The observed on-orbit background of the ACIS instrument on the Chandra X-ray Observatory

We have analyzed data acquired during the Orbital Activation and Checkout (OAC) and Guest Observer (GO) phases of the Chandra X-ray Observatory (CXO) mission in order to characterize the background of the Advanced CCD Imaging Spectrometer (ACIS) instrument produced by energetic particles. The ACIS instrument contains 8 Front-Illuminated (FI) CCDs and 2 Back-Illuminated (BI) CCDs. The Fl and BI CCDs exhibit dramatically different responses to enhancements in the particle flux. The FI CCDs show relatively little increase in the overall count rate, typical increases are 1 - 5 cts s(-1) above the quiescent level; the BI CCDs can show large excursions to as high as 100 cts s(-1) above the quiescent level. The durations of these intervals of enhanced background are also highly variable ranging from 500 s to 10(4) s. These periods of enhanced background are sometimes but not always associated with increased particle flux when Chandra is near the radiation belts. We see evidence for a weak correlation with the low-energy electron channel of the. Electron, Proton, Helium Instrument particle detector (EPHIN) instrument on-board Chandra. We present some of the most extreme examples of these background enhancements identified to this point in the mission. The spatial distribution of these enhanced background events is more intense in the middle of the detectors and fainter towards the top and bottom, inconsistent with what one would expect for the vignetting pattern of the telescope and more consistent with an isotropic illumination. The spectral distribution isa continuum across the entire bandpass from 0.3 to 10.0 keV and can not be characterized by a simple power-law plus constant model. The two BI CCDs on Chandra respond similarly to these enhanced background rates, offering the possibility of using data from the off-axis BI CCD to understand the background of the on-axis BI CCD. The traditional techniques of distinguishing background events from x-ray events are ineffective in excluding these background events. Therefore, we suggest that observers might want to consider more aggressive on-board event filtering to reduce the contribution of these background events to the finite ACIS telemetry limit.