"Snowballs" in the WFC3-IR Channel: Characterization

We have searched WFC3-IR channel data from two different epochs in order to identify and characterize “snowballs”. These snowballs are transient, extended sources with unknown origins that appear in FPA165 ground testing data at rates between 0.4 and 0.8 snowballs per hour of exposure time. A snowball affects between 11 and 34 pixels, and contains between 200,000 and 900,000 e-. With their behavior mimicing that of cosmic ray impacts, CALWF3 should be able to remove snowballs from WFC3-IR data. That, combined with snowballs’ low rate of occurrence, implies that snowballs should have a minimal impact on science observations.