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 \pm 1.9 \times 10 ^ { -4 } at –70 km s ^ { -1 } , and 8.5 \pm 2.0 \times 10 ^ { -4 } at –40 km s ^ { -1 } , with corresponding spin temperatures of 6000 \pm 1700 K and 4800 \pm 1600 K , respectively .
The volume filling factor for the WNM appears to be fairly high ( f \approx 0.4 ) .

Subject headings : ISM : structure and atoms – Galaxy : fundamental parameters – Radio Lines : ISM