The Neil Gehrels Swift Observatory

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 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
XRT localization
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.