The Most Remote Gamma-Ray Burst: ESO Telescopes Observe “Lightning” in the Young Universe

One of the currently most active fields of astrophysics is the
study of the mysterious events known as "gamma-ray
bursts". They were first detected in the late 1960’s by
instruments on orbiting satellites. These short flashes of energetic
gamma-rays last from less than a second to several minutes.

Despite much effort, it is only within the last few years that
it has become possible to locate the sites of some of these events
(e.g. with the Beppo-Sax
satellite). Since the beginning of 1997, astronomers have
identified about twenty 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. 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). "Gamma-ray bursts" are in fact by far the most
powerful events since the Big Bang that are known in the Universe.

While there are indications that gamma-ray bursts originate in
star-forming regions within distant galaxies, the nature of such
explosions remains a puzzle. Recent observations with large
telescopes, e.g. the measurement of the degree of polarization of
light from a gamma-ray burst in May 1999 with the VLT (ESO PR 08/99), are now beginning to cast some light on this
long-standing mystery.

A gamma-ray burst was detected on January 31, 2000, by an
international network of satellites (Ulysses,
NEAR and Konus) via the InterPlanetary Network (IPN) [2].
It was designated GRB 000131 according to the date of the
event. From geometric triangulation by means of the measured, exact
arrival times of the signal at the individual satellites, it was
possible to determine the direction from which the burst came. It was
found to be from a point within a comparatively small sky area (about
50 arcmin2 or 1/10 of the apparent size of the Moon), just inside the
border of the southern constellation Carina (The Keel).

Follow-up observations were undertaken by a group of European
astronomers [1]
with the ESO Very Large
Telescope at the Paranal Observatory. A
comparison of several exposures with the FORS1 multi-mode instrument
at the 8.2-m VLT ANTU telescope during the nights of February 3-4 and
5-6 revealed a faint, point-like object that was fading rapidly –
this was identified as the optical counterpart of the gamma-ray burst
(the "afterglow"). On the second night, the R-magnitude
(brightness) was found to be only 24.4, or 30 million times fainter
than visible with the unaided eye in a dark sky.

It was also possible to observe it with a camera at the 1.54-m
Danish Telescope at the La Silla
Observatory, albeit only in a near-infrared band and with a
1-hour exposure. Additional observations were made on February 8 with
the SOFI multi-mode instrument at the ESO 3.58-m
New Technology Telescope (NTT) at La Silla. The observations
were performed partly by the astronomers from the group, partly in
"service mode" by ESO staff at La Silla and Paranal.

The observations showed that the light from the afterglow was
very red, without blue and green light. This indicated a
comparatively large distance and, assuming that the light from the
explosion would originally have had the same colour (spectral
distribution) as that of optical counterparts of other observed
gamma-ray bursts, a photometric redshift of 4.35 to 4.70 was
deduced [3].

An accurate measurement of the redshift – hence the distance –
requires spectroscopic observations. A spectrum of GRB 000131
was therefore obtained on February 8, 2000, cf. PR Photo
28c/00. At this time, the brightness had decreased further and
the object had become so faint (R-magnitude 25.3) that a total of 3
hours of exposure time was necessary with VLT ANTU + FORS1 [4].
Still, this spectrum is quite "noisy".

The deduced photometric redshift of GRB 000131 predicts
that a "break" will be seen in the red region of the
spectrum, at a wavelength somewhere between 650 and 700 nm. This
break is caused by the strong absorption of light in intergalactic
hydrogen clouds along the line of sight. The effect is known as the
"Lyman-alpha forest" and is observed in all remote
objects [5].

As PR Photo 28c/00 shows, such a break was indeed found
at wavelength 670.1 nm. Virtually all light at shorter wavelengths
from the optical counterpart of GRB 000131 is absorbed by
intervening hydrogen clouds. From the rest wavelength of the
Lyman-alpha break (121.6 nm), the redshift of GRB 000131 is then
determined as 4.50, corresponding to a travel time of more than 90%
of the age of the Universe.

The measured redshift of 4.50 makes GRB 000131 the most
distant gamma-ray burst known (the previous, spectroscopically
confirmed record was 3.42). Assuming an age of the Universe of the
order of 12 – 14,000 million years, the look-back time indicates that
the explosion took place around the time our own galaxy, the Milky
Way, was formed and at least 6,000 million years before the solar
system was born.

GRB 000131 and other gamma-ray bursts are believed to
have taken place in remote galaxies. However, due to the huge
distance, it has not yet been possible to see the galaxy in which the
GRB 000131 event took place (the "host" galaxy).
From the observed fading of the afterglow it is possible to estimate
that the maximum brightness of this explosion was at least 10,000
times brighter than the host galaxy.

The present team of astronomers has now embarked upon a detailed
study of the surroundings of GRB 000131 with the VLT. A main
goal is to observe the properties of the host galaxy.

From the observations of about twenty optical counterparts of
gamma-ray bursts identified until now, it is becoming increasingly
clear that these very rare events are somehow related to the death
of massive, short-lived stars. But despite the accumulating
amount of excellent data, the details of the mechanism that leads to
such dramatic explosions still remain a puzzle to astrophysicists.

