The problem seems to be in the likelihood (i.e., observed_data), not in the sigma. The initial value of sigma is e^{-7.36189677}=0.00063, which is small but positive and has a prior logp of e^{-.77}=0.46301306831.
The problem seems to be in the likelihood (i.e., observed_data), not in the sigma. The initial value of sigma is e^{-7.36189677}=0.00063, which is small but positive and has a prior logp of e^{-.77}=0.46301306831.