2014MNRAS.441.2691Q

We searched the light curves of over 40 000 stars in the Two Micron All Sky Survey (2MASS) calibration data base, spanning approximately 4yr, for objects that have significant day-long dimming events. We also searched the multi-colour light curves for red-dimming events that could be due to transient extinction. In the colour-independent sigma-limited search, we found 46 previously unrecognized eclipsing binaries, 6 previously unrecognized periodic variable stars likely to be intrinsic pulsators and 21 young stellar objects in the ρ Ophiuchus star formation region previously studied by Parks et al. An additional 11 objects exhibited dimming events, and most of these are unclassified. The search for red-dimming events primarily reveals a population of low-luminosity active galaxies that become bluer when they are brighter, and variable young stellar objects exhibiting high cross-correlation coefficients between colour and brightness. The young stellar objects primarily exhibit brightness and colour variations in the direction of interstellar extinction whereas the active galaxies can have a bowed distribution in colour and magnitude with reduced variation in colour when the object is brightest. Among the objects that are usually quiescent (not strongly variable), we failed to find any dimming events deeper than 0.2 mag and lasting longer than a day. Two of the young stellar objects, however, dimmed by 0.2mag for longer than a day without strong colour variation.