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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 2 of 3

The hypernova connection

Based on a careful study of these spectra, the astronomers are now
presenting their interpretation of the GRB 030329 event in a research
paper appearing in the international journal "Nature" on Thursday,
June 19. Under the prosaic title "A very energetic supernova
associated with the gamma-ray burst of 29 March 2003", no less than
27 authors from 17 research institutes, headed by Danish astronomer
Jens Hjorth conclude that there is now irrefutable evidence of a
direct connection between the GRB and the "hypernova" explosion of a
very massive, highly evolved star.

This is based on 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 is moving at
record speed, testifying to the enormous power of the explosion.

Hypernovae are rare events and they are probably caused by explosion
of stars of the so-called Wolf-Rayet" type [4]. These WR-stars were
originally formed with a mass above 25 solar masses and consisted
mostly of hydrogen.  Now in their WR-phase, having stripped
themselves of their outer layers, they consist almost purely of
helium, oxygen and heavier elements produced by intense nuclear
burning during the preceding phase of their short life.

"We have been waiting for this one for a long, long time", says Jens
Hjorth, "this GRB really gave us the missing information. From these
very detailed spectra, we can now confirm that this burst and
probably other long gamma-ray bursts are created through the core
collapse of massive stars.  Most of the other leading theories are
now unlikely." 

A "type-defining event"

His colleague, ESO-astronomer Palle Moeller, is equally content:
"What really got us at first was the fact that we clearly detected
the supernova signatures already in the first FORS-spectrum taken
only four days after the GRB was first observed - we did not expect
that at all. As we were getting more and more data, we realised that
the spectral evolution was almost completely identical to that of the
hypernova seen in 1998. The similarity of the two then allowed us to
establish a very precise timing of the present supernova event".

The astronomers determined that the hypernova explosion (designated
SN 2003dh [2]) documented in the VLT spectra and the GRB-event
observed by HETE-II must have occurred at very nearly the same time.
Subject to further refinement, there is at most a difference of 2
days, and there is therefore no doubt whatsoever, that the two are
causally connected. 

"Supernova 1998bw whetted our appetite, but it took 5 more years
before we could confidently say, we found the smoking gun that nailed
the association between GRBs and SNe" adds Chryssa Kouveliotou of
NASA. "GRB 030329 may well turn out to be some kind of 'missing link'
for GRBs."

In conclusion, GRB 030329 was a rare "type-defining" event that will
be recorded as a watershed in high-energy astrophysics.

What really happened on March 29 (or 2,650 million years ago)?

Here is the complete story about GRB 030329, as the astronomers now
read it.

Thousands of years prior to this explosion, a very massive star,
running out of hydrogen fuel, let loose much of its outer envelope,
transforming itself into a bluish Wolf-Rayet star [4]. The remains of
the star contained about 10 solar masses worth of helium, oxygen and
heavier elements.

In the years before the explosion, the Wolf-Rayet star rapidly
depleted its remaining fuel. At some moment, this suddenly triggered
the hypernova/gamma-ray burst event. The core collapsed, without the
outer part of the star knowing. A black hole formed inside,
surrounded by a disk of accreting matter. Within a few seconds, a jet
of matter was launched away from that black hole.

The jet passed through the outer shell of the star and, in
conjunction with vigorous winds of newly formed radioactive nickel-56
blowing off the disk inside, shattered the star. This shattering, the
hypernova, shines brightly because of the presence of nickel.
Meanwhile, the jet plowed into material in the vicinity of the star,
and created the gamma-ray burst which was recorded some 2,650 million
years later by the astronomers on Earth. The detailed mechanism for
the production of gamma rays is still a matter of debate but it is
either linked to interactions between the jet and matter previously
ejected from the star, or to internal collisions inside the jet
itself. 

This scenario represents the "collapsar" model, introduced by
American astronomer Stan Woosley (University of California, Santa
Cruz) in 1993 and a member of the current team, and best explains the
observations of GRB 030329.

"This does not mean that the gamma-ray burst mystery is now solved",
says Woosley. "We are confident now that long bursts involve a core
collapse and a hypernova, likely creating a black hole. We have
convinced most skeptics.  We cannot reach any conclusion yet,
however, on what causes the short gamma-ray bursts, those under two
seconds long."

 - Continued -

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