On the Afterglow and Host Galaxy of GRB 021004: A Comprehensive Study with theHubble Space Telescope

(Abridged) We report on HST observations of the late-time afterglow and hostgalaxy of GRB021004 (z=2.33). Although this GRB is one of the best observed sofar, there is substantial disagreement between different interpretations ofdata sets on this burst in the literature. We have observed the field ofGRB021004 with the HST at multiple epochs from 3 days until almost 10 monthsafter the burst. With STIS spectroscopy we cover the spectral region 600-1700AAin the restframe. From the limit on the flux recovery bluewards of theLyman-limit we constrain the HI column density to be above 1x10^18 cm-2 Basedon ACS and NICMOS imaging we find that the afterglow evolved achromaticallywithin the errors. The color changes observed by other authors during the firstfour days must be related to a stochastic phenomenon superimposed on anafterglow component with a constant spectral shape. This achromaticity impliesthat the cooling break has remained on the blue side of the optical part of thespectrum for at least two weeks after the burst. The optical-to-X-ray slope isconsistent with being the same at 1.4 and 52.4 days after the burst. Thisindicates that the cooling frequency is constant and hence, according tofireball models, that the circumburst medium has a constant density profile.The late-time slope of the lightcurve is in the range alpha_2 = 1.8-1.9, and isinconsistent with a single power-law. This could be due to a late-timeflattening caused by the transition to non-relativistic expansion or due toexcess emission (a lightcurve `bump') 7 days after burst. The host is astarburst galaxy with no evidence for dust and with strong Ly-alpha emission.The afterglow was located very close (~100 pc) to the center of the hostsuggesting that the progenitor was associated with a circumnuclear starburst.Comment: 8 figures, to appear in the Astrophysical Journa