Abstract
Scuola Normale Superiore, I–56100 Pisa, Italy.(version: 2 February 2005)ABSTRACTWe present the first high resolution (R=20000–45000, corresponding to 14km/s at 4200˚A to 6.6 km/s at 9000 ˚A ) observations of the optical afterglowof Gamma Ray Bursts. GRB 020813 and GRB 021004 were observed byUVES@VLT 22.19 hours and 13.52 hours after the trigger, respectively. Thesespectra show that the inter–stellar matter of the GRB host galaxies is complex,with many components contributing to each main absorption system, andspanning a total velocity range of up to about 3000 km/s. Several narrowcomponents are resolved down to a width of a few tens of km/s. In the case ofGRB 021004 we detected both low and high ionization lines. Combined withphotoionization results obtained with CLOUDY, the ionization parameters ofthe various systems are consistent with a remarkably narrow range with noclear trend with system velocity. This can be interpreted as due to densityfluctuations on top of a regular R
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