Search

The objectives and principles are described of a single spectroscopic imaging package that can provide effective imaging in the hard X- and gamma-ray ranges. Called the High-Energy Solar Physics (HESP) mission instrument for solar investigation, the device is based on rotating modulation collimators with germanium semiconductor spectrometers. The instrument is planned to incorporate thick modulation plates, and the range of coverage is discussed. The optics permit the coverage of high-contrast hard X-ray images from small- and medium-sized flares with large signal-to-noise ratios. The detectors allow angular resolution of less than 1 arcsec, time resolution of less than 1 arcsec, and spectral resolution of about 1 keV. The HESP package is considered an effective and important instrument for investigating the high-energy solar events of the near-term future efficiently.

A primary scientific objective for solar flare research during the rapidly approaching maximum in solar activity is the imaging of gamma-ray and hard x ray sources of solar flare emissions. These goals will be pursued by the Gamma Ray Imaging Device (GRID) instrument, one of three instruments recently selected for NASA's Max '91 Solar Balloon Program. The GRID instrument is based on the technique of Fourier transform imaging and utilizes scanning modulation grid collimator optics to provide full-Sun imaging with 1.9-arcsecond resolution over the energy range from 20 to 700 keV at time resolutions from 0.1 to 2 s. The GRID telescope will employ 32 subcollimators, each composed of a matched pair of high-Z collimator grids separated by 5.2 meters and a phoswich scintillation spectrometer detector having no spatial resolution. The subcollimators and integrally-mounted fine aspect system are contained within a telescope canister which will be pointed to 0.1 degree accuracy and cyclically scanned to produce source modulation. The 32 subcollimators provide a uniform distribution of grid slit orientations and a logarithmic distribution of slit spacings corresponding to angular dimensions of 1.9 arcseconds to several arcminutes. The instrument is several orders of magnitude more sensitive than the HXIS instrument on the Solar Maximum Mission (SMM) and nearly 10 times more sensitive than any similar instrument scheduled to fly during the next solar maximum. The payload, designed for long-duration high-altitude balloon capability, is scheduled for its first science flight (8 to 14 days duration) from the Antarctic in January of 1992.

This study compares flare source volumes inferred from impulsive hard X-rays and microwaves with those derived from density sensitive soft X-ray line ratios in the O VII spectrum. The data for this study were obtained with the SMM Hard X-ray Burst Spectrometer, Earth-based radio observatories, and the SOLEX-B spectrometer on the P78-1 satellite. Data were available for the flares of 1980 April 8, 1980 May 9, and 1981 February 26. The hard X-ray/microwave source volume is determined under the assumption that the same electron temperature or power law index characterizes both the source of hard X-rays and the source of microwaves. The O VII line ratios yield the density and volume of the 2 x 10 to the 6th K plasma. For all three flares, the O VII source volume is found to be smallest at the beginning of the flare, near the time when the impulsive hard X-ray/microwave volume reaches its first maximum. At this time, the O VII volume is three to four orders of magnitude smaller than that inferred from the hard X-ray/microwave analysis. Subsequently, the O VII source volume increases by one or two orders of magnitude then remains almost constant until the end of the flare when it apparently increases again.

Flare energetics and dynamics were studied using observations of simple impulsive spike bursts. A large, homogeneous set of events was selected to enable the most definite tests possible of competing flare models, in the absence of spatially resolved observations. The emission mechanisms and specific flare models that were considered in this investigation are described, and the derivations of the parameters that were tested are presented. Results of the correlation analysis between soft and hard X-ray energetics are also presented. The ion conduction front model and tests of that model with the well-observed spike bursts are described. Finally, conclusions drawn from this investigation and suggestions for future studies are discussed.

This study compares flare source volumes inferred from impulsive hard X-rays and microwaves with those derived from density sensitive soft X-ray line ratios in the O VII spectrum. The data for this study were obtained with the SMM Hard X-Ray Burst Spectrometer, Earth-based radio observatories, and the SOLEX-B spectrometer on the P78-1 satellite. Data were available for the flares of 1980 April 8, 1980 May 9, and 1981 February 26. The hard X-ray/microwave source volume is determined under the assumption that the same electron temperature or power law index characterizes both the source of hard X-rays and the source of microwaves. The O VII line ratios yield the density and volume of the 2 X 10 to the 6th K plasma. For all three flares, the O VII source volume is found to be smallest at the beginning of the flare, near the time when the impulsive hard X-ray/microwave volume reaches its first maximum. At this time, the O VII volume is three to four orders of magnitude smaller than that inferred from the hard X-ray/microwave analysis. Subsequently, the O VII source volume increases by one or two orders of magnitude then remains almost constant until the end of the flare when it apparently increases again.

