Astronomers Find Farthest Known Gamma-Ray Burst with ESO VLT(Forwarded)

ESO Education and Public Relations Dept.

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Contact:
Guido Chincarini
Osservatorio Astronomico di Brera (INAF), Italy
Phone: +39 39 999 1157

Under Embargo until September 12, 2005, 00:00 CET

ESO Press Release 22/05

Star Death Beacon at the Edge of the Universe

Astronomers Find Farthest Known Gamma-Ray Burst with ESO VLT

An Italian team of astronomers has observed the afterglow
of a Gamma-Ray Burst that is the farthest known ever. With
a measured redshift of 6.3, the light from this very remote
astronomical source has taken 12,700 million years to reach
us. It is thus seen when the Universe was less than 900
million years old, or less than 7 percent its present age.

"This also means that it is among the intrinsically brightest
Gamma-Ray Burst ever observed", said Guido Chincarini from
INAF-Osservatorio Astronomico di Brera and University of
Milano-Bicocca (Italy) and leader of a team that studied
the object with ESO's Very Large Telescope. "Its luminosity
is such that within a few minutes it must have released 300
times more energy than the Sun will release during its
entire life of 10,000 million years."

ESO PR Photo 27a/05 ESO PR Photo 27b/05
The Distant Gamma-Ray Burst GRB 050904 Observed in
GRB 050904 Various Bands

Caption: ESO PR Photo 27a/05 shows the position of the
distant Gamma-Ray Burst GRB 050904. The image is a
colour-composite based on images obtained with ISAAC on
the VLT in three different bands in the near-infrared (J,
H and K). ESO PR Photo 27b/05 presents observations of
the Gamma-Ray Burst GRB 050904 in different filters in
the visible (z- and I-band) and near-infrared (J-, H-,
and K-band). The observations were done with FORS2 and
ISAAC on the 8.2m Antu (UT1) telescope, which is part of
ESO's VLT at Paranal. The burst is seen in the lower middle.
It is obvious that the GRB is much fainter in the I-band
and becomes more and more brighter as the wavelength
increases. This indicates the object is at a high redshift,
hence, is very far away.

Gamma-ray bursts (GRBs) are short flashes of energetic gamma-
rays lasting from less than a second to several minutes. They
release a tremendous quantity of energy in this short time
making them the most powerful events since the Big Bang. It
is now widely accepted that the majority of the gamma-ray
bursts signal the explosion of very massive, highly evolved
stars that collapse into black holes.

This discovery not only sets a new astronomical record, it
is also fundamental to the understanding of the very young
Universe. Being such powerful emitters, these Gamma Ray Bursts
serve as useful beacons, enabling the study of the physical
conditions that prevailed in the early Universe. Indeed,
since GRBs are so luminous, they have the potential to
outshine the most distant known galaxies and may thus probe
the Universe at higher redshifts than currently known. And
because Gamma-ray Burst are thought to be associated with
the catastrophic death of very massive stars that collapse
into black holes, the existence of such objects so early in
the life of the Universe provide astronomers with important
information to better understand its evolution.

The Gamma-Ray Burst GRB050904 was first detected on September
4, 2005, by the NASA/ASI/PPARC Swift satellite, which is
dedicated to the discovery of these powerful explosions.

Immediately after this detection, astronomers in observatories
worldwide tried to identify the source by searching for the
afterglow in the visible and/or near-infrared, and study it.

First observations by American astronomers with the Palomar
Robotic 60-inch Telescope failed to find the source. This
sets a very stringent limit: in the visible, the afterglow
should thus be at least a million times fainter than the
faintest object that can be seen with the unaided eye
(magnitude 21). But observations by another team of American
astronomers detected the source in the near-infrared J-band
with a magnitude 17.5, i.e. at least 25 times brighter than
in the visible.

This was indicative of the fact that the object must either
be very far away or hidden beyond a large quantity of
obscuring dust. Further observations indicated that the
latter explanation did not hold and that the Gamma-Ray Burst
must lie at a distance larger than 12,500 million light-years.
It would thus be the farthest Gamma-Ray Burst ever detected.

ESO PR Photo 27c/05 ESO PR Photo 27d/05
GRB 050904 at Two Epochs Spectral Energy Distribution of
GRB 050904

Caption: ESO PR Photo 27c/05 shows the observation of the
Gamma-Ray Burst GRB 050904 at two different epochs, i.e.
about one day after its discovery with the Swift satellite
(left), and four days after (right), in the near-infrared
J-band. The burst becomes quickly fainter as can be seen
by comparing the two images. ESO PR Photo 27d/05 shows the
magnitude of the Gamma-Ray Burst GRB 050904 as observed with
FORS2 and ISAAC in the various filters. The bandpasses of
the ESO filters are overplotted as well as the best-fitting
template which allowed the astronomers to measure the
photometric redshift. The clear drop of the flux of the
object in the I-band compared to the others is the telltale
signature of a high-redshift object.

Italian astronomers forming the MISTICI collaboration [1] then
used Antu, one of four 8.2-m telescopes that comprise ESO's
Very Large Telescope (VLT) to observe the object in the near-
infrared with ISAAC and in the visible with FORS2. Observations
were done between 24.7 and 26 hours after the burst.

Indeed, the afterglow was detected in all five bands in which
they observed (the visible I- and z-bands, and the near-
infrared J, H, and K-bands). By comparing the brightness of
the source in the various bands, the astronomers could deduce
its redshift and, hence, its distance. "The value we derived
has since then been confirmed by spectroscopic observations
made by another team using the Subaru telescope", said Angelo
Antonelli (Roma Observatory), another member of the team.

More information

Preliminary communication about these results have been done
via the "Gamma Ray Bursts Coordinates Network".

Note

[1]: The MISTICI collaboration consists of astronomers from
Osservatorio Astronomico di Roma (INAF), Osservatorio
Astronomico di Brera (INAF), Osservatorio Astronomico di
Arcetri (INAF), Universita` degli Studi di Milano - Bicocca,
International School for Advanced Studies (SISSA) and
Observatori Astronomic of Universitat de Valencia (Spain).
In particular, Angelo Antonelli, Daniele Malesani, Vincenzo
Testa, Paolo D'Avanzo, Stefano Covino, Alberto Fernandez-Soto,
Gianpiero Tagliaferri, Guido Chincarini, Sergio Campana,
Massimo Della Valle, Felix Mirabel, and Luigi Stella were
notably active with the data analysis and observations. Prof.
Guido Chincarini is the Italian Principal Investigator of the
Italian research on GRBs related to the Swift satellite, which
is funded by the Italian Space Agency (ASI).

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United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25

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