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
- Historic mag.
- Mean, historic magnitude of the alerting source, in Gaia's G band before
- 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
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. 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:
- A small fraction of Gaia data do not get downlinked to the ground,
especially during high source-density scans, and especially for faint
- Also, infrequently, there can be issues in the daily processing
which may result in the occasional photometric measurement being
delayed, or omitted.
- 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.
- 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.