The Temporal Characteristics of the Chandra X-ray Observatory High Energy Particle Background

It was observed early on in the Chandra X-ray Observatory mission that the background rates of the Advanced CCD Imaging Spectrometer (ACIS) were highly variable on both short and long timescales. We present analysis of lightcurves of the high energy (> 15 keV) ACIS background spanning most of the mission lifetime. These high energy events are not produced by astrophysical X-rays, but by the particle background, primarily energetic protons. Temporal characteristics of both the quiescent background and background flares will be discussed as well as correlations with Chandra’s orbital position and the solar activity cycle. The strength and frequency of background flares appear cyclic with a quasi-annual period, most likely from the motion of Chandra’s orbit through the geomagnetic environment. The Chandra X-ray Observatory, the third of NASA’s great observatories in space, was launched just past midnight on July 23, 1999, aboard the space shuttle Columbia. After a series of orbital maneuvers Chandra reached its final, highly elliptical, orbit. Chandra’s orbit, with a perigee of 10,000 km, an apogee of 140,000 km and an initial inclination of 28.5◦, transits a wide range of particle environments, from the radiation belts at closest approach through the magnetosphere and magnetopause and past the bow shock into the solar wind. The Advanced CCD Imaging Spectrometer (ACIS), one of two focal plane science instruments on Chandra, utilizes charge-coupled devices (CCDs) of two types, frontand back-illuminated (FI and BI). The BI CCDs are more sensitive at low X-ray energies, but are also less efficient at rejecting background events and more sensitive to background flares. Soon after launch it was observed that the background rates on ACIS were highly variable on both short and long timescales. Understanding the temporal characteristics of the ACIS particle background will allow better modeling and removal of the instrumental background from astrophysical data.