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 still bright. To exploit this benefit, the William Herschel Telescope (WHT) and the Nordic Optical Telescope (NOT) 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.

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 10^4 cm-3.

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 NOT.

More information:

Fynbo, J.P.U., et al., 2006, “Probing Cosmic Chemical Evolution with Gamma-Ray Bursts: GRB060206 at z=4.048”,      http://xxx.unizar.es/abs/astro-ph?papernum=0602444

IMAGE CAPTIONS:

[Figure 1: http://www.ing.iac.es/PR/press/DLA_NOT.gif ] 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. Credit: Isaac Newton Group of Telescopes, La Palma

[Figure 2: http://www.ing.iac.es/PR/press/finestructure_WHT.gif ] Portions of the ISIS spectrum showing the OI, 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. Credit: Isaac Newton Group of Telescopes, La Palma