This week we look again, at one of the Gaia alerts that occurred in one of our fields. This one, Gaia22bfj, was published on April 1st (no joke) in the NGC2264 target field, the Christmas Tree Cluster. The object is a known young star, listed under the name ESO-HA 393, i.e. as been identified as an emission line star. The object is quite faint, but we have some Halpha images, that support this classification. The Halpha brightness varies between one and two magnitudes above the R-band brightness, thus there is a clear excess, i.e. a strong Halpha emission line.
The source has been flagged up by Gaia due to an about 1.5mag dimming event in the latest set of data points. Looking at the Gaia data one finds that there have been other short dimming events, about 1mag deep. Generally the source varies by about half a magnitude around its average.
A detailed look at our data displayed above, confirms this behaviour. Over the first six years of HOYS data one can see a slight (0.1mag) increase in the average brightness. The last year of data clearly shows an on average half magnitude drop compared to the previous year. This is most likely in part the reason the object got flagged up, as the shorter dimming events are no deeper than the ones in previous years. It will be interesting to see if this is the start of a longer term dimming event or if the object returns to the brightness it had before.