Gaia Photometric Alerts

Meaning of columns:

Unique name of the alert. Please use this name when referring to the Gaia data of the alert in publications.
IAU Transient Name Server identifier of the Gaia alert.
Time of observation of the event that triggered the alert, in TCB.
Time of publication, in UTC.
RA (deg.)
Right ascension of the alerting source, in degrees, in the IRCS frame.
Dec. (deg.)
Declination of the alerting source, in degrees, in the IRCS frame.
Magnitude, in Gaia's G band of the alerting source at the time of the alert.
Historic mag.
Mean, historic magnitude of the alerting source, in Gaia's G band before the alert.
Historic scatter
Observed variation of magnitude (standard deviation of measurements) of the alerting source, in Gaia's G band.
Type of transient event.
Checked if the alert has any RVS (Radio Velocity Spectrometer) spectrum available.

The time of the triggering event is the instant at which Gaia detected a significant change from a constant magnitude, and that depends more on Gaia's scanning law than on astrophysical events in the source. Notably, for eruptive events, the time of peak brightness may be either after or before the triggering time of the alert.

Time of observation is in barycentric coordinate time (TCB) rather than in UTC. Time of publication is in UTC.

The sky position may either refer to a source in Gaia's own catalogue, or to a source in an external catalogue (e.g. SDSS) used as a reference for combining Gaia observations. Where the position comes from Gaia's catalogue, it is derived from a single, Gaia observation at the triggering point of the alert; this is not an astrometric measurement to the full precision of the Gaia main mission.

Magnitudes are in Gaia's "G" band. These are unfiltered, white-light observations in which the pass-band is defined by the instrument response. The magnitudes in the table are derived from a preliminary calibration of the photometry. The forthcoming photometric-catalogue from the Gaia mission will provide more accurate magnitudes based on a proper calibration.



For each Alert, on its webpage, we present the lightcurves in a figure, and also via a downloadable CSV file. Our published lightcurves use a timestamp extracted from the reference time encoded in the transitId, i.e. the time of observation in the first strip of detectors in the astrometric field (AF1, see e.g. Fig 4 in Gaia collaboration; Prusti et al. 2016). The TransitId is a numeric field which encodes a number of values: the reference pixel coordinates in AF1, the telescope, and the CCD row in which the object was observed. It uniquely identifies the transit of a source on the focal plane. We convert this AF1 timestamp to Barycentric Coordinate Time (TCB: hence the conversion includes the light travel time from the spacecraft to the Solar system barycentre).

The column headings are: Date(TCB), JD(TCB), average mag (i.e. mag averaged across all CCD strips). Note that it takes approximately 44 seconds for a source to cross 9 AF CCDs

In the lightcurve data, we have epochs where no numeric value is given for the magnitude. In this instance we report two distinct cases with a text label in the CSV file, and a different symbol in the figure.

Gaia is predicted to have scanned the sky at this location at the Barycentric time given (TCB). A null detection does not necessarily mean that no detection was made by Gaia. There are a number of reasons why we may report a null value in the lightcurve:
  1. A small fraction of Gaia data do not get downlinked to the ground, especially during high source-density scans, and especially for faint sources.
  2. Also, infrequently, there can be issues in the daily processing which may result in the occasional photometric measurement being delayed, or omitted.
  3. Additionally the predicted scanning times are calculated at lower spatial resolution than the Gaia spatial resolution, which can lead to edge cases with incorrect values for the predicted time.
  4. Finally, some individual Gaia observations are associated with the wrong source by the Initial Data treatment software. When we spot these cases, we try to repair the gaps in the lightcurve by combining the data of multiple sources.
In this case we do have a Gaia detection record, so we know that a source existed brighter than the Gaia detection limit G~20.7. However, the flux measurement derived is not reliable, and can not be used. Sometimes there are untrusted measurements for all sources within a window of time lasting some days, for example when the spacecraft was decontaminated.