One of the very good things about Dynamics AX 4.0: there is no axdat.udb file anymore! All information about users online etc... is kept in the database.
The table that holds the users' data is called SYSCLIENTSESSIONS.
The table that holds the AOS' data is called SYSSERVERSESSIONS.
It seems to me that SysClientSessions will contain the sessions that had been logged in on the current day (no matter if they are still logged in or not). Currently logged in sessions will have the field STATUS set to 1.