The high energy X-ray timing experiment on XTE

The High Energy X-ray Timing Experiment (HEXTE) is one of three science instruments of NASA's X-ray Timing Explorer (XTE) mission, which was launched into low earth orbit in late 1995. Its energy range of 15-250 keV overlaps with the OSSE and BATSE detectors of the Compton Gamma Ray Observatory (CGRO). The HEXTE consists of eight NaI scintillation detectors grouped into two independent clusters, each with its own data processing electronics. State-of-the art detector resolution is achieved with the help of an automatic gain control system. Like OSSE, the HEXTE permits on-the-fly correction of internal background by beam-switching to blank fields adjacent to the source on the sky. We present here a description of the HEXTE, and demonstrate some aspects of its performance by a spectral simulation of a bright Galactic source with cyclotron absorption. While the HEXTE's large collecting area yields the high event rates essential for temporal studies, the sensitivity to faint sources (such as active galaxies) is limited by systematic effects in the residual background, which we discuss in some detail.