RefiningChandra/ACIS Subpixel Event Repositioning Using a Backside‐illuminated CCD Model

Subpixel event repositioning (SER) techniques have been demonstrated tosignificantly improve the already unprecedented spatial resolution of ChandraX-ray imaging with the Advanced CCD Imaging Spectrometer (ACIS). Chandra CCDSER techniques are based on the premise that the impact position of events canbe refined, based on the distribution of charge among affected CCD pixels. ACISSER 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. Toimprove the event counting statistics, we modified the ACIS SER algorithms toinclude 2-pixel split events and single pixel events, using refined estimatesfor photon impact locations. Furthermore, simulations that make use of ahigh-fidelity backside illuminated (BI) CCD model demonstrate that mean photonimpact positions for split events are energy dependent leading to furthermodification of subpixel event locations according to event type and energy,for BI ACIS devices. Testing on Chandra CCD X-ray observations of the OrionNebula Cluster indicates that these modified SER algorithms further improve thespatial resolution of Chandra/ACIS, to the extent that the spreading in thespatial distribution of photons is dominated by the High Resolution MirrorAssembly, rather than by ACIS pixelization.Comment: 23 pages, 8 figures, 2nd version, submitted to Ap