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The mergers of compact binaries with at least one neutron star component are the potential leading sites of the production and ejection of $r$-process elements. Discoveries of galactic binary pulsars, short gamma-ray bursts, and gravitational-wave detections have all been constraining the rate of these events, while the gravitational wave plus broadband electromagnetic coverage of binary neutron star merger (GW170817) has also placed constraints on the properties (mass and composition) of the merger ejecta. But uncertainties and ambiguities in modeling the optical and infrared emission make it difficult to definitively measure the distribution of heavy isotopes in these mergers. In contrast, gamma rays emitted in the decay of these neutron-rich ejecta may provide a more direct measurement of the yields. We calculate the gamma production in remnants of neutron star mergers, considering two epochs: a kilonova epoch, lasting about two weeks, and a much later epoch of tens and hundreds of thousands of years after the merger. For the kilonova epoch, when the expanding ejecta is still only partially transparent to gamma radiation, we use 3D radiative transport simulations to produce the spectra. We show that the gamma-ray spectra associated with beta- and alpha-decay provide a fingerprint of the ejecta properties and, for a sufficiently nearby remnant, may be detectable, even for old remnants. We compare our gamma spectra with the potential detection limits of next generation detectors, including the Lunar Occultation Explorer (LOX), the All-sky Medium Energy Gamma-ray Observatory (AMEGO), and the Compton Spectrometer and Imager (COSI). We show that fission models can be discriminated via the presence of short-lived fission fragments in the remnant spectra., Comment: 14 pages, 11 figures

We explore the impact of pulsar electromagnetic dipole and fallback accretion emission on the luminosity of a suite of kilonova models. The pulsar models are varied over pulsar magnetic field strength, pulsar lifetime, ejecta mass, and elemental abundances; the fallback models are varied over fallback accretion rate and ejecta mass. For the abundances, we use Fe and Nd as representatives of the wind and dynamical ejecta, respectively. We simulate radiative transfer in the ejecta in either 1D spherical or 2D cylindrical spatial geometry. For the grid of 1D simulations, the mass fraction of Nd is 0, $10^{-4}$, or $10^{-3}$ and the rest is Fe. Our models that fit the bolometric luminosity of AT 2017gfo (the kilonova associated with the first neutron star merger discovered in gravitational waves, GW170817) do not simultaneously fit the B, V, and I time evolution. However, we find that the trends of the evolution in B and V magnitudes are better matched by the fallback model relative to the pulsar model, implying the time dependence of the remnant source influences the color evolution. Further exploration of the parameter space and model deficiencies is needed before we can describe AT 2017gfo with a remnant source., 15 pages, 10 figures, accepted to ApJ. Simulation data available (under "Source term study") at: https://ccsweb.lanl.gov/astro/transient/transients_astro.html

We report on our serendipitous discovery with the XMM-Newton Observatory of a luminous X-ray emitting cluster of galaxies that is located behind the Andromeda galaxy (M31). X-ray emission from the cluster was detected previously by ROSAT, and cataloged as RX J0046.4+4204, but it was not recognized as a galaxy cluster. The much greater sensitivity of our XMM-Newton observations revealed diffuse x-ray emission that extends at least 5 arcmin and has a surface brightness profile that is well fit by the alpha-beta model with beta = 0.70 +/- 0.08, a core radius r_c = 56 arcsec +/- 16, and alpha = 1.54 +/- 0.25. A joint global spectral fit of the EPIC/MOS1, MOS2, and PN observations with Mewe-Kaastra-Liedahl plasma emission model gives a cluster temperature of 5.5 +/- 0.5 keV. The observed spectra also show high significance iron emission lines that yield a measured cluster redshift of z = 0.290 with a 2% accuracy. For a cosmological model with H_0 = 71 km s^{-1} Mpc^{-1}, Omega_M = 0.3 and Omega_{Lambda} = 0.7 we derive a bolometric luminosity of L_x=(8.4 +/- 0.5)*10^{44}$ erg/s. This discovery of a cluster behind M31 demonstrates the utility of x-ray surveys for finding rich clusters of galaxies, even in directions of heavy optical extinction., Comment: ApJ in press, updated to match the accepted version

We present a comprehensive report on the high-energy properties of the γ-ray blazar PKS 0208-512 as observed with EGRET and the Imaging Compton Telescope (COMPTEL) of the Compton Gamma Ray Observatory (CGRO), spanning the entire nine-year mission (1991-2000). More high-significance detections were recorded with EGRET of PKS 0208-512 (nine at greater than 6 σ confidence) than for any other γ-ray blazar. On timescales of weeks to months, PKS 0208-512 is one of the five most variable of the γ-ray blazars. We find a statistically significant correlation between intensity and spectral index for PKS 0208-512, and this source constitutes the best example of spectral hardening with intensity within the EGRET database of blazar observations. Under the assumption of isotropic Eddington-limited emission, we infer a black hole mass of approximately 6 × 107 M☉ at the nucleus of PKS 0208-512. Gamma-ray transparency arguments, however, clearly support the hypothesis of relativistic beaming in PKS 0208-512, with a minimum relativistic Doppler factor for the jet of δ 3.2 at GeV energies. We have also carried out a comprehensive reanalysis of the COMPTEL data associated with PKS 0208-512 at MeV energies and reexamined the earlier reports of excess 1-3 MeV emission from this source. For individual CGRO viewing periods, we find upper limits only for MeV emission from PKS 0208-512. Our reanalysis of the original COMPTEL data for the period 1993 May-June leads us to conclude that the significance of the original reported detection is marginal at best and that there is no compelling evidence at present for the existence of a distinct flaring state for PKS 0208-512 at MeV energies.

We present a compilation of data for all 258 gamma-ray —ares detected above 300 keV by the Gamma Ray Spectrometer (GRS) aboard the Solar Maximum Mission satellite. This gamma-ray —are sample was collected during the period from 1980 February to 1989 November; covering the latter half of the 21st solar sunspot cycle and the onset of the 22d solar sunspot cycle. We describe the SMM/GRS instrument, its in-orbit operation, and the principal data reduction methods used to derive the gamma-ray —are properties. Utilizing measurements for 185 —ares that were sufficiently intense to allow the derivation of gamma-ray spectra, we present an atlas of time pro—les and gamma-ray spectra. The —are parameters derived from the gamma-ray spectra include bremsstrahlung —uence and best-—t power-law parameters, narrow nuclear line —uence, positron annihilation line —uence, neutron capture line —uence, and an indi- cation of whether or not emissions greater than 10 MeV were present. Since a uniform methodology was adopted for deriving the parameters, this atlas should be very useful for future statistical and correlative studies of solar —ares. Subject headings: catalogsgamma rays: burstsSun: —ares

We present Compton Gamma Ray Observatory (CGRO) EGRET measurements of an outburst of greater than 100 MeV gamma-ray emission from the massive X-ray binary system Centaurus X-3 that occurred during an interval of rapid spin-down by the X-ray pulsar. For the 1994 October outburst, the phase-averaged 30 MeV- 10 GeV emission is best fitted by a power law with index α = 1.81 ± 0.37 and an integral flux above 100 MeV of (9.2 ± 2.3) × 10-7 photons cm-2 s-1, corresponding to a phase-averaged luminosity in GeV gamma rays of ~5 × 1036 ergs s-1. Our phase analysis, employing contemporaneous X-ray pulse observations by CGRO BATSE, indicates modulation of the gamma-ray emission at the pulsar's spin frequency with a significance level higher than 99.5%. Straightforward interpretation of the EGRET measurements requires at least sporadic acceleration of GeV particles within the Cen X-3 binary system. The observations also suggest that Galactic X-ray binary systems may constitute a class of highly variable GeV gamma-ray sources.

We describe observations of the quasi-stellar object (QSO) NRAO 190 during a gamma-ray flare from 1994 August 9 to 1994 August 29. This QSO was serendipitously detected by the EGRET instrument on the Compton Gamma Ray Observatory in a gamma-ray flare with a luminosity at least 10 times that of its quiescent state. Optical, radio, and microwave data were obtained during or near the gamma-ray observations. The historical behavior of this object places it in the category of bright, flat-spectrum radio sources with strong optical variability that appear to form the largest class of non-Galactic high-energy gamma-ray sources. During the gamma-ray flare the source is observed with l(E > 100 MeV) = 8.4 ± 1.2 × 10-7 photons s-1 cm-2. A single power-law model gives a best-fit photon index of γ = -1.83 ± 0.14. Little evidence for major radio variability is seen during the flare or immediately afterward, although there is some increase in the 10-100 GHz flux over the next several months. There may be a slight hardening of the radio spectrum. In the optical region there are significant fluctuations on timescales of 1 day or less, although the overall optical luminosity is within the range of previous measurements. Optical observations a few weeks after the gamma-ray observations show a drop of about 60% and reduced variability. A contemporary optical spectrum shows that the source may be slightly harder than seen in a previously published spectrum. Radio monitoring of the source over the year subsequent to the flare has shown a very substantial drop in the flux at many frequencies.

