Swift Observation Types and Target IDs

There are three fundamental observation timescales for Swift:

Snapshot: This is the time spent observing the same position without a break. Because of observing constraints, this is shorter than a single orbit of the Earth (which lasts ~96 minutes). The configuration of the instruments (e.g. mode and/or filter) can change during a snapshot.
Observation or Observation Segment: An observation is a cluster of snapshots of the same target. The length of an observation will vary, depending on the planning of the science timeline.
Monitoring Campaign: Some targets will be observed several times, depending on the evolution of the flux, the sensitivity of the instruments and the science aims. The different observations of a single target make up a monitoring campaign. There is no maximum time set for the number of observations of a target, or the duration of the monitoring campaign, but it expected that GRB targets will generally be monitored for about a month.

A sample monitoring campaign

Objects will be assigned both a (unique) 24-bit Target ID number and an 8-bit number for the Observation Segment. The allocation of Target IDs and Observation Segments is summarised in the table below (though users should be aware that the convention is subject to change).

0	Reserved for spacecraft: safehold or spacecraft fails to assign a number.
1-6	Used for safe pointings.
7-999	Reserved for spacecraft; currently not planned to be used.
1 000-9 999	Unassigned.
10 000-19 999	Reserved for BAT catalogue (known sources).
20 000-29 999	GRBs from other missions, or discovered from ground analysis of BAT data.
30 000-39 999	Non-GRB observations (TOOs and other targets) uploaded from the ground.
40 000-49 999	This is used for 1) non-GRB observations (TOOs and other targets) uploaded from the ground, and 2) Reassigned Target IDs for safe pointings. The reassignment occurs on the ground to remove the degeneracy of the several sky positions used for safe pointing for which only 6 Target IDs are available and assigned onboard.
50 000-59 999	Calibration observations performed during the mission.
60 000-69 999	Non-science observation.
70 000-79 999	Pre-launch calibration.
80 000-89 999	Unassigned.
90 000-99 999	Guest Investigator observations.
100 000-16 777 214	BAT triggers. Target IDs are assigned to all triggers, even if they are judged onboard not to be good GRBs.
16 777 215 (=$FFFFFF)	Reserved to indicate that the reporting subsystem does not know the correct Target ID.

Every observation is tagged with an Observation Number, which is the 32-bit concatenation of the Target ID and Observation Segment discussed above. There is also the Sequence Number, which is the decimal representation of the 32-bit Observation Number, in the form of an 11 digit value; the first 8 digits are for the Target ID and the remaining 3 for the Observation Segment. It is these 11 digits, the Observation ID, which are used for the naming of the data directories, possibly with a processing version appended: i.e. sw[obsid].[ver] (from the US site; or [obsid] from the UK site). Using the example in the diagram below, the Target ID is 100001 and the observation segment is 2, since it is the second observation of the target in question. Thus, the 11 digit sequence number is 00100001002.

Target ID, segment, observation and sequence numbers have been marked on this sample monitoring campaign.

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Swift Observation Types and Target IDs