Is possible compute the bandwidth of a Narrowband Twin-T Passive Notch Filter?
Is possible compute the bandwidth of a Narrowband Twin-T Passive Notch
  Filter?





The above is the generalized transfer function for a twin T notch filter taken from here (which also provides a useful calculator. If you want the 3 dB frequencies you can rejig the formula and make it simpler because in your equations the resistors are either R or 2R. Ditto the capacitors.

Then solve for the 3 dB points and compute the bandwidth of the notch.

The picture of the response looks like this (R=1 kohm, C = 1nF): -



And not like what you have shown in your question.