Flare energetics and dynamics were studied using observations of simple impulsive spike bursts. A large, homogeneous set of events was selected to enable the most definite tests possible of competing flare models, in the absence of spatially resolved observations. The emission mechanisms and specific flare models that were considered in this investigation are described, and the derivations of the parameters that were tested are presented. Results of the correlation analysis between soft and hard X-ray energetics are also presented. The ion conduction front model and tests of that model with the well-observed spike bursts are described. Finally, conclusions drawn from this investigation and suggestions for future studies are discussed.

The X-ray emitting system GX 339-4 contains one of the prime candidates for a stellar mass-sized black hole. Determining the observational similarities and differences between the members of this group is of value in specifying which characteristics can be used to identify systems containing a black hole, especially those for which no mass determination can be made. The first observations of the E greater than 20 keV spectrum of GX 339-4 during a transition between luminosity states are reported here. The hard spectral state is the lower luminosity state of the system. GX 339-4 has a power-low spectrum above 20 keV which pivots during transitions between distinct luminosity states. The only other X-ray sources known to exhibit this behavior, Cyg XR-1 and (probably) A0620-00, are leading candidates for systems containing a black hole component based on their measured spectrocopic mass function.

The overall flare process involves phenomena characterized as the impulsive and gradual phases, following the X-ray signature first recognized by Kane (1969). In addition, evidence exists for a pre-flare phase in some flares, and recent Solar Maximum Mission data have shown that a post-flare phase, in which extensive and energetically important coronal activity occurs, may also exist. The data to describe the pre-flare and post-flare phases are insufficient to place them properly into an overall picture of the energetics, aside from noting that these phases may indeed be significant from the energetics point of view. What is presently known is reviewed and comments are made about the possible interactions among the flare structures involved.

The ratio of the flux of 15.10-MeV gamma rays to the flux of 4.438-MeV gamma rays resulting from excitation of the corresponding states in C-12 as a sensitive measure of the spectrum of the exciting particles produced in solar flares and other cosmic sources. These gamma rays are produced predominantly by interactions with C-12 and O-16, both of which are relatively abundant in the solar photosphere. Gamma ray production cross sections for proton interactions have been reported previously for all important channels except for the production of 15.10-MeV gamma rays from O-16. The first reported measurement of the 15.10-MeV gamma ray production cross section from p + O-16 is presented here. The University of Maryland cyclotron was employed to produce 40-, 65-, and 86-MeV protons which interacted with CH2 and BeO targets. The resultant gamma ray spectra were measured with a high-purity germanium semiconductor detector at 70, 90, 110, 125, and 140 degrees relative to the direction of the incident beam for each proton energy. Other gamma ray lines resulting from direct excitation and spallation reactions with C-12 and 0-16 were observed as well, and their gamma ray production cross sections described.

This paper describes a Fourier transform telescope designed to image solar flare X-rays and gamma rays at energies up to 1 MeV with arcsec resolution. The imaging technique makes use of a bigrid collimator divided into a number of smaller areas called subcollimators. The grids in each subcollimator consist of a set of linear apertures so configured that each subcollimator provides a measurement of a single Fourier component of the angular distribution of the source. The imaging concept is therefore a mathematical analog to aperture synthesis in radio astronomy. For X-ray and gamma-ray astronomy, this approach has significant advantages in terms of relaxed requirements for astronomy, this approach has significant advantages in terms of relaxed requirements for position sensitivity in the detector and for control of grid alignment in the large scale telescope structure. The concept of the Fourier transform telescope will be illustrated with numerical parameters of a version now under study for the Pinhole/Occulter Facility.

The present paper is concerned with an analysis of new observations of impulsive hard X-ray and microwave bursts. The results are compared with predictions of a thermal flare model. A description is presented of the thermal model with conduction-front confinement, taking into account the development and previous applications of the model, the high-energy limit of confinement and its implications for microwave emission, the formation of a test based on the revised predictions of the model, and derived parameters. The observations and the test of the model are discussed, giving attention to instrumentation, event selection, selection of a homogeneous sample of impulsive rises,. observed and derived parameters for each rise, a correlation analysis of observed and predicted rise times, uncertainties in the measurements, consistency checks, and a comparison with results of Crannell et al. (1978).

