Ask for CaC once across subdomains

Hi, I was hoping to get some help on something that has been bothering me for a while. I have 4 subdomains - site1.mydomain.com, site2.mydomain.com, site3.mydomain.com, site4.mydomain.com

Each of the subdomains have their own access policy and they are all cac enforced. When a person comes to one of the subdomains they are asked to present their cac., so a user at site3 will present his cac and then when he goes to site2 he is asked to present his cac credentials again.  Is there a way I could make it that the user is only asked for his cac token once across the same domain when he gets his first initial login; basically carry his authorization across individual subdomains?  I only need the user to present himself once after the initial login and then after that the user should be able to have carte blanche access.

Any help is greatly appreciated, thanks!