Gamma-ray bursts (GRBs) are
powerful flashes of energetic gamma-rays lasting from less
than a second to several minutes. They release a tremendous
amount of energy in this short time making them the most
powerful events in the Universe. In the explosion, two jets
of very fast-moving material are ejected, as depicted in
this artist's illustration. (Credit: Image courtesy of ESO)
Gamma-ray bursts (GRBs) are powerful flashes of energetic
gamma-rays lasting from less than a second to several minutes.
They release a tremendous amount of energy in this short time
making them the most powerful events in the Universe. They are
thought to be mostly associated with the explosion of stars that
collapse into black holes.
The gamma-ray burst GRB 090423 was detected by the NASA/STFC/ASI
Swift satellite during the morning (CEST) of Thursday 23 April
2009. The 10 second burst was located in the constellation of
Leo (the Lion). It was soon being followed by a whole range of
telescopes on the ground, including the 2.2-metre ESO/MPG
telescope at La Silla and ESO’s Very Large Telescope (VLT) at
Paranal, both in Chile.
VLT infrared observations, made 17 hours after the burst
detection, allowed astronomers to establish the distance to the
explosion. “We find that the light coming from the explosion has
been stretched, or redshifted, considerably by the expansion of
the Universe”, says Nial Tanvir, the leader of the team who made
the VLT observations. “With a redshift of 8.2 this is the most
remote gamma-ray burst ever detected, and also the most distant
object ever discovered — by some way.”
Because light moves at a finite speed, looking farther into the
Universe means looking back in time. The explosion occurred when
the Universe was about 600 million years old, less than 5
percent of its current age. It is believed that the very first
stars only formed when the Universe was between 200 and 400
million years old.
“This discovery proves the importance of gamma-ray bursts in
probing the most distant parts of the Universe”, says Tanvir.
“We can now be confident that even more remote bursts will be
found in the future, which will open a window to studying the
very first stars and the ultimate end of the Dark Age of the
The previous record holder for the most distant GRB — first
detected by Swift last year and then also studied with the VLT
— had a redshift of 6.7. The blast, designated GRB 080913, arose
from a star exploding about 200 million years after GRB090423.
The previous most distant object known in the Universe confirmed
spectroscopically is a galaxy with a redshift of 6.96.
The ISAAC observations at the VLT were done on behalf of an
international collaboration by N. Tanvir (U. Leicester, UK), A.
Levan (U. Warwick, UK), K. Wiersema (U. Leicester, UK), J. Fynbo
and J. Hjorth (Dark Cosmology Centre, Copenhagen, Denmark), and
P. Jakobsson (Reykjavik, Iceland).
The GROND observations with the 2.2-metre ESO/MPG telescope at
La Silla were made by F. Olivares, T. Krühler, J. Greiner and R.
Filgas (Max Planck Institute for Extraterrestrial Physics,
Gamma-ray bursts are discovered by telescopes in space. After
releasing their intense burst of high-energy radiation, they
become detectable for a short while in the optical and in the
near-infrared. This ‘afterglow’ fades very rapidly, making
detailed analysis possible for only a few hours after the
gamma-ray detection. This analysis is important in particular in
order to determine the GRB's distance and, hence, intrinsic
Gamma-ray bursts are the universe's most luminous explosions.
Most occur when massive stars run out of nuclear fuel. As their
cores collapse into a black hole or neutron star, gas jets —
driven by processes not fully understood — punch through the
star and blast into space. There, they strike gas previously
shed by the star and heat it, which generates short-lived
afterglows in many wavelengths.