5.7.4 Detailed Notes

In the list of alerts prepared for Gaia DR3, two transient events are duplicated, and are retained in the table to match the events published on the science alerts website (they are also discussed in Hodgkin et al. (2021)). The duplicated events are: Gaia16ade$\equiv$Gaia16aey, and Gaia16acr$\equiv$Gaia16adx. This arose when they were assigned different sourceIds by IDT and hence were treated as different events within GSA. Since this occurred, we have implemented traps in our system (based on positional crossmatch) to avoid this kind of duplication. Ideally, we would have assigned the second sourceId (and its transits) as a mixed-in source to the first one. But once published, we decided to leave things as they stood.

There are a total of 130 alerts published by our system (and so on the GSA website) which were not included in Gaia DR3.

123 out of the 130 are missing because they were detected right at the beginning of operations, during the verification phase. Our nominal transient detectors were completely overwhelmed by artefacts - and so we could not trust them until we had better filters. For this first phase, we started running a temporary detector in parallel, which grouped transits together positionally, and ignored sourceId. They have no primary sourceId associated with the alert, and thus could not be included in the data release.

The remaining 7 alerts have no entry for their primary (alerting) transitId in the match table generated by IDU (i.e. these transits have been discarded in Gaia DR3). Thus they are not included here.

There are two main routes for an alert to be classified: (i) via a spectroscopic classification which is shared via an ATEL, TNS, or through some other survey included in our local archive (see Section 5.7.2 and Figure 5.53), and (ii) from the team in Warsaw who are classifying microlensing events from photometric lightcurve modelling, combined with spectroscopic follow-up (see Wyrzykowski et al. (2020)). Note that the GSA microlensing events included in Gaia DR3 are independent from those described in Section 10.9.

We have not added the classifications to the data tables included in Gaia DR3, because they are dynamic and never complete. Instead we encourage interested users to extract the information from https://gsaweb.ast.cam.ac.uk/alerts. For completeness we give the full list (45 classes) in Table 5.11. The default class for all alerts is unknown, until otherwise determined.