The afterglow and the host galaxy of GRB 011211

Abstract

We present optical, near-infrared, and X-ray observations of the optical afterglow (OA) of the X-ray rich, longduration 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 1 = −0:95 0:02, and a post-break slope of 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 = −0:56 0:19 reddened by an SMC-like extinction law with a modest AV = 0:08 0:08 mag. By comparison, from the XMM-Newton X-ray data at around the same time, we find a decay index of X = −1:62 0:36 and a spectral index of X = −1:21+0:10 −0:15. Interpolating between the UV-optical and X-ray implies that the cooling frequency is located close to 1016 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. [see online version for correct notation]