Detection of H I 21 centimeter absorption by the warm neutral medium

We have detected H I 21 cm 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) x 10(-4) at -70 km s(-1) and (8.5 +/- 2.0) x 10(-4) at -40 km s(-1), with corresponding spin temperatures of 6000 +/- 1700 and 4800 +/- 1600 K, respectively. The volume filling factor for the WNM appears to be fairly high (f approximate to 0.4).