We present 21-cm absorption measurements towards 12 radio continuum sources with previously identified thermally-unstable warm neutral medium ( WNM ) .
These observations were obtained with the Expanded Very Large Array ( EVLA ) and were complemented with the HI emission spectra obtained with the Arecibo Observatory .
Out of 12 sources , HI absorption was detected along 5 lines of sight ( seven new absorption features in total ) , resulting in a detection rate of \sim 42 % .
While our observations are sensitive to the WNM with a spin temperature T _ { s } < 3000 K , we detected only two wide absorption lines with T _ { s } = 400 - 900 K. These temperatures lie above the range allowed for the cold neutral medium ( CNM ) by the thermal equilbrium models and signify the thermally unstable WNM .
Several absorption features have an optical depth of only a few \times 10 ^ { -3 } .
While this is close or lower than what is theoretically expected for the CNM , we show that these weak lines are important for constraining the fraction of the thermally unstable WNM .
Our observations demonstrate that , for the first time , high bandpass stability can be achieved with the VLA , allowing detection of absorption lines with a peak optical depth of \sim 10 ^ { -3 } .