TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1611 SUBJECT: GRB 021004: optical spectroscopy DATE: 02/10/09 21:14:08 GMT FROM: Nial Tanvir at IofA U.Cambridge Isabel Salamanca(1), Evert Rol(1), Ralph Wijers (1), Sara Ellison(2), Lex Kaper(1), and Nial Tanvir (3) (1) University of Amsterdam (2) Universidad Catolica de Chile (3) University of Hertfordshire, report: We obtained a spectrum with the double arm spectrograph ISIS at the 4.2-m WHT telescope at La Palma (Spain) on October 4. A partial calibration of the blue arm spectrum (2 Angstrom spectral resolution) reveals the presence of four absorption components at detected in CIV at redshifts 2.295, 2.298, 2.320, and 2.327, i.e. spanning a total velocity range of approximately 3000 km/s. The latter three components are also clearly detected in SiIV, with a marginal detection of SiIV at z=2.295. The lines around 4000-4050 Angstrom can be then identified as Ly-alpha (for which we also detect the emission component reported earlier); there is a close match for the components at z=2.320 and 2.327, whilst the two lowest redshift Lya components are blended. The interpretation of this 4-component absorption system could be of absorption local to the burst source, or of cosmological components. If all arise local to the burst, we must be viewing through ejecta in a supernova-like explosion. After a day, supernova ejecta are so opaque that we cannot see through them, and so we must be viewing the results of an explosion that occurred weeks or more before the burst, as in the supranova model. The narrowness of the components then must mean that the ejecta have already cooled and partly condensed into filaments. It is a challenge to have this happen within weeks, and given the small filling factor inherent to filamentation, to have our line of sight intersect four filaments. If cosmological, we see our line of sight pass through 4 mass concentrations of a magnitude well in excess of lyman limit systems in quasar absorption line terms. These would all be local to the host galaxy in cosmological terms, but with velocity separations large for even a present-day massive cluster. Given that only 10-20% of quasars at redshift 1 - 2 have even one such system, one concludes nonetheless that seeing four systems implies a physical relation between them. Thus we would be seeing a very large cosmic structure at z=2.3 with an extent of 3000 km/s in velocity space. This is very difficult to get in any cosmology, and would push cosmological models to very low values of Omega_matter. Further high-resolution spectroscopy is required to resolve the velocity structure, and to determine the abundances and thermodynamic state of these absorption systems, in order that we may understand the nature of this remarkable system. In local models, one might find variability in the Lyman-alpha emission. Since the source is now fading past R=21, these observations are most urgent. This message may be cited.