What type of backups are kept on site at the computer center for immediate recovery purposes?

They are known as working copy backups

Working copy backups also sometimes known as shadow copies are full or partial backups kept at the computer center for immediate recovery purposes. This type of backup uses Journaled Filing Systems (JFS) that consist of a log file of all transactions and changes that have happened within a set period of time. If a crash happens, the OS can refer to the log files to see the transactions.