We have detected HI 21cm line absorption by the Warm Neutral Medium (WNM) using the Westerbork Synthesis Radio Telescope. The absorption was detected toward Cygnus A at LSR velocities of -40 and -70 km s^{-1}. These two velocity ranges were previously identified as being relatively free of cold absorbing clouds. The measured optical depth for the WNM along the line of sight to Cygnus A is 8.9+/-1.9 x10^{-4} at -70 km s^{-1}, and 8.5+/-2.0 x10^{-4} at -40 km s^{-1}, with corresponding spin temperatures of 6000+/-1700 K and 4800+/-1600 K, respectively. The volume filling factor for the WNM appears to be fairly high (f = 0.4).
@article{arxiv.astro-ph/9805172,
title = {Detection of HI 21cm Absorption by the Warm Neutral Medium},
author = {C. L. Carilli and K. S. Dwarakanath and W. M. Goss},
journal= {arXiv preprint arXiv:astro-ph/9805172},
year = {2009}
}