Ambiguity in defaultable term structure models

We introduce the concept of no-arbitrage in a credit risk market under ambiguity considering an intensity-based framework. We assume the default intensity is not exactly known but lies between an upper and lower bound. By means of the Girsanov theorem, we start from the reference measure where the intensity is equal to $1$ and construct the set of equivalent martingale measures. From this viewpoint, the credit risky case turns out to be similar to the case of drift uncertainty in the $G$-expectation framework. Finally, we derive the interval of no-arbitrage prices for general bond prices in a Markovian setting.