The detection and present follow-up observations of GRB
000131 highlight the new possibilities for studies of the
extremely distant (and very early) Universe, now possible by means of
gamma-ray bursts.

When observed with the powerful instruments at a large
ground-based telescope like the VLT, this incredibly bright class of
cosmological objects may throw light on the fundamental processes of
star formation in the infant universe. Of no less interest is the
opportunity to analyse the chemical composition of the gas clouds at
the epoch galaxies formed, by means of the imprints of the
corresponding absorption lines on the afterglow spectrum.

In this context, it would be extremely desirable to obtain very
detailed (high-dispersion) spectra of the afterglow of a future
gamma-ray burst, soon after the detection and while it is still
sufficiently bright.

It would for instance be possible to observe a gamma-ray burst
like GRB 000131 with the UVES spectrograph at VLT KUEYEN at
the moment of maximum brightness (that may have been about magnitude
16). An example of chemical studies of clouds at intermediate
distance by means of a more nearby quasar is shown in HETE-2
(High Energy Transient Explorer 2) gamma-ray burst satellite
on October 9, 2000, is a major step in this direction. Under optimal
conditions, a relative accurate sky position of a gamma-ray burst may
henceforth reach the astronomy community within only 10-20 seconds of
the first detection by this satellite.

The research described in this press release is the subject of a
scientific article by the team, entitled "VLT
Identification of the optical afterglow of the gamma-ray burst GRB
000131 at z = 4.50"; it will appear in a special
VLT-issue (Letters to the Editor) of the European journal
Astronomy & Astrophysics (December 1, 2000). The results
are being presented today (October 18) at the joint CNR/ESO meeting on
"Gamma-Ray Burst in the Afterglow Era" in Rome,
Italy. Note also the related article in the ESO
Messenger (No. 100, p. 32, June 2000).

[1]: The team consists of Michael Andersen (University of
Oulu, Finland), Holger Pedersen, Jens Hjorth, Brian Lindgren
Jensen, Lisbeth Fogh Olsen, Lise Christensen (University of
Copenhagen, Denmark), Leslie Hunt (Centro per l’Astronomia
Infrarossa e lo Studio del Mezzo, Florence, Italy), Javier
Gorosabel (Danish Space Research Institute, Denmark), Johan
Fynbo, Palle Møller (European Southern Observatory), Richard
Marc Kippen (University of Alabama in Huntsville and
NASA/Marshall Space Flight Center, USA), Bjarne Thomsen
(University of Århus, Denmark), Marianne Vestergaard (Ohio
State University, USA), Nicola Masetti, Eliana Palazzi
(Instituto Tecnologie e Studio Radiazoni Extraterresti, Bologna,
Italy) Kevin Hurley (University of California, Berkeley, USA),
Thomas Cline (NASA Goddard Space Flight Center, Greenbelt,
USA), Lex Kaper (Sterrenkundig Instituut “Anton
Pannekoek", the Netherlands) and Andreas O. Jaunsen
(formerly University of Oslo, Norway; now ESO-Paranal).

[2]: Detailed reports about the early observations of this
gamma-ray burst are available at the dedicated
webpage within the GRB Coordinates Network
website.

[3]: The photometric redshift method makes it possible to
judge the distance to a remote celestial object (a galaxy, a quasar,
a gamma-ray burst afterglow) from its measured colours. It is based
on the proportionality between the distance and the velocity along
the line of sight (Hubble’s law) that reflects the expansion of the
Universe. The larger the distance of an object is, the larger is its
velocity and, due to the Doppler effect, the spectral shift of its
emission towards longer (redder) wavelengths. Thus, the measured
colour provides a rough indication of the distance. Examples of this
method are shown in ESO PR 20/98 (Photos 48a/00 and 48e/00).

[4]: In fact, the object was so faint that the positioning of
the spectrograph slit had to be done in "blind" offset,
i.e. without actually seeing the object on the slit during the
observation. This very difficult observational feat was possible
because of excellent preparations by the team of astronomers and the
very good precision of the telescope and instrument.

[5]: The "Lyman-alpha forest" refers to the
crowding of absorption lines from intervening hydrogen clouds,
shortward of the strong Lyman-alpha spectral line at rest
wavelength 121.6 nm. Good examples in the VLT ANTU + FORS1 spectra of
distant quasars are shown in ESO PR Photos 14a-c/99 and, at much higher dispersion, in
a spectrum obtained with VLT KUEYEN + UVES, cf. PR Photo 28a/00: The photo is based on
three 8-min exposures obtained with VLT ANTU and the multi-mode FORS1
instrument. The optical filters were B (seeing 0.9 arcsec; here
rendered as blue), V (0.8 arcsec; green) and R (0.7 arcsec; red). The
field measures 6.8 x 6.8 arcmin2. North is up and East is left.

Holger Pedersen
Astronomical Observatory
Copenhagen University
DK-2100 Copenhagen Ø
Denmark

Tel.: +45 3532 5980
email: holger@astro.ku.dk

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