Refining Chandra/ACIS Subpixel Event Repositioning Using a Backside-illuminated CCD Model

Subpixel event repositioning (SER) techniques have been demonstrated to significantly improve the already unprecedented spatial resolution of Chandra X-ray imaging with the Advanced CCD Imaging Spectrometer (ACIS). Chandra CCD SER techniques are based on the premise that the impact position of events can be refined, based on the distribution of charge among affected CCD pixels. ACIS SER models proposed thus far are restricted to corner split (3 and 4 pixel) events and assume that such events take place at the split-pixel corners. To improve the event-counting statistics, we modified the ACIS SER algorithms to include 2 pixel split events and single-pixel events, using refined estimates for photon impact locations. Furthermore, simulations that make use of a high-fidelity backside-illuminated (BI) CCD model demonstrate that mean photon impact positions for split events are energy dependent, leading to further modification of subpixel event locations according to event type and energy, for BI ACIS devices. Testing on Chandra CCD X-ray observations of the Orion Nebula Cluster indicates that these modified SER algorithms further improve the spatial resolution of Chandra/ACIS, to the extent that the spreading in the spatial distribution of photons is dominated by the high-resolution mirror assembly, rather than by ACIS pixelization.