Targets of Opportunity (TOOs) are astronomical events observable by XMM-Newton, which cannot be predicted and scheduled on the time scale of a year, yet are scientifically sufficiently important to justify interrupting the overall XMM-Newton programme. Because these events often need rapid reaction times, TOO can only be observed on a best effort basis. The overall responsibility lies with the Project Scientist, who may coordinate his decision with a chairperson of the OTAC. The Project Scientist makes the decision for an interruption of the programme.
The XMM-Newton project recognizes two kinds of TOOs: anticipated and unanticipated TOOs, that are described in the following sections: