AAS 200th meeting, Albuquerque, NM, June 2002
Session 25. Gamma Rays, GRB Afterglows
Oral, Monday, June 3, 2002, 2:00-3:30pm, Ballroom A

## [25.06] Small-Scale Density Variations in the Environment of the Gamma-Ray Burst GRB 011211

S.T. Holland (Notre Dame), I. Soszynski (Warsaw University Observatory), M.D. Gladders (Carnegie Observatories), L.F. Barrientos (Pontificia Universidad Católica de Chile), P. Berlind, D. Bersier (Harvard--Smithsonian CfA), P.M. Garnavich (Notre Dame), S. Jha, K.Z. Stanek (Harvard--Smithsonian CfA)

We present early-time optical photometry and spectroscopy of the optical afterglow of the gamma-ray burst GRB 011211. The afterglow has a redshift of 2.140 ±0.001 and follows a power-law decay with f \propto t0.83 \pm 0.04 for approximately two days after the burst. There is evidence for a break between 1.5 and 2.7 days after the burst and a late-time slope increasing to \ge 1.4. The total energy in the burst was 1.6--2.4 \times 1050 erg, consistent with the Frail et al. (2001, ApJL, 562, L55) standard'' value of (5 ±2) \times 1050 erg. Comparing the observed colour of the optical afterglow with predictions of the standard beaming model suggests that the rest-frame V-band extinction in the host galaxy is less than approximately 0.03 mag. The magnitude of the break in the light curve, and the observed total energy, suggest that the burst expanded into an ambient medium that is homogeneous on large scales with a local particle density between \approx 0.1 and 10 cm-3. We find that the R-band optical decay deviates from a power law at the 95% confidence level \approx 0.5 days after the burst. The magnitude and duration of these deviations are consistent with density fluctuations on spatial scales of \approx 30--200 pc in the circumburst medium within \approx 0.05--0.20 pc of the gamma-ray burst's progenitor.

STH and PMG acknowledge support from the NASA LTSA grant NAG5--9364. DB has been supported by NSF grand AST--9979812.