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Observation definition

Observation definition

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.

Figure 2.1: GRB Monitoring

Target Id, Observation segment and Sequence number

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
1000-9999 Unassigned
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
80000-89999 Unassigned
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.

Figure 2.2: Sequence number assigment when a trigger is followed by a slew

Figure 2.3: Sequence number assigment when a trigger is not followed by a slew
Therefore, in the no-slew case, the observation will only contain data from the BAT, with none from the NFI.

next up previous contents
Next: Archive structure Up: Swift Archive Guide Previous: Archive   Contents
Lorella Angelini 2007-04-03