Snowballs Artifact

See also: Understanding Exposure Times

A data artifact known as a "snowball" is caused by a large cosmic ray impact in one of the near IR detectors. 

While the vast majority of cosmic ray impacts directly affect only one or 2 pixels, there are less common events that affect hundreds of pixels. These large events are colloquially termed snowballs. A very rough estimate of the rate is around one snowball every 20 seconds in each 2Kx2K near infrared detector. While a similar event was seen in the near infrared detectors on the ground, the frequency was an order of magnitude lower and the amount of charge deposited was also significantly smaller. The primary similarity is that they are large radiation events that affect a circular set of pixels.

There are also large radiation events that affect the MIRI detectors. Because they do not have a generally circular morphology they tend to not be termed snowballs but are called "shower" events. The rate of occurrence of the MIRI shower events is at least a factor of two lower than the rate of snowballs in the near infrared detectors. At this time we are treating both types of events similarly. 

The current Webb data reduction pipeline handles the first order effects from cosmic ray events but the large number of electrons that result from a snowball event have secondary effects that are not currently corrected in the pipeline.

Figure 1 shows a closeup of a slope image (see Stage 1 Detector Processing) centered on the location of a very large snowball. This slope image is from a long NIRSpec dark exposure and thus is very sensitive to any perturbation in the calculated slope. Thus, this is a worst case example; images with significant background rates will see much smaller effects.

Figure 1. Cutout of a slope image centered on the location of a very large snowball

Residual effects of a snowball

Alternating light and dark regions are caused by various effects.  See text for details.
Since dark exposures  accumulate counts in a highly linear fashion, they provide a sensitive probe of deviations caused by the event.  Four regions can be identified. 

  • The central region inside the white circle indicates pixels that saturated when the particle hit the detector.
  • The white ring of pixels just outside of the saturated pixels represents pixels that saw a discontinuity in the count rate when the event occurred but did not saturate. The discontinuity was flagged as a JUMP by the pipeline and was not included in the slope calculation. They show the excess slope due to charge spilling from the saturated region in subsequent groups of the integration.
  • The large darker region outside the white ring is caused by pixels that were flagged as a jump when the snowball occurred but then had below average count rates because photo-electrons were temporarily trapped by small defects in the detector pixels. 
  • Finally, the outer white halo represent pixels that saw an increase in the accumulated charge when the snowball event occurred but were uncorrected  because the increase was below the pipeline's detection threshold.  

Although the halo regions are clearly indicated in dark exposures, they could be masked in images with significant astrophysical or other backgrounds.

Figure 2. An initial look at snowballs sizes

Size Distribution of Snowballs in NIRCAM

This histogram provides a first look at the size distribution of typical snowball events from data obtained in-flight for the NIR detectors.
Currently, the best mitigation strategy for snowballs is to dither exposures with as many dither steps as possible. A four-point or larger dither will allow the pipeline outlier detection routine to significantly improve the final combined image. 

Work is in progress to add a new algorithm to the calibration pipeline for flagging the regions affected by snowballs.

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Originally published