Normal and unusual transient events in IRAC images

Abstract

The Spitzer Space Telescope Infrared Array Camera (IRAC) is a four-channel camera that uses two pairs of 256 x 256 pixel InSb and Si:As IBC detectors to provide simultaneous images at 3.6, 4.5, 5.8, and 8 microns. IRAC experiences a flux of cosmic rays that produce transient events in images from each of the arrays, with 5-7 pixels per second being affected in an IRAC integration. The vast majority of these transient events can be adequately characterized so they can be effectively detected and flagged by a pipeline software module. However, because of the nature of the arrays and their arrangement in the camera structure, a small fraction of the cosmic ray hits on IRAC produce transients with unusual morphologies which cannot be characterized in a general way. We present nominal cosmic ray rates observed for IRAC on-orbit and rates observed during a period of elevated solar proton flux following a series of X-class solar flares in late 2003. We also present a guide for observers to help identify unusual transient events in their data. We comment on the physical nature of the production of many o9f these unusual transients and how this mechanism differs from the production of "normal" transient events.