The High Energy X-ray Timing Experiment (HEXTE) is one of three scientific 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 field of view. The field of view of each cluster is switched on-and off-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-flight 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.

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We present the 2-60 keV spectrum of the supernova remnant Cassiopeia A measured using the Proportional Counter Array and the High Energy X-ray Timing Experiment on the Rossi X-ray Timing Explorer satellite. In addition to the previously reported strong emission-line features produced by thermal plasmas, the broad-band spectrum has a high-energy "tail" that extends to energies at least as high as 120 keV. This tail may be described by a broken power law that has photon indices of Γ 1 = 1.8 +0.5 −0.6 and Γ 2 = 3.04 +0.15 −0.13 and a break energy of E b = 15.9 +0.3 −0.4 keV. We argue that the high-energy component, which dominates the spectrum above about 10 keV, is produced by synchrotron radiation from electrons that have energies up to at least 40 TeV. This conclusion supports the hypothesis that Galactic cosmic rays are accelerated predominantly in supernova remnants.

We present the results of a 17 ks Chandra ACIS-S observation of the nearby dwarf spiral galaxy NGC 4395. Chandra affords the first high-quality, broadband X-ray detection of the active nucleus of this object that is uncontaminated by nearby sources in the field. We find the nuclear X-ray source to be unresolved and confirm the rapid, large-amplitude variability reported in previous studies. The light curve appears to show $11 cycles of a quasi-periodic oscillation with a period of $400 s. If associated with an orbital feature near the innermost stable orbit of the accretion disk, this period would constrain the black hole mass to be M < 9 ; 10 5 M . The X-ray spectrum indicates absorption by an ionized medium, and the spectral shape appears to vary over the course of our observation. Contrary to prior reports, however, the spectral variations are uncorrelated with changes in the hard X-ray flux. It is possible that the short-term spectral variability we observe results from column density fluctuations in the ionized absorber. A power-law fit to the spectrum above 1 keV yields a photon index of À % 0:6, much flatter than that typically observed in the spectra of Seyfert 1 galaxies. We have ruled out photon pile-up as the cause of the flat spectrum. Even when complex spectral features are considered, the photon index is constrained to be À < 1:25 (90% confidence). Comparing our results with previous determinations of the photon index (À ¼ 1:46 and 1.72), we conclude that the slope of the primary continuum varies significantly on timescales of 1 yr or less. The extreme flatness and dramatic long-term variability of the X-ray spectrum are unprecedented among active galactic nuclei.

We identify the extended Einstein IPC X-ray source, 1E0657-56, with a previously unknown cluster of galaxies at a redshift of z = 0.296. Optical CCD images show the presence of a gravitational arc in this cluster and galaxy spectra yield a cluster velocity dispersion of 1213 +352 −191 km s −1 . X-ray data obtained with the ROSAT HRI and ASCA indicate that 1E0657-56 is a highly luminous cluster in which a merger of subclusters may be occurring. The temperature of the hot gas in 1E0657-56 is kT = 17.4 ± 2.5keV, which makes it an unusually hot cluster, with important cosmological implications.Subject headings: galaxies: clusters:individual (1E0657-56) -intergalactic medium -large-scale structure of the universe -X-rays: galaxies