Gamma-ray bursts (GRBs) are very bright flashes of radiation, that are
detected approximately 100 times per year by satellites. For a long time
it was a mystery where these flashes originated from and how they were produced.
In 1997 astronomers using the William Herschel Telescope discovered that GRBs
show a so-called "afterglow": radiation in other wavelengths following the
gamma-rays. This afterglow can be studied with telescopes from the Earth,
and allowed astronomers to find that GRBs originate in the violent deaths of
massive stars, in star-forming galaxies far away.
GRBs have proven to be excellent probes of the distant
Universe. The high luminosities of GRB afterglows allow absorption line
studies of the interstellar medium at high redshift up to redshifts larger than six.
The decrease in brightness of GRB afterglows means that a rapid
response is essential: the afterglow can be "caught" when it is
To exploit this benefit, the Nordic Optical Telescope (NOT) and the
William Herschel Telescope (WHT) have
implemented rapid response GRB programmes.
Early in the morning of February the 6th, 2006 a GRB was detected by the Swift
satellite. The GRB was at that time high in the sky over La Palma and the
weather was good. Within 15 minutes the NOT was pointed towards this burst
by the Danish GRB follow-up group. Using ALFOSC a bright optical afterglow
was discovered in the R band. Directly after the detection had been made,
a low-resolution spectrum was acquired using the same instrument. The latter spectrum rapidly
determined the redshift of GRB 060206 at z=4.048.
Meanwhile, the WHT had been alerted through our collaboration of the NOT
and WHT, involving GRB follow-up teams from the Netherlands, the United
Kingdom and Denmark. Starting at just 1.6 hours after the burst a medium-resolution
spectrum could be obtained using WHT's ISIS spectrograph.
Figure 1. Portion of the NOT ALFOSC spectrum showing the Damped
Lyman-alpha line at the GRB redshift and the best fitting profile. A column density
of neutral hydrogen log N
(HI)=20.85 ± 0.10 is derived. [ GIF
The combination of the NOT and WHT provides a unique window on this afterglow.
The low resolution and broad
wavelength coverage of the NOT spectrum allowed an accurate
determination of the column density of neutral hydrogen (HI), redshifted to optical
wavelengths. The high resolution of the WHT spectra meant an accurate
study of metal lines in the spectrum was possible. A large number of metal
lines are found in the spectra,
including (forbidden) fine-structure lines. Based on the measurement of
the neutral hydrogen column density and the metal content from weak,
unsaturated singly-ionised sulphur (SII) lines, a metallicity of
[S/H] = -0.84 ± 0.10, or ~0.14 times solar metallicity, was derived.
This is in fact one of the highest
metallicities measured from absorption lines at redshift around 4. From the very high
column densities for the forbidden singly-ionised silicon (SiII*), neutral oxygen (OI* and
and OI**) lines the researchers infer very high densities in the system,
significantly larger than 104 cm-3.
Figure 2. Portions of the ISIS spectrum
showing the OI, OI*
, SiII, SiII*
and SII lines.
It is clearly visible that there are four discrete velocity components, with velocity differences up to ~500 km/s. [ GIF
The high-resolution spectra also allows the astronomers to study the kinematics of the
absorption systems: several different, discrete velocity systems can be
distinguished, with velocities up to 500 km/s. Most surprising however,
was the tentative detection of molecular hydrogen in the ISIS
spectrum. This is the very first detection of molecular lines in an
optical GRB afterglow spectrum. Especially remarkable is the fact that
this possible detection has been done with a 4-metre telescope, proving that medium size
telescopes can compete when response times are short. The joint study of the afterglow of GRB 060206 with WHT ISIS and NOT
ALFOSC shows the power of a multi-national collaboration coordinating GRB
follow-up at the Roque de Los Muchachos Observatory on La Palma.
The authors acknowledge
the indispensable assistance given by both observers and staff at WHT and
More information: Fynbo, J.P.U., et al., 2006, "Probing Cosmic Chemical Evolution with Gamma-Ray Bursts: GRB060206 at z=4.048",astro-ph/0602444.