In the present study of the microwave and hard X-ray characteristics of 13 solar flares emitting microwave fluxes greater than 500 solar flux units, simultaneous 3-35 GHz and hard X-ray observations were conducted in the 30-500 keV energy range. An analysis is conducted to determine whether the same distribution of energetic electrons can explain both emissions; tests for any correlations between them yield results suggesting that optically thick microwave emission, near the peak frequency, originates in the same electron population that produces the hard X-rays. A single temperature model and a multitemperature model were tested for consistency with the coincident X-ray and microwave spectra at microwave burst maximum; neither model, however, attempts to explain the high frequency component of the microwave spectrum.

During the three years between satellite launch in June 1975 and turn-off in October 1978, the high energy X-ray spectrometer on board OSO-8 observed nearly all of the COS-B gamma-ray source positions given in the 2CG catalog (Swanenburg et al., 1981). An X-ray source was detected at energies above 20 keV at the 6-sigma level of significance in the gamma-ray error box containing 2CG342 - 02 and at the 3-sigma level of significance in the error boxes containing 2CG065 + 00, 2CG195 + 04, and 2CG311 - 01. No definite association between the X-ray and gamma-ray sources can be made from these data alone. Upper limits are given for the 2CG sources from which no X-ray flux was detected above 20 keV.

Expressions which give the effective color temperatures and corresponding emission measures for solar X-ray events observed with instruments onboard any of the GOES satellites are developed. Theoretical spectra were used to simulate the solar X-ray input at a variety of plasma temperatures. These spectra were folded through the wavelength dependent transfer functions for the two GOES detectors. The resulting detector responses and their ratio as a function of plasma temperature were then fit with simple analytic curves. Over the entire range between 5 and 30 million degrees, these fits reproduce the calculated color temperatures within 2% and the calculated emission measures within 5%. With the theoretical spectra, similar expressions for any pair of broadband X-ray detectors whose sensitivities are limited to wavelengths between 0.2 and 100 A are calculable.

Observations of the X-ray binary Cen XR-3 in the 20-120 keV energy range by means of OSO 8's high energy X-ray spectrometer, during July 16-19, 1975, and July 5-14 and 28-29, 1978, indicate that the source was in a high luminosity state during 1975 and a low luminosity one in 1978. While mean orbital light curves appear similar in shape in both years, orbit-to-orbit intensity variations are noted. Spectral, luminosity, and the 4.84 sec modulation are characterized. Cen XR-3 may be a system in which mass transfer by Roche lobe overflow, and by accretion from a stellar wind, are both effective in the production of observable X-ray radiation.

The design, development, and balloon-flight verification of a payload for observations of gamma-ray emission from solar flares are reported. The payload incorporates a high-purity germanium semiconductor detector, standard NIM and CAMAC electronics modules, a thermally stabilized pressure housing, and regulated battery power supplies. The flight system is supported on the ground with interactive data-handling equipment comprised of similar electronics hardware. The modularity and flexibility of the payload, together with the resolution and stability obtained throughout a 30-hour flight, make it readily adaptable for high-sensitivity, long-duration balloon fight applications.

The microwave and hard X-ray charateristics of 13 solar flares that produced microwave fluxes greater than 500 Solar Flux Units were analyzed. These Great Microwave Bursts were observed in the frequency range from 3 to 35 GHz at Berne, and simultaneous hard X-ray observations were made in the energy range from 30 to 500 keV with the Hard X-Ray Burst Spectrometer on the Solar Maximum Mission spacecraft. The principal aim of this analysis is to determine whether or not the same distribution of energetic electrons can explain both emissions. Correlations were found between respective temporal characteristics and, for the first time, between microwave and hard X-ray spectral characteristics. A single-temperature and a multi-temperature model from the literature were tested for consistency with the coincident X-ray and microwave spectra at microwave burst maximum. Four events are inconsistent with both of the models tested, and neither of the models attempts to explain the high-frequency part of the microwave spectrum. A model in which the emissions above and below the peak frequency originate in two different parts of a diverging magnetic loop is proposed. With this model the entire microwave spectrum of all but one of the events is explained.