We present X-ray observations of the nearby BL Lac PKS 2155-304 taken when it was undergoing a GeV/TeV gamma-ray outburst. During the outburst we measured X-ray fluxes in the 2–10 keV band that are the largest ever observed for PKS 2155-304. Comparison of these November 1997 measurements and other X-ray observations made contemporaneously with GeV or TeV gamma-ray observations indicate that X-ray and gamma-ray emissions are correlated. Measurements with X-ray all-sky monitors such as the ASM/RXTE and MOXE can therefore signal the presence of outbursts at gamma-ray energies from PKS 2155-304.

We present X-ray data of the middle-aged radio pulsar PSR B0355+54. The XMM-Newton and Chandra observations show not only emission from the pulsar itself, but also compact diffuse emission extending ∼50″ in the opposite direction to the pulsar’s proper motion. Our analysis also indicates the presence of fainter diffuse emission extending ∼5′ from the point source. The morphology of the diffuse component is similar to the ram-pressure confined pulsar wind nebulae detected for other sources. We find that the compact diffuse component is well-fitted with a power-law, with an index that is consistent with the values found for other pulsar wind nebulae. The core emission from the pulsar can be characterized with a thermal plus power-law fit, with the thermal emission most likely originating in a hot polar cap.

The primary scientific mission of the Black Hole Finder Probe (BHFP), part of the NASA Beyond Einstein program, is to survey the local Universe for black holes over a wide range of mass and accretion rate. One approach to such a survey is a hard X-ray coded-aperture imaging mission operating in the 10--600 keV energy band, a spectral range that is considered to be especially useful in the detection of black hole sources. The development of new inorganic scintillator materials provides improved performance (for example, with regards to energy resolution and timing) that is well suited to the BHFP science requirements. Detection planes formed with these materials coupled with a new generation of readout devices represent a major advancement in the performance capabilities of scintillator-based gamma cameras. Here, we discuss the Coded Aperture Survey Telescope for Energetic Radiation (CASTER), a concept that represents a BHFP based on the use of the latest scintillator technology., Comment: 12 pages; conference paper presented at the SPIE conference "UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XIV." To be published in SPIE Conference Proceedings, vol. 5898

We report on the first XMM-Newton observation of the Vela-like pulsar PSR B2334+61. Spectral analysis reveals soft X-ray emission, with the bulk of the photons emitted at energies below ~1.5 keV. We find that the spectrum has a thermal origin and is well-fitted with either a blackbody or a magnetized, pure H atmospheric model. In the latter case, for a neutron star with a radius of 13 km and a magnetic field of 10e13 G, the best-fit gives an hydrogen column density nH = 0.33 x 10^22 cm^-2 and an effective temperature T_eff^infinity = 0.65 x 10^6 K, as measured at Earth. A comparison of the surface temperature of PSR B2334+61 obtained from this fit with cooling curves favor a medium mass neutron star with M ~ 1.45 solar masses or M ~ 1.6 solar masses, depending on two different models of proton superfluidity in the interior. We do not detect any pulsed emission from the source, and determine an upper limit of 5% for the modulation amplitude of the emission on the pulsar's radio frequency., Comment: 14 pages, 1 figure, 1 table, accepted for publication in ApJ

Fast variability of optical objects is an interesting though poorly explored subject in modern astronomy. Real-time data processing and identification of transient celestial events in the images is very important for such study as it allows rapid follow-up with more sensitive instruments. We discuss an approach which we have developed for the RAPTOR project, a pioneering closed-loop system combining real-time transient detection with rapid follow-up. RAPTOR's data processing pipeline is able to identify and localize an optical transient within seconds after the observation. The testing we performed so far have been confirming the effectiveness of our method for the optical transient detection. The software pipeline we have developed for RAPTOR can easily be applied to the data from other experiments., Comment: 10 pages, 7 figures, to appear in SPIE proceedings vol. 4845

We review evidence for the first detection of GeV gamma-rays from the massive x-ray binary system Centaurus X-3. The CGRO/EGRET detection of an outburst of gamma-ray emission from the direction of Cen X-3 occurred during an interval of rapid spindown by the x-ray pulsar. Our phase analysis of photon arrival times, employing the contemporaneous x-ray pulse observations by CGRO/BATSE, indicates modulation of the gamma-ray emission at the pulsar spin frequency with a significance level higher than 99.5%. Straightforward interpretation of the observations suggests that accreting pulsars may constitute a class of highly variable GeV gamma-ray emission.

