what do you mean by "online"?
Logged on to your Teradata system?
dbc.LogonoffV keeps the history of sessions, there's a row with LogonDate/LogonTime for Event = 'Logon' and another for Event = 'Logoff' with LogDate/LogTime plus the LogonDate/LogonTime.
You can try
Eg. active sessions today between 9:00:00 and 9:30:00
SELECT username, sessionno,
MAX(CASE WHEN event ='logon' THEN logtime ELSE NULL END) AS time_start,
MAX(CASE WHEN event ='logoff' THEN logtime ELSE NULL END) AS time_end
WHERE logdate = CURRENT_DATE
HAVING time_start <= 093000
AND time_end >= 090000
GROUP BY 1,2,3