### gaia data release 3 documentation

Table containing all Gaia Photometric Science Alerts triggered in the period 25-07-2014 to 28-05-2017, i.e. the time span covered by DR3.
A Gaia Photometric Science Alert is known to the community by its name Gaiannxyz where nn is the year number and xyz is an incrementing, alphabetic sequence starting at aaa. Science alerts have their details published on the WWW. Given the name of an alert, the URL for the published details can be derived, e.g. https://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia18ahj for alert Gaia18ahj.
The source_id associated to the alert may have other alternative matches in Gaia DR3. These other identifiers are listed in table alerts_mixedin_sourceids.

Columns description:

source_id : Unique source identifier (unique within a particular Data Release) (long)

A unique single numerical identifier of the source obtained from gaia_source (for a detailed description see gaia_source.source_id).

transit_id : Alerting transit identifier (long)

The transit_id is a unique identifier assigned to each detected (and confirmed) source as it transits the Gaia focal plane. Each time a given source is detected as Gaia scans and re-scans the sky a new transit_id will be created to badge that apparition. Hence the along–scan time and the across–scan position along with the telescope in which the source was detected are used to form a unique integer with which to label the transit.

The several features of a detection that are encoded in transit_id can be easily retrieved using bit masks (&) and shifts ($>>$) as follows:

• On-Board Mission Time line [ns]
= 204800 * ((transit_id $>>$ 17) & (0x000003FFFFFFFFFF))

• Field-of-view = 1 + (transit_id $>>$ 15) & 0x03 [1 for ‘preceding’ and 2 for ‘following’ fields-of-view respectively]

• CCD row = (transit_id $>>$ 12) & 0x07 [dimensionless, in the range 1 to 7]

• Across-scan ‘reference acquisition pixel’ in strip AF1 = (transit_id) & 0x0FFF [pixels] (this is the across-scan centre of the AF1 window and is odd if immediately below the mid-point of the window and even if immediately above)