Is it possible to authenticate users across sub-domains when the authentication takes place at a sub-domain instead of the parent domain?

For example:

User logs into site1.parent.com, and then we need to send them to reporting.parent.com.

Can I authenticate them to the reporting site even though the log-in occured at a sub-domain?

So far all of the research I have done has users logging into the parent domain first and then each sub-domain has access to the authentication cookie.

c# asp.net forms-authentication
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