Trend of Dark Rates of the COS and STIS NUV MAMA Detectors

Abstract. The dark rate of the STIS NUV MAMA detector was about an orderof magnitude higher after SM4 repair than anticipated, with an initial rate of 0.01count sec −1 pixel −1 .Measurementsoverthepastyearshowadual-componentex-ponential decline with e-folding timescales of≈ 30 and 300 days. The most recentmeasurements show a rate of 2.8 × 10 −3 count sec −1 pixel −1 .ThedarkrateoftheCOS NUV detector started at a very low value of 6×10 −5 count sec −1 pixel −1 ,andhas displayed a steady increase, approaching the ground-tested level of 3.7 × 10 −4 count sec −1 pixel −1 .Still,therateofCOSNUVdetectorisconsiderablylowerthanthat of its STIS counterpart. The rates for both detectors aresensitivetodetectorand tube temperatures, and the rate fluctuations can be fit withanempiricalmodel.1. IntroductionDuring the Hubble servicing mission in 2009, the new COS instrument was installed and theSTIS revived. As part of the Cycle 17 calibration programs, wehavecollecteddarkimagesregularly and have monitored the dark rates of the FUV and NUVdetectors. In general, thedark rates of the FUV detectors, both COS and STIS, are steadyand consistent with pre-SM4 estimates. The NUV detectors, however, display significant changes. We model themeasurements of dark rates with existing parameters that were derived for STIS (Proffitt,et al. 2010) over its many year operation. Note that the COS NUVdetectorisasparedevice for its STIS counterpart, considered as nearly identical. However, their dark ratesfollow opposite trends.2. ObservationsThe STIS dark images were obtained in ACCUM mode. Since all thedatapointsareused, the dark rates include contributions from cosmic raysand other effects. The COSdark images, on the other hand, were obtained in TIME-TAG mode, thus offering timeresolution. During the SAA (South Atlantic Anomaly) passages, the dark rate becomesconsiderably higher than average. We used the time variationofdarkratestoidentifyand exclude data during SAA passages. When the average dark rate rises to a value morethan twice the normal dark rate or declines from such a high level, a flag was set for SAA.Further iterations over rates at di fferent intervals yieldedanimprovedidentificationoftheSAA passage, and the data during that period were excluded from our study.