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 trigger an autonomous slew followed by a programmed sequence of observations with the narrow-field, focusing telescopes. The slew time for the Swift baseline is less than approximately 90 s. XRT and UVOT observations begin while the burst is still in progress for approximately 30% of the bursts.
Time = time since Gamma-Ray Burst, for both columns in this table.
|Time||Spacecraft Event||Ground Event|
|0 s||BAT detects GRB|
|10 s||Slew begins|
|20 s||BAT location distributed via GCN|
|90 s||XRT and UVOT acquire GRB|
|95 s||XRT image obtained|
|BAT lightcurve distributed via GCN|
|120 s||XRT location distributed via GCN|
|250 s||UVOT finding chart obtained|
|300 s||Optical finding chart distributed via GCN|
|1200 s||XRT spectrum obtained|
|1212 s||XRT spectrum distributed via GCN|
|7200 s||UVOT pre-planned filter sequence complete|
|∼104 s||Ground station pass||Burst data downloaded|
|New observing program uploaded|
The initial GRB position is determined by the BAT, but 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 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 an automatic quality checking (<1 sec), the message is routed to the GCN for delivery to the community. This takes approximately 20 s for BAT positions and approximately 120 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 as well as other science. This plan is uploaded daily on business days. The MOC is able to generate and upload a new schedule within approximately four hours if the need arises.