Swift Observing Strategy
The Swift strategy is to slew to each new GRB position as soon as possible and to follow all the GRB afterglows as long as they are visible. To see the earliest phase of the afterglow, new BAT positions will trigger an autonomous slew followed by a programmed sequence of observations with the focusing telescopes. The slew time for the Swift baseline ranges from 20 to 70 s. Simulations show XRT and UVOT observations begin while the burst is still in progress for 30% of all bursts.Time = time since Gamma-Ray Burst, for both columns, below.
| Time | S/C Event | Time | Ground Event |
|---|---|---|---|
| 0 s | GRB | ||
| 10 s | Slew begins | 20 s | BAT location distributed |
| ~ 50 s | GRB acquired | ||
| 55 s | XRT image | 70 s | XRT location distributed |
| 150 s | UVOT finding chart | 240 s | Optical finding chart distributed |
| 1200 s | XRT Spectrum | 1210 s | XRT spectra distributed |
| 7200 s | UVOT filters complete | ||
| ~ 104 s | Ground station pass | All burst data, new observing program uploaded |
The initial GRB position is normally determined by the BAT. Positions uploaded from other satellites can also trigger a slew. The spacecraft software plans and executes the autonomous slew. A separate processor (the Observing Priority Processor) will calculate a Figure of Merit (FoM) for each new burst based on information from the instruments and the spacecraft and determine when to slew to a new position. This 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.
Each of the three Swift instruments rapidly produces alert messages after a GRB is detected. To ensure prompt delivery, these messages are sent through the TDRSS on-demand system to the Mission Operations Center (MOC) (which can also be used to uplink an external burst trigger). After automatic quality checking (< 1 sec), the message is routed to the GCN for delivery to the community. Based on the similar process already in place for BACODINE on CGRO, we estimate the entire delivery process will take 12 s for BAT positions and ~ 60 s for XRT positions.
When Swift is not engaged in prompt observations of the most recent bursts, it will follow a schedule uploaded from the ground. This schedule will provide for long term follow-up of GRB afterglows and other science. It will be a 1 week plan, that will be uploaded each working day and as needed. The MOC will be able to generate a new schedule in < 2 hours, a capability already demonstrated by RXTE.
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