Convert normalized frequency to real frequency in AR model
Matlab generally uses normalized frequencies in radians/sample. If the signal is real-valued then these frequencies are in the interval $[0,\pi]$ (as you can see in the plot). The Matlab frequency variable $w$ is normalized as

$$w=\frac{2\pi f}{f_s}$$

where $f_s$ is the sampling frequency. So $w=\pi$ corresponds to $f=f_s/2$.