We present results of an ongoing program to monitor at millimeter wavelengths a select sample of bright, variable gamma-ray blazars with the 37-m Haystack radio telescope. Our primary objective during the 1996–1997 observing season was to follow the blazars 3C 279 and PKS 0528+134, each the subject of intense multiwavelength observing campaigns in parallel with observations carried out with the Compton Gamma Ray Observatory (CGRO). Our secondary objective was to monitor those active galactic nuclei (AGN) identified as particularly strong candidates for gamma-ray flares based on their CGRO detection history. Quasi-weekly monitoring sessions were carried at frequencies of 43 and 86 GHz with the Haystack telescope using new beam-switching instrumentation. In particular, we have applied a promising new technique (termed “multiple drift scanning”) that has been demonstrated to reliably and repeatedly measure continuum source fluxes to levels of a few hundred milliJanskys at 43 GHz, and to ∼1–1.3 Jy at 86 GHz. ...

We discuss the results of the analysis of multi-wavelength data for the afterglows of GRB 081007 and GRB 090424, two bursts detected by Swift. One of them, GRB 081007, also shows a spectroscopically confirmed supernova, SN 2008hw, which resembles SN 1998bw in its absorption features, while the maximum luminosity is only about half as large as that of SN 1998bw. Bright optical flashes have been detected in both events, which allows us to derive solid constraints on the circumburst-matter density profile. This is particularly interesting in the case of GRB 081007, whose afterglow is found to be propagating into a constant-density medium, yielding yet another example of a GRB clearly associated with a massive star progenitor which did not sculpt the surroundings with its stellar wind. There is no supernova component detected in the afterglow of GRB 090424, likely due to the brightness of the host galaxy, comparable to the Milky Way. We show that the afterglow data are consistent with the presence of both forward- and reverse-shock emission powered by relativistic outflows expanding into the interstellar medium. The absence of optical peaks due to the forward shock strongly suggests that the reverse shock regions should be mildly magnetized. The initial Lorentz factor of outflow of GRB 081007 is estimated to be \Gamma ~ 200, while for GRB 090424 a lower limit of \Gamma > 170 is derived. We also discuss the prompt emission of GRB 081007, which consists of just a single pulse. We argue that neither the external forward-shock model nor the shock-breakout model can account for the prompt emission data and suggest that the single-pulse-like prompt emission may be due to magnetic energy dissipation of a Poynting-flux dominated outflow or to a dissipative photosphere., Comment: 15 pages, 8 figures, accepted for publication in the Astrophysical Journal

Clusters of galaxies may emit high‐energy gamma rays due to a number of emission mechanisms including cosmic ray interactions with the intracluster medium and the annihilation of dark matter. We report on a search of measurements from the EGRET instrument aboard the COMPTON Observatory for gamma‐ray emission from clusters of galaxies. This cluster survey includes a preliminary analysis of data from both pointed observations and the EGRET all‐sky survey. Current upper limits are given and the cosmological implications of the observations are briefly discussed.

We present observations taken with the SMM Gamma‐Ray Spectrometer of a giant flare that occurred on 1989 September 29 at a position located approximately 10° beyond the solar limb. The spectral measurements show a rich gamma‐ray spectrum with a remarkably strong neutron capture line at 2.22 MeV. We briefly examine several possible interpretations of the measurements. We conclude that the best interpretation is that the gamma‐ray emission had a spatially extended component which subtended more than 30° on the solar surface in addition to the spatially compact component that normally dominates the total flare fluence. We suggest that the ‘‘back diffusion’’ of solar energetic particles resident on open field lines may have generated the extended component.

We have initiated an observing program with the 37‐m Haystack radio telescope to monitor at millimeter wavelengths the continuum flux levels of known and probable extragalactic γ‐ray sources, primarily blazars. These observations are being conducted in parallel with those carried out by the Compton Gamma Ray Observatory, as part of a large multiwavelength monitoring campaign of extragalactic γ‐ray sources. These observations are intended to place constraints on the predictions of theoretical models which postulate correlations between high‐ and low‐energy emission from γ‐ray quasars and similar objects.First operational tests of a new beam‐switching capability for continuum flux measurement with the Haystack telescope were successfully carried out over two observing sessions during the winter and spring of 1993. A number of γ‐ray blazars and similar targets, of varying source strength, have been observed to date. Initial sensitivity estimates suggest that, under good observing conditions and with moderate...

Ultraviolet observations of the gamma‐ray blazar 3C 279 were carried out in July 1991 with the International Ultraviolet Explorer (IUE) satellite, 28 days after the outburst of intense gamma‐ray emission detected from this source with the high‐energy EGRET instrument aboard the Compton Gamma Ray Observatory (CGRO). IUE observations were conducted over the wavelength range 1200–3200 A (5–10 eV), and are compared with archival UV measurements spanning the period 1988 to 1991. This set of observations was analyzed with a uniform, standarized data‐reduction procedure. No significant variability in the UV spectrum of 3C 279 is noted over timescales of hours, though variations at the 3σ level or higher exist on longer timescales of months to years. The UV observations which most closely bracket the γ‐ray flare detected by EGRET show ∼3σ differences in flux and spectral index, consistent with (but not definitive proof of) variations in the UV flux during the period of the γ‐ray flare. Statistically mariginal evidence of a monotonic correlation is found between UV spectral index and the UV continuum flux observed from 3C 279 in that a harder spectrum is associated with increased flux.

We show that if the reported detections of TeV and PeV gamma‐ray signals from x‐ray binaries are correct, then it is likely that sensitive low energy gamma‐ray spectral measurements by NASA’s Gamma‐Ray Observatory will show evidence for a narrow neutron capture line at 2.22 MeV. We argue that such a detection would provide an unambiguous signature of proton or heavier ion accleration within the binary system.

Ultraviolet observations of the gamma-ray balzar 3C 279 were carried out in 1991 July with the International Ultraviolet Explorer (IUE) satellite, 28 days after the outburst of intense gamma-ray emission detected from this source with the high-energy Energetic Gamma Ray Experiment Telescope (EGRET) instrument aboard the Compton Gamma-Ray Observatory. IUE observations were conducted over the wavelength range 1200-3200 A (5-10 eV) and are compared with archival UV measurements spanning the period 1988-1991. This set of observations was analyzed with a uniform, standardized data-reduction procedure. No significant variability in the UV spectrum of 3C 279 is noted over timescales of hours, though variations at the 3 sigma level or higher exist on longer timescales of months to years. The UV observations which most closely bracket the gamma-ray flare detected by EGRET show approximately 3 sigma differences in flux and spectral index, consistent with (but not definitive proof of) variations in the UV flux period of the gamma-ray flare. Statistically marginal evidence for a monotonic correlation is found between UV spectral index and the emission observed from 3C 279, in that a harder spectrum is associated with increased UV flux.

The properties of solar flare continuum emission at energies >300 keV are discussed. Emphasis is placed on observations made during the 21st Solar Maximum by γ-ray detectors aboard the Solar Maximum Mission and Hinotori satellites. The statistical properties of high-energy flares are presented, including their size-frequency distribution, spectral-index distribution, position distribution, and associated soft X-ray size. The temporal structure of the high-energy continuum is reviewed as well as attempts to model the structure by two-step acceleration and particle trapping. Evidence for the directivity of flare radiation is presented and statistical and stereoscopic analysis techniques are compared and contrasted. The first observations of flare γ-rays at energies > 10 MeV are examined. We show that the very high-energy emission must be a mixture of pion-decay radiation and primary electron bremsstrahlung. Finally, we present high-energy observations from the extended phase of the giant 3 June, 1982 flare which seem to require a new acceleration component.

A 3-min outburst of very-high energy (VHE) quanta, E > 1012 eV, was recently detected from the X-ray pulsar Hercules X-1 (ref. 1). The outburst occurred at a time during the 35-day X-ray modulation that is associated with X-ray turn-on. Temporal analysis of the outburst showed the same 1.24-s modulation that is observed at X-ray energies. Subsequent monitoring of the system at ultra-high energies (UHE) by the Fly's Eye2 yielded evidence for a 40-min outburst of quanta with energies E>1×l014TeV. This UHE outburst also exhibited a 1.24-s modulation. Here we assume the quanta to be γ-rays and show how the interaction of these UHE particles with a precessing accretion disk can explain the observed γ-ray ‘light’ curve. We also discuss the possibility that the emitting UHE particles are accelerated by shocks in an accretion flow as well as the constraints that can be placed on the acceleration mechanism.

Since launch in early 1980 the Gamma-Ray Spectrometer (GRS) onboard the Solar Maximum Mission (SMM) satellite has monitored the sun at gamma-ray energies. In addition to observations of solar flares, cosmic gamma-ray bursts, and precipitating radiation belt electrons, the instrument has detected a new class of high-energy transient events that cannot be attributed to any of these phenomena. The duration of these transients can range from 1 second to more than 10 minutes. The average event rate between 1980 and 1986 was about five per month. However, in February 1987 this rate increased by more than a factor of 25 and continued at this high level until June 1988. These transients can be subdivided into three classes: (i) 0.511-megaelectron volt annihilation line events, (ii) particle events, and (iii) broad-band photon continuum-like events. Evidence is presented that these transients are not of natural origin. It is found that the most likely sources of these events are reactors in earth orbiting satellites. Apart from the threat these reactors pose upon accidental reentry, the reactor-generated transients may have a deleterious effect on cosmic observations obtained with gamma-ray detectors in low earth orbit.

It is argued that the efficiency of particle acceleration in the Cyg X‐3 system can be determined from observations. Direct interpretation of the observed gamma radiation at Eγ≳1012 eV shows that the luminosity in high energy particles (E≳1013 eV) is comparable to the total luminosity of the system. It is concluded that the particles are accelerated with an efficiency of order unity.

SMM gamma-ray spectrometer data are examined to look for gamma-ray emission coincident with the about-10-s neutrino burst from SN 1987A. The absence of a detectable signal suggests that the energy radiated into MeV gamma rays by neutrino decay (or any other process) is less than 10 to the -10th of that in supernova neutrinos above 9 MeV. The results are used to set a direct limit on the lifetime of any massive neutrino type generated in the core collapse leading to SN 1987A.

The generation of relativistic electrons and positrons in thermal material surrounding a γ‐ray source is examined. This in situ generation of particles can resolve many of the problems associated with the quiescent emission from non‐thermal radio stars. The most luminous radio star, Cyg X‐3, is used as a paradigm throughout.

Samorski and Stamm1 have reported and Lloyd-Evans et al.2 have recently confirmed the detection of high-energy quanta, presumably γ rays, with energies E>2×1015 eV from Cygnus X-3. These ultra-high energy (UHE) γ rays were detected with extensive air shower arrays and included four events with E>1016 eV. Temporal analyses1,2 of the events indicate that the flux is modulated with a 4.8-h period and is sharply pulsed. Here (1) we discuss the implications of these γ-ray detections and suggest that autocorrelating the air shower data may be the best way to determine the intrinsic width of the γ-ray pulses; (2) we argue that the radiating particles are accelerated by a pulsar and that if they are accelerated according to any pulsar mechanism we know of, then they must be ions; (3) we note that if the ions are accelerated to 1016 eV by a large amplitude Deutsch wave, then the gravitational wave luminosity Lg should exceed that of the Crab pulsar by a factor of ∼5×105, and the spin-down time should be ∼80 yr (requiring a truly remarkable object); and (4) we show that the ions can be accelerated in the near zone but only if, contrary to the standard view, pair production does not greatly reduce the vacuum potential drop in the near zone. We note that near-zone acceleration could be confirmed by detection of curvature radiation.

The data base consisting of flares detected by the gamma-ray spectrometer (GRS) on board the Solar Maximum Mission (SMM) satellite is used to study the directivity of high-energy radiation. A number of observations are presented that, strongly indicate that the high-energy emission from flares is anisotropic. They are the following: (1) the fraction of events detected at energies above 300 keV near the limb is higher than is expected for isotropically emitting flares; (2) there is a statistically significant center-to-limb variation in the 300 keV to 1 MeV spectra of flares detected by the SMM GRS; (3) the 25-200 keV hard X-ray spectra measured during the impulsive phase by the SMM GRS show a center-to-limb variation; and (4) nearly all of the events detected at above 10 MeV are located near the limb.