The gradual soft X-ray emissions associated with a homogeneous set of solar flares have been investigated in the context of a thermal model proposed to explain the impulsive components. The parametric techniques which successfully characterized the hard X-ray and microwave observations are employed in an event-by-event analysis to test for quantitative and correlative relationships between the impulsive and gradual emissions. The results of this investigation are consistent with the hypothesis that the hard X-ray and microwave emissions are produced by bulk heating of a common thermal source. The quantitative relationships require an additional source to explain the soft X-ray observations, consistent with previous results. Correlations between the energetics of the impulsive and gradual emissions, identified in the present work, provide the first clear evidence that their energizing mechanisms are related.

(Previously announced in STAR as N82-18085)

The X-ray source 4U 0900-40 (= Vela XR-1) was observed with the high-energy X-ray spectrometer on OSO 8 for one week in 1976 and three weeks in 1978. Spectra of the source are presented above 16 keV. No systematic difference exists between the X-ray eclipse centers and the eclipse centers predicted from optical ephermerides. Short period intrinsic variability in the system's X-ray intensity may be related to changes in the Compton scattering optical depth in the system and does not require sporadic mass transfer via Roche lobe overflow. The 282 s modulation in the source's X-ray flux above 21 keV consists of two essentially similar pulses per period, most easily interpreted as arising from the two different magnetic poles of a rotating neutron star. The secondary appears to be a spherically accreting, magnetic neutron star.

The X-ray source Cyg X-3 (=4U2030+40) was observed with the high energy X-ray spectrometer on OSO-8 for two weeks in 1975 and in 1976 and for one week in 1977. No change in spectral shape and intensity above 23 keV was observed from year to year. No correlation is observed between the source's intensity and the phase of the 34.1 day period discovered by Molteni, et al. (1980). The pulsed fraction of the 4.8 hour light curve between 23 and 73 keV varies from week to week, however, and the magnitude of the pulsed fraction appears to be correlated with the 34.1 day phase. No immediate explanation of this behavior is apparent in terms of previously proposed models of the source.

The nucleus of the Seyfert galaxy NGC 4151 was observed during May and June 1977 at X-ray energies from 20 to 200 keV using the high-energy X-ray spectrometer on OSO 8 at radio frequencies of 2695 and 8085. Data were also taken in the visual, and at infrared wavelengths of 1 to 2 microns, and from 2 to 6 keV using the OSO 8 proportional counter. Optical data showed variability by a factor of 2 within the observing period, which is comparable to the variability in the 2 to 6 keV X-ray flux. No variability, however, was measured at infrared or radio frequencies. The composite spectrum from radio to gamma-ray is plotted, and it is found that the optical and X-ray fluxes varied during the observing period, although it is not known if they varied concurrently.

The gamma-ray line produced at 0.51-MeV was studied and is shown to be the result of either of free annihilation of positrons with electrons or of the decay of positronium by 2-photon emission. Positron annihilation from the bound state of positronium may also proceed by 3-photon emission, resulting in a continuum with energies up to 0.51-MeV. Accurate calculations of the rates of free annihilation and positronium formation in a solar-flare plasma are presented. Estimates of the positronium-formulation rates by charge exchange and the rates of dissociation and quenching are also considered. The temperature and density dependence of the ratio of 3-photon to 2-photon emission was obtained. It is shown that when the ratio of free electrons to neutral atoms in the plasma is approximately unity or greater, the Doppler width of the 0.51-MeV line is a function of the temperature of the annihilation region. For the small ion densities characteristics of the photosphere, the width is predominantly a function of the density.

An actively-shielded, high-energy X-ray telescope was launched onboard OSO-8 on 21 June 1975. The primary objectives of this experiment are the measurements of the energy spectrum of discrete cosmic X-ray sources in the range 20 keV to 3 MeV and of the temporal variations in the intensity of each source detected with a time resolution of 0.3 msec. This detector provides the highest duty factor and the finest time resolution of any of its kind for observations over a period of up to 10 days. The background spectrum of this detector in orbit was monitored continuously since shortly after launch. The minimum detectable source strength is estimated to be between 10,000 and 100,000 photons/sq-cm-sec keV, limited primary by the effects of induced radioactivity. From 16 July through 18 July 1975, the X-ray binary, Cen X-3, was observed with the hard X-ray telescope.

Anisotropies of charged particles accelerated in solar flares can be studied by observing Doppler shifts of selected gamma-ray lines. The spectral shape of the 6.1-MeV line of O-16 is calculated. If the accelerated particles are isotropic, the line remains centered at an emitting-nucleus rest-frame energy of 6129.4 keV, and its width (FWHM) is about 100 keV. However, for particle anisotropies that may be produced in solar flares, the line is shifted to lower energies by about 30 to 40 keV.

The gamma ray line at 0.51 MeV originates from the annihilation of positrons. When a fraction of the positrons annihilate from bound states of positronium, the 0.51-MeV line is accompanied by a continuum of 3-gamma annihilation radiation at energies up to 0.51 MeV. Accurate calculations of the rates of free annihilation and positronium formation in a solar flare plasma are presented and positronium formation by charge exchange is discussed. The observability of the 3-gamma annihilation is increased by the inherent delay in the production and slowing down time of the positrons. It was concluded that such radiation could be detected at times late in solar gamma ray events when the continuum and prompt line emissions have essentially disappeared.

A search of MIT/OSO-7 data was made for evidence of X-ray emission from flares of UV Ceti flare stars. Observations from McDonald Observatory were used to identify the times of optical flares. The only instance of coincident coverage occurred on 1974 January 21 UT at 03:43:26 GMT for delta m(u)=0.86 flare of YZ CMi. No radio coverage of this particular event was obtained. Upper limits (3 sigma) of 0.8, 1.0, and 0.7 photons/sq cm-sec on the observed X-ray flux were set for the energy ranges greater than or approximately equal to 15, greater than or approximately equal to 3, and 1-10 keV, respectively.

Anisotropies of charged particles accelerated in solar flares were studied by observing Doppler shifts of selected gamma-ray lines. The spectral shape was calculated of the 6.1-MeV line of O-16. If the accelerated particles are isotropic, the line remains centered at e sub 0 = 6129.4 keV, and its width (FWHM) is about 100 keV. For particle anisotropies that may be produced in solar flares, the line is shifted to lower energies by about 30 to 40 keV.

Time-varying features of an electron-photon cascade are considered, particularly those associated with radio emission. The cosmic ray shower is represented as a superposition of collinear 10 GeV electron-photon cascades launched at different heights in the atmosphere. Actual simulations are performed for only 10 cascades at each of 40 heights and the results are scaled to represent the total number of cascades required. The apparent angular motions of the cascade particles as detected by antennas located at various positions up to 300 m from the shower axis are simulated. The radio pulse waveform and the corresponding frequency spectrum are obtained from these motions.

Results of a Monte-Carlo simulation of electron-photon cascades are presented with particular emphasis on cascade lateral spread. The effects of single and multiple Coulomb scattering are represented by giving the particles suitable displacements in position and angle at each interaction point. Some of the more oblique particle tracks are first divided into a number of steps between interaction points and the scattering procedure is applied to each step separately. To represent the particle defections due the geomagnetic field, a field of 0.3 gauss is supposed present and the appropriate helical path is imposed on each particle between interaction points. The results are in good agreement with the calculations of Nordheim et al., with simulations using the Berger-Seltzer electron photon transport program and with experimental measurements in water.

Techniques are being investigated to aid in distinguishing between electrons and the large background of interacting protons which simulate electron-induced electromagnetic cascades. For cosmic ray primaries, incident on the HECRE ionization spectrometer, statistical criteria are employed to test the cascade curves with the incident energy and starting point as free parameters. The physical significance of the distribution of apparent cascade starting points is being studied using Monte Carlo 100-GeV protons. The proposed use of a CsI detector module (totally-active and high Z) to further discriminate against protons is described.

The most intense hard X-ray source in the confused region in Scorpius has been identified as 4U 1700-37 (=HD 153919). Observations extending over three binary periods in 1978 September were carried out with the high-energy X-ray spectrometer on OSO 8. The 3.4 day modulation is seen above 20 keV with the intensity during eclipse being consistent with zero flux. The photonumber spectrum from 20 to 150 keV is well represented by a single power law with a photonumber spectral index of -2.77 + or - 0.35 or by a thermal bremsstrahlung spectrum with kT = 27 (+15, -7)keV. The counting rate above 20 keV outside of eclipse shows no evidence for the 96.8 minute X-ray modulation previously reported at lower energies. Despite the difficulties that exist in reconciling both the lack of periodic modulation in the emitted X-radiation and the orbital dynamics of the system with our currently accepted theories of the evolution and physical properties of neutron stars, the observed properties of 4U 1700-37 are all consistent with the source being a spherically accreting neutron star rather than a black hole.

