
afterglow") was detected in the same direction by means of a 40-inch
telescope at the Siding Spring Observatory (Australia) and also in
Japan. The gamma-ray burst was designated GRB 030329, according to
the date.

And within 24 hours, a first, very detailed spectrum of this new
object was obtained by the UVES high-dispersion spectrograph on the
8.2-m VLT KUEYEN telescope at the ESO Paranal Observatory (Chile). It
allowed to determine the distance as about 2,650 million light-years
(redshift 0.1685).

Continued observations with the FORS1 and FORS2 multi-mode
instruments on the VLT during the following month allowed an
international team of astronomers [1] to document in unprecedented
detail the changes in the spectrum of the optical afterglow of this
gamma-ray burst. Their detailed report appears in the June 19 issue
of the research journal "Nature".

The spectra show the gradual and clear emergence of a supernova
spectrum of the most energetic class known, a "hypernova". This is
caused by the explosion of a very heavy star - presumably over 25
times heavier than the Sun. The measured expansion velocity (in
excess of 30,000 km/sec) and the total energy released were
exceptionally high, even within the elect hypernova class.

From a comparison with more nearby hypernovae, the astronomers are
able to fix with good accuracy the moment of the stellar explosion.
It turns out to be within an interval of plus/minus two days of the
gamma-ray burst.  This unique conclusion provides compelling evidence
that the two events are directly connected.

These observations therefore indicate a common physical process
behind the hypernova explosion and the associated emission of strong
gamma-ray radiation. The team concludes that it is likely to be due
to the nearly instantaneous, non-symmetrical collapse of the inner
region of a highly developed star (known as the "collapsar" model).

The March 29 gamma-ray burst will pass into the annals of
astrophysics as a rare "type-defining event", providing conclusive
evidence of a direct link between cosmological gamma-ray bursts and
explosions of very massive stars.

The full text of this Press Release, with two photos (ESO PR Photos
17a-b/03) and all related links, is available (after the lifting of
the embargo) at:

http://www.eso.org/outreach/press-rel/pr-2003/pr-16-03.html

What are Gamma-Ray Bursts?

One of the currently most active fields of astrophysics is the study
of the dramatic events known as "gamma-ray bursts (GRBs)". They were
first detected in the late 1960's by sensitive instruments on-board
orbiting military satellites, launched for the surveillance and
detection of nuclear tests.  Originating, not on the Earth, but far
out in space, these short flashes of energetic gamma-rays last from
less than a second to several minutes.

Despite major observational efforts, it is only within the last six
years that it has become possible to pinpoint with some accuracy the
sites of some of these events. With the invaluable help of
comparatively accurate positional observations of the associated
X-ray emission by various X-ray satellite observatories since early
1997, astronomers have until now identified about fifty short-lived
sources of optical light associated with GRBs (the "optical
afterglows"). 

Most GRBs have been found to be situated at extremely large
("cosmological") distances. This implies that 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 massive stars. This was originally based on
the probable association of one unusual gamma-ray burst with a
supernova ("SN 1998bw", also discovered with ESO telescopes, cf. ESO
PR 15/98). More clues have surfaced since, including the association
of GRBs with regions of massive star-formation in distant galaxies,
tantalizing evidence of supernova-like light-curve "bumps" in the
optical afterglows of some earlier bursts, and spectral signatures
from freshly synthesized elements, observed by X-ray observatories.

VLT observations of GRB 030329

On March 29, 2003 (at exactly 11:37:14.67 hrs UT) NASA's High Energy
Transient Explorer (HETE-II) detected a very bright gamma-ray burst.
Following identification of the "optical afterglow" by a 40-inch
telescope at the Siding Spring Observatory (Australia), the redshift
of the burst [3] was determined as 0.1685 by means of a
high-dispersion spectrum obtained with the UVES spectrograph at the
8.2-m VLT KUEYEN telescope at the ESO Paranal Observatory (Chile).

The corresponding distance is about 2,650 million light-years. This
is the nearest normal GRB ever detected, therefore providing the
long-awaited opportunity to test the many hypotheses and models which
have been proposed since the discovery of the first GRBs in the late
1960's.

With this specific aim, the ESO-lead team of astronomers [1] now
turned to two other powerful instruments at the ESO Very Large
Telescope (VLT), the multi-mode FORS1 and FORS2
camera/spectrographs. Over a period of one month, until May 1, 2003,
spectra of the fading object were obtained at regular rate, securing
a unique set of observational data that documents the physical
changes in the remote object in unsurpassed detail.

 - Continued -

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