The Figure of Merit

The Figure of Merit is the on-board instrument flight software module that is used to coordinate the observations of pre-planned and autonomous targets. When Swift is not observing a transient source it will perform pre-planned observations. During this time the BAT continually processes event data in order to detect transient sources. When a transient is detected the BAT determines its location and sends a notification to the MOC via the TDRSS link. Then the FoM evaluates the merit of this transient against the current observation to determine if the current observation should be interrupted in favor of observing the transient source. When the FoM decides that it is appropriate to slew to the new burst the FoM will request that the spacecraft slew to the new position and begin observations.

The design of the FoM is determined by the top-level science requirements of the Swift mission and is designed to maximize the amount of science data returned. The top-level FoM science requirements are listed below.

• Minimize the time delay to slew to a new burst (the design goal is less than 0.4 seconds for FoM processing).

• Not significantly degrade the observations of GRB afterglows by Swift. The predicted number of XRT afterglow photons shall be greater than 80% of the total predicted for an optimally efficient system.

• Multi-orbit automated observations of new BAT source locations to maximize the XRT and UVOT observing efficiency during the critical early phase of the afterglow.

• Maximize observing time for the uploaded program whenever an target is not being observed.

• Automated termination of automated observing of a target at completion of a pre-determined observing strategy.

• Pre-specified uploaded observations to override automated observations.

• Automated observations triggered by TDRSS commands.

• Allow the current automated target to override new GRBs based on input from the instruments.

• Allow the extension of automated observing time based on input from the XRT and UVOT.

• Allow for adjustment of the fraction of observing time allocated to the automated and uploaded programs for extended automated observations.

• Allow for termination of automated observing by command from the ground.

• Allow for a workable mechanism to implement new automated observations after launch.

• The capability to disable slew requests to the spacecraft by command.

The FoM software is provided by the science team and primarily affects the observing efficiency. The baseline observing strategy produced acceptable performance in the simulations, but we expect this strategy to be adjusted as we learn more about GRBs. The FoM can also accommodate more focused studies of specific GRB questions.