Abstract

The Rapid-Response Mode (RRM) at ESO's Very Large Telescope (VLT) allows for rapid automatic observations of any highly variable target such as Gamma-Ray Burst (GRB) afterglows. This mode has been available for various instruments at the VLT since April 2004, and can be easily implemented for any new instumentation. Apart from discussing the operational side of this mode, we also present VLT/UVES GRB afterglow spectra observed using the RRM, which show clear variability of absorption lines at the redshift the GRB host galaxy. Without the RRM this variability would not have been observed. Using photo-excitation and -ionization modelling, we show that this varibility is due to the afterglow flux exciting and ionizing a gas cloud at distances varying from tens of parsecs to kiloparsecs away from the GRB. © 2010 Copyright SPIE - The International Society for Optical Engineering.