X-ray spectral diagnostics of the immediate environment of GRB 991216 D. R. Ballantyne, E. Ramirez-Ruiz, D. Lazzati and L. Piro A&A, 389 3 (2002) L74-L77 Published online: 01 July 2002 DOI: 10.1051/0004-6361:20020852