EPOXI: HRI-IR GARRADD DATA ARTIFACT NOTE                                      
                                                                              
This document details an artifact found in the HRI-IR spectral data of        
comet C/2009 P1 Garradd.                                                      
                                                                              
During the initial analyses of the comet Garradd data (Feaga et al. 2014),    
the EPOXI Science Team detected a strong emission feature between 2.75        
and 2.85 microns, peaking at 2.8 microns, longward of the typical water       
band detected at 2.67 microns. This feature was not seen in other DIF         
HRI-IR cometary flyby data. The feature had a distinct spatial profile        
that is different from both the water and carbon dioxide distribution for     
comet Garradd.                                                                
                                                                              
The feature is not present above the noise in the scans acquired on the       
first day of HRI-IR observations of Garradd (DOY 86), but is present in       
a subset of scans acquired on the second day of observing (DOY 93). The       
data were examined more closely in smaller temporal chunks to isolate         
the introduction of an artifact into the data. The artifact is strong         
between scans with observation ID 4000033 through 4000063, acquired on        
2 April 2012 from 12:18 through 16:06. No obvious bad pixels were found       
in the individual frames and the location of the comet moved by 15 pixels     
along the slit during the set of scans. As such, a single bad pixel or        
group of pixels should not dominate the spatially coaligned and resistant-    
meaned data presented in Feaga et al. (2014) where the feature is             
clearly seen. No cosmic ray hits were noted in the scans near the comet's     
position along the slit on DOY 93 either, but it is possible that a           
damaging cosmic ray hit the detector between scans on 2012 April 2 and        
created an artifact at 2.8 microns.                                           
                                                                              
An artifact like this, stable excess noise lasting for several hours has      
been seen in data from other phases of the mission, specifically during       
EPOCh. Noise caused by defects in the vicinity of p-n junctions in HgCdTe     
detector HAWAII-1R multiplexers, like the sensor on the HRI-IR, is well       
documented (Bacon et al. 2005). This noise can manifest as a region of        
neighboring pixels measuring a steady signal and is the most likely cause     
of the 2.8 micron feature.                                                    
                                                                              
Bacon, C. M., et al. 2005. Burst noise in the HAWAII-1RG multiplexer.         
Proceedings of the SPIE, 5902, 116.                                           
10.1117/12.624191                                                             
                                                                              
Feaga, L. M., et al. 2014. Uncorrelated volatile behavior during the 2011     
apparition of comet C/2009 P1 Garradd. The Astronomical Journal, 147, 24.     
doi:10.1088/0004-6256/147/1/24