The results from extended high energy X-ray observations of Scorpius X-1 from the OSO 8 satellite are reported. The source was observed for a total of 15 days in 1975, 1977, and 1978. Simultaneous 10.7 GHz and 4.75 GHz radio data were obtained during the 1978 observation, and low energy X-ray data during the 1975 and 1978 observations. Detailed studies of the data reveal a lack of any correlation between the high energy X-rays and the other energy ranges. A 3 sigma upper limit of 22% was obtained for any modulation of the high energy flux with the binary period. No high energy tail was observed at any time.

Results are presented from repeated scans of the galactic center region made with the high-energy X-ray detector on OSO 8 during the five days of Sept. 20-25, 1978. The observations show a strong flux of high-energy excluding GX 1 + 4 and GX 3 + 1. The photon-number spectrum of this flux is harder than that of GX 1 + 4 measured during the same interval.

The present understanding of the impulsive phase of a solar flare, characterized by short-duration bursts of impulsive hard X-ray, EUV, optical and radio emission indicating the release of energetic electrons is reviewed. Observations of the spectral distribution of impulsive hard X-ray bursts and of Type III and radio continuum bursts are presented and interpreted in terms of energetic electron distributions, and impulsive EUV, XUV, soft X-ray and optical observations, which provide a lower limit to total energy release during the impulsive phase, are discussed. The role of energetic electrons in exciting the hard X-ray, EUV and microwave emissions is considered, and thin-target, thick-target, partial-precipitation and thermal models of impulsive phase electron acceleration are evaluated in light of the observations. It is noted that available data do not allow discrimination between a thermal or a nonthermal electron distribution, on which depends the proportion of flare energy supplied by the energetic electrons, and that data favors models which permit at least partial electron precipitation. Future observational and theoretical work is indicated.

Some results from the analysis of a set of multiply impulsive hard X-ray and microwave solar bursts are presented, showing that some bursts can exhibit widely different magnetic-field strengths at different times. Two categories of microwave spectral behavior are identified: those events during which the microwave turnover frequency (MTF) and spectral shape (SS) remain the same from peak to peak, and those during which the MTF and SS change significantly. These categories correspond to two classes of multiply impulsive bursts: those for which the emission can be characterized by a constant magnetic field and therefore a single source region, and those in which groups of component spikes appear to originate in regions of different magnetic-field strengths, corresponding to separate source regions which flare sequentially. With regard to the latter type, examples are presented, the discrete flaring regions are examined, and their spatial separations are estimated.

The most intense hard X-ray source in the confused region in Scorpius is identified as 4U1700-37. The 3.4-day modulation is seen above 20 keV with the intensity during eclipse being consistent with zero flux. The photon-number spectrum from 20 to 150 keV is well represented by a single power law with a photo-number spectral index of -2.77 + or - 0.35 or by a thermal bremsstrahlung spectrum with kT = 27 96.8-min X-ray modulation previously reported at lower energies. Despite the difficulties in reconciling both the lack of periodic modulation in the emitted X-radiation and the orbital dynamics of the system with theories of the evolution and physical properties of neutron stars, the observed properties of 4U1700-37 are all consistent with the source being a spherically accreting neutron star rather than a black hole.

Observations of the galactic center region made with the high energy X-ray detector on OSO-8 are discussed. A strong hard X-ray which was detected during these observations from the vicinity of the galactic center are examined. The counting rate spectrum and the photon number spectrum of the flux are determined. Comparisons with the high energy X-ray fluxes observed from sources in the region by others are discussed.

Hercules X-1 was observed from August 30 to September 10, 1977, by using the high-energy X-ray scintillation spectrometer on board the OSO 8 satellite. The observation, during which the source was monitored continually for nearly an entire ON-state, covered the energy range from 16 to 280 keV. Pulsed-flux measurements as a function of binary orbit and binary phase are presented for energies between 16 and 98 keV. The pulsed flux between 16 and 33 keV exhibited a sharp decrease following the fourth binary orbit and was consistent with zero pulsed flux thereafter. Only weak evidence was found for temporal variation in the pulsed flux between 33 and 98 keV. The pulsed spectrum has been fitted with a power law, a thermal spectrum without features, and a thermal spectrum with a superposed Gaussian centered at 55 keV. The latter fit has the smallest value of chi-square per degree of freedom, and the resulting integrated line intensity is approximately 0.0015 photon/sec per sq cm for a width of 3.1 (+9.1, -2.6) keV. This result, while of low statistical significance, agrees with the value observed by Truemper (1978) during the same ON-state.