Arbitrage-Free Pricing of Credit Index Options: The No-Armageddon Pricing Measure and the Role of Correlation after the Subprime Crisis

Abstract

In this work we consider three problems of the standard market approach to the pricing of credit index options: the definition of the index spread is not valid in general, the payoff considered leads to a pricing which is not always defined, and the candidate numeraire to define a pricing measure is not strictly positive, which would lead to an inequivalent pricing measure. We give to the three problems a general mathematical solution, based on a novel way of modelling the flow of information through the definition of a new subfiltration. Using this subfiltration, we take into account consistently the possibility of default of all names in the portfolio, that is neglected in the standard market approach. We show that, while this mispricing can be negligible for standard options in normal market conditions, it can become highly relevant for different options or in stressed market conditions. In particular, we show on 2007 market data that after the subprime credit crisis the mispricing of the market formula compared to the no arbitrage formula we propose has become financially relevant even for the liquid Crossover Index Options.