A person who is an authorized user on a credit card usually has the right to make charges, but is not a co-holder of the account - so it is not immediately clear that you would be responsible for paying any portion of the account balance, unless you contracted to do so when the card was issued to you. Of course, if this was meant to build your credit you may well have agreed to bear responsibility - you can't build your credit simply by having a credit card off of somebody else's account, for which you bear no further responsibility.

If in fact you were joint account holders, you probably are jointly responsible for the debt - and with his responsibility discharged by bankruptcy the creditors would typically come after you as a matter of course.

Contact the original creditors, and ask that they send proof of your obligation on the account. (You may wish to look at this article on the Fair Debt Collection Practices Act.)