ISO ammonia line absorption reveals a layer of hot gas veiling Sgr B2 C. Ceccarelli, J.-P. Baluteau, M. Walmsley, B. M. Swinyard, E. Caux, S. D. Sidher, P. Cox, C. Gry, M. Kessler and T. Prusti A&A, 383 2 (2002) 603-613 Published online: 15 February 2002 DOI: 10.1051/0004-6361:20011765