Frequently the data in our survey can be used to follow-up newly variable stars identified in data from the Gaia satellite – the so-called Gaia alerts. These objects are flagged up as they suddenly change their behaviour. This usually means they undergo a sudden increase or decrease in brightness. Our data does of course also allow to study the behaviour of the object in detail prior the discovery as variable. This week, we look at a newly announced Gaia alert in one of our fields.

The source we look at is Gaia21euy, which is situated in the Christmas Tree Cluster (NGC2264). It was identified as undergoing a sudden drop of 1mag in brightness six days ago. Looking at the Gaia light curve, one can see that the star usually varies by about 0.1mag (10%), and that it had a short 0.4mag drop in April 2019 – which did not trigger an alert.

Our light curve of the source shows that there are many more of these dips. We show a zoom into the V, R, and I-band HOYS data from last winter. One finds at least two of those about 1mag deep dips, which typically do not last more than one to a few days. This is the main reason that there are only two visible in the Gaia data. Gaia only observes each target roughly once per month, and hence tends to miss these short term events.