One of the key features of Swift is its ability to rapidly respond to the detection of a new transient source. As soon as a new, unknown transient is detected by the BAT the BAT will localize it to within 1-3 arcminutes. This position is distributed by the GCN within approximately 20 seconds of the initial detection. If the on-board software determines that follow-up observations should be made the spacecraft will automatically slew so that the transient is within the FoVs of the XRT and UVOT.
Swift's low orbit and pointing constraints prevent it from having a continuous viewing zone. To accommodate this Swift observing is broken into snapshots of approximately 20-30 minutes each. Figure 3 shows an example of the overall observing strategy.
The first observation segment will consist of an automated set of observations. The following observation segments are pre-planned and will be uploaded from the ground. Swift will observe each transient with all three of its telescopes and follow them until the automated observing sequence ends, another transient is detected, or a new observing sequence is uploaded from the ground.
Swift will slew to the location of an uncataloged transient source that is detected by the BAT as soon as possible after the initial detection. Automatic slewing to known sources is currently disabled (with a few exceptions for sources that are deemed to be unusually interesting). However, if a known source brightens above a certain threshold a message is sent to the ground and distributed through the Gamma-Ray Burst Coordinates Network (GCN). The slew time for Swift depends on the distance from the initial pointing to the final pointing, but the typical slew rate is approximately one degree per second. On-orbit experience has shown that XRT and UVOT observations with the narrow-field instruments begin during the prompt emission phase for some bursts. Table 2 shows an example sequence and approximate time scales for distributing alerts of transient sources detected by Swift. These times are approximate and depend on the details of Swift's initial pointing relative to the transient, the slew time, ground communications, and other factors. Table 2 is intended to provide an overview of a typical detection sequence, assuming a slew time of 80 seconds.
The initial GRB position is normally determined by the BAT. However, if a burst is not detected by the BAT--because it occurred in a part of the sky where the BAT is not looking, or it falls below BAT's detection threshold--then the Swift Observatory Duty Scientist can initiate at Target of Opportunity observation by uploading a position from another satellite to Swift. In either case (a BAT-detected position or an uploaded position the spacecraft software plans and executes an autonomous slew to the new target. A ``Figure of Merit'' (FoM) is calculated for each new burst based on information from the three instruments and the spacecraft. If the FoM is larger than that of the current observation then the current observation is interrupted and the spacecraft slews to the position of the new transient.
Each of the three Swift instruments (BAT, XRT, and UVOT) produces rapid alert messages after a GRB is detected (see Table 2). To ensure prompt delivery of transient detections to the ground these messages are sent through the Tracking and Data Relay Satellite System (TDRSS) to the Mission Operations Center (MOC) at the Pennsylvania State University (PSU). After automatic quality checks (which requires less than one second) the message is routed to the GCN for delivery to the astrophysics community. The entire delivery process takes approximately 20 seconds for the initial BAT positions (3 arcminute radius error circle) and approximately 70 seconds for XRT positions (5 arcsecond radius error circle).
When Swift is not engaged in prompt observations of GRBs it will follow a schedule uploaded from the ground. This schedule will provide for long-term follow-up observations of GRB afterglows, and for other science. A one- or two-day plan will be uploaded each working day, and at other times as needed. The MOC can generate a new schedule in less than two hours should the need arise.