The typical Swift observing strategy for a Gamma Ray Burst (GRB)/afterglow consists of a cluster of `snapshots' (defined below) which are aimed to follow the GRB and its afterglow evolution. Soon after an event is triggered by the BAT, the Figure of Merit (FoM) algorithm, part of the observatory software for autonomous operations, decides if it is worth requesting a slew maneuver in order to point the narrow field instruments (NFIs) on Swift (XRT and UVOT) in the direction of the trigger. The initial follow-up is automatically set at the spacecraft level, i.e., the FoM initiates the observation using a standard configuration of modes and filters for the NFIs. These types of observation are named Automatic Targets (ATs). Observations of GRBs can be also triggered by ground commands based on observations made by other satellites. Once the trigger has occurred (based either on Swift or other satellites), a monitoring schedule is planned and uploaded from the ground. The monitoring of the GRB/afterglow will typically be initiated as soon as possible after its detection and will last until the afterglow reaches a specified minimum flux or until a specified time interval has elapsed. It is also possible that the follow-up of a particular trigger may be abandoned by Swift because other targets are deemed to be more interesting.
The onboard software tags the data using two numbers.
The first number identifies the target which is being observed, hereafter
the Target ID, and this, therefore, is unique for each object. The second
number identifies the observation which forms part of the monitoring
campaign on a specific target, hereafter the Observation Segment.
The Target ID is a 24-bit number, and the Observation Segment is an 8-bit
number. The instruments will tag their science data in the telemetry
with a 32-bit number where the most significant 8-bits represent the
Observation Segment, and the least significant 24 bits are the Target ID.
This representation is hereafter named the Observation Number, and is
given as a HEX value. The Swift data are tagged Swift with an eleven
digit, Sequence Number, which is equivalent of the observation number
expressed in decimal format.
The Sequence Number is a concatenation of the Target ID
(the first 8 digits, in decimal format) and the Observation Segment (the
last 3 digits, in decimal format). The Sequence Number will be present
as a parameter in all database tables associated with Swift data and in
the as-flown timeline as well.
The number allocation for the target Id follows the scheme :
|0||Reserved for the spacecraft: safehold or spacecraft fails to assign a number|
|1-6||Used for safe pointings|
|10000-19999||Reserved for the BAT catalog (known sources)|
|20000-29999||GRBs from other missions or discovered from ground analysis of BAT data|
|30000-34999||Non-GRB observations, TOO and other targets uploaded from the ground.|
|Note this range is also used for TOO in the Guest Investigator program|
|35000-39999||Fill-in targets uploaded from the ground|
|40000-49999||Reassigned targets for safe pointing. The reassignment occurs on ground to remove|
|the degeneracy of the several sky positions used for safe pointing for which|
|only 6 target id are allocated on board|
|50000-59999||Calibration observation performed during the mission. The calibration targets|
|can be already included in other subgroups of target id|
|60000-64999||Non Science observation. Allocated to use by the SDC|
|65000-68999||Non Science observation. Allocated to use by the SOT In the PPT|
|69000-69999||Non Science observation. Allocated to use by the FOM during FOM activation.|
|70000-70999||Non Science observation. Allocated to use by the SOT In the PPT|
|71000-73999||Pre-launch calibration 'observation'|
|(Not relevant for flight data but reported for completness)|
|74000-79999||Non Science observation. Allocated to use by the SOT In the PPT|
|90000-99999||Target ID reserved for Guest Investigator observations|
|1000000-16777214||BAT trigger. Target ID is assigned to every BAT trigger even if they are judged|
|on board not to be good GRBs. These numbers are also used for triggers of new|
|non-GRB BAT transients, or if flux variations of a known source already in the BAT|
|catalog generate a trigger. Subsequent PPT observations on these sources will use|
|the same target ID.|
The segment number instead has the following values :
Using the above prescription, for a target ID of 100001 and an observation segment of 2 the sequence number associated to an observation is therefore
Typically, at any time all three instruments will tag their data with the same observation/sequence number. However, this may not be true around the time of a trigger and will depend on whether or not the spacecraft slews to observe the newly discovered target. While the BAT will always produce data tagged with the target ID associated to the newly discovered source, the NFI will tag the data with the target ID of the new discovered source only if the spacecraft slews to the new target. If there is no slew, the BAT will change its observation number to agree with that of the NFI after its has run the modes used when a new target is discovered. Figure 2 and 3 show how the sequence number are assigned to the data in the case of slew (Fig 2.2) or no-slew (Fig 2.3), respectively.