This week we keep with ‘tradition’ and look at one of the recently (yesterday, October 18) announced Gaia photometric alerts in one of our target fields. The source in question is Gaia22efx, located in the Rosette Nebula, NGC 2244. The source as been identified due to brightening in a known YSO. Indeed the object is located in the Rosette nebula, based on it’s parallax of 1.54mas, i.e. a distance of about 650pc.

The Gaia light curve shows that even though the object has gotten brighter over the last 5yr, it has only been flagged up now. Indeed it has been at the brightness it is now (within one percent or so), for the entire last year. In the figure above we show the R and I-band data of the source from HOYS. As you can see, there is a group of data points for each winter, when the source is observable.

Similar to the long term Gaia data, we also see the same trend of increasing brightness over the last five years. The data look quite noisy. However, note that the total rise in brightness is only 0.1mag, i.e. ten percent. Thus, the uncertainties in the data, which are typically a few percent, do look quite prominent. As always: The data plotted on the website, has not yet gone through the final photometry calibration, which removes some of the scatter due to colour terms caused by slightly varying filters.

Since the change in brightness is quite small, it is difficult to judge what causes the variation. Usually the change in colour can be used to estimate the cause of the variability. Due to the small changes, the colour change will be very small, and thus most likely governed by the noise in the data. However, these kind of variations are either caused by small long term changes in the mass accretion rate or the line of sight extinction. If I had to bet, my money would be on the latter. Most likely what we are seeing is the orbital motion (into / out of the line of sight to the central star) of material far out in the accretion disk.