as tr o - ph / 9 71 03 28 28 O ct 1 99 7 In - ight Performance of The High Energy X - Ray Timing Experiment on theRossi X - ray Timing

The High Energy X-ray Timing Experiment (HEXTE) is one of three scientiic instruments aboard the Rossi X-ray Timing Explorer (RXTE), which was launched on December 30, 1995. RXTE performs timing and spectral studies of bright x-ray sources to determine the physical parameters of these systems. The HEXTE consists of two independent clusters of detectors, each cluster containing four NaI(Tl)/CsI(Na) phoswich scintillation counters sharing a common 1 FWHM eld of view. The eld of view of each cluster is switched on-and oo-source to provide near real-time background measurements. The net open area of the eight detectors is 1600 cm 2 , and each detector covers the energy range 15?250 keV with an average energy resolution of 15.4% at 60 keV. The in-ight performance of the HEXTE is described, the light curve and spectrum of the Crab Nebula/Pulsar is given, and the 15?240 keV spectrum of the weak source, active galaxy MCG 8-11-11 is presented to demonstrate the weak source spectral capabilities of HEXTE.