The afterglow and the host galaxy of GRB 011211

We present optical, near-infrared, and X-ray observations of the optical afterglow (OA) of the X-ray rich, long-duration gamma-ray burst GRB 011211. Hubble Space Telescope (HST) data obtained 14, 26, 32, and 59 days after the burst, show the host galaxy to have a morphology that is fairly typical of blue galaxies at high redshift. We measure its magnitude to be R=24.95 ±0.11. We detect a break in the OA R-band light curve which is naturally accounted for by a collimated outflow geometry. By fitting a broken power-law to the data we find a best fit with a break 1.56 ±0.02 days after the burst, a pre-break slope of (alpha)1=-0.95 ±0.02, and a post-break slope of (alpha)2=-2.11 ±0.07. The UV-optical spectral energy distribution (SED) around 14 hours after the burst is best fit with a power-law with index (beta)=-0.56 ±0.19 reddened by an SMC-like extinction law with a modest AV=0.08 ±0.08 mag. By comparison, from the XMMNewton X-ray data at around the same time, we find a decay index of (alpha)X=-1.62 ±0.36and a spectral index of (beta)X=-1.21 +0.10 -0.15. Interpolating between the UV-optical and X-ray implies that the cooling frequency is located close to ~10^16 Hz in the observer frame at the time of the observations. We argue, using the various temporal and spectral indices above, that the most likely afterglow model is that of a jet expanding into an external environment that has a constant mean density rather than a wind-fed density structure. We estimate the electron energy index for this burst to be p~2.3.