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Cosmic Gamma-Ray Bursts K. HURLEY University of California, Berkeley and Centre d'Etude Spatiale des Rayonnements, Toulouse, France ABSTRACT A review of the cosmic gamma-ray burst phenomenon is presented. Both the light curies and the energy spectra of these short transient events display a great diversity. However, rapid rise times and periodiaties sometimes observed in the light curies suggest a compact object origin. Similarly, absorption and emission features in the energy spectra argue stronger in favor of this interpretation. Counterpart lo gamma-bursters in other energy ranges, such as optical and soft X-ray, have still not been identified, however, leading to a large uncertainty in the distances to bursters. Although gamma-ray burst sources have not yet been observed to repeat, numerous bursts from three objects which may be related to the gamma-bursters, called Soft Gamma Repeaters, have been recorded; there is weak evidence that they may be relatively distant on a galactic scale. Future missions, par~cluarly those emphasizing high energy, time, and/or spatial resolution, as well as a multiwavelength approach, are likely to advance our understanding of this enigmatic phenomenon INTRODUCTION Cosmic ga~nma-ray bursts (GRBs) are brief transient events witch du- rations ~ the lO's of milliseconds to minutes range. The emission in this short internal consists of photons with energies from several keV to lO's of MeV and above, and generally most of the power is in gamma rays of energies around an MeV, making GRB energy spectra among the hardest of all astrophysical objects. The spatial distribution of GRBs is apparent 204

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HIGH-ENERGY ASTROPHYSICS 205 isotropic and, with only 3 known exceptions, each GRB source has been observed to burst only once. A lower limit to the time between bursts from a single source may be estimated from the data to be at least several years. Despite numerous deep searches for burster counterparts at many wavelengths, no unambiguous candidate for a quiescent counterpart has yet been identified; as a result, the distances to bursters are completely unlmown It now seems indisputable that some fraction, perhaps 20% at least, of the GRB sources are neutron stars, most likely in our galaxy. This conclu- sion follows from the observation of line features in GRB energy spectra. While hard evidence for a galactic origin is still lacking for the majority of the bursts observed, it seems likely that advances in instrumentation in the near future will provide unambiguous data to resolve this uncertainly. Some 500 GRBs have been detected since the announcement of their discovery (Klebesadel e! al. 1973~. Since 1978, most have been observed with dedicated instruments aboard fiche spacecraft of many nations: U.S. experiments on the Pioneer Venus Orbiter and Internadonal-Sun-Earth- ~plorer 3; Soviet and Franco-Soviet detectors on the earth-orbi~cing Prog- noz 6,7, and 9 missions, and on the interplanetary Venera 11, 12, 13 and 14, and Phobos 1 and 2 probes; and U.S. experiments on the German Helios-2 mission. Currently operating experiments include those aboard Pioneer Venus Orbiter, the Mir station, and U.S.-Japanese investigations aboard the Ginga satellite. This paper will review only the essential aspects of the GRB phe- nomenon, with emphasis on the more recent results. An in-depth treat- ment of the subject, including both experiment and theory, has appeared recently (Liang and Petrosian 1986), as well as a more detailed review article (Hurley 1989~. In what follows, GRBs will be introduced by their time histones, which pronde some evidence for a compact object origin. The energy spectra of bursts are then presented, and they will be seen to demonstrate practically unambiguoush,r that the origin of some GRBs involves neutron stars. Counterpart searches will be reviewed briefly, and the statistical properties of bun sters will then be treated. One section is devoted to a review of the three known repeating bursters (the Soft Gamma Repeaters). In the concluding section, some models will be mentioned, and future prospects assessed. TIME HISTORIES A gamma-ray burst may last from about 30 ms to 1000 s. Three examples are shown In Figures la-c. When the time histories are relatively simple (e.g., Figure la), e-fold~g rise and decay times may be calculated (Barat et al. 1984a); they range from 10 to 1000 me, with some tendency to

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206 AMERICAN AND SOYlET PERSPECTIVES 60 ~0 30 Go C) 10 o vENERl-13 9 nor 1!382 C.~-298 KElI l rll . 0 0.1 0.2 0.3 TIME , ~ o FIGURES lam Alma histories of three gamma bursts observed By Franco-Soviet expen- ments aboard the interplanetary Venera 13 and 14 spacecraft. Raw counts in 16, 32 and 500 ms intervals, in an energy band several hundred keV wide, are plotted as a function of time. Dashed lines indicate background levels. cluster about 500 and 100 me, respectively. The longer events (e.g., Figures lb, lc3 tend to be more complex, displaying many peaks which appear in a chaotic fashion: there is generally little resemblance between peaks, no apparent relation between their amplitudes, and no clearly defined periodicities. It is possible that the short, apparently simple events have a complex underlying structure which only becomes apparent where they are observed with better statistics (Laros et al. 1985~. The wide diversity of time history shapes and durations is perhaps a clue to an equally wide diversity of mechanisms which are responsible for them, or even to a number of source types. It is also possible that we have

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HIGH-ENERGY ASTROPHYSICS 2~)7 1 1 i ~0 (A 60 Car o 50 GO 30 20 10 o Figure lb 800 400 Go ( ) 200 o Figure lc tIEIJERA-14 -1 1 1 ~ 11 MAY 1982B 51-352 KEN 1 1 1 1 1 o O 1 1 1 . 5 TIME, 1 11111 1 1111111111111illllllllll111111111111111 ' UENERA-14 31 LARCH 1982 50-300 KEN n ~ 10 _UL ~ - 1lllllIllllllllllllllllllllllllIllllllllllIlllllllllllllllllllllllllllllllll 15 30 45 60 35 TIME, S to

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208 AMERICAN AND SOVIET PERsPECTnzES not exhausted the burster morphologies with current instruments, since it can be shown that present experiments discriminate strongly against short bursts (Kuznetsov et al. 1987~. As there is no obvious reason why such bursts should not exist, the durations may span an even greater range. In discussing bursts, one exceptional event must be mentioned: that of March 5, 1979. As far as the time history is concerned, it was noteworthy (Cline et al. 1980) first, because of its rise time, < 0.2 me, the shortest observed to date and second, because of a clearly resolved 8 s periodicity which lasted for several minutes. These characteristics are indicative of a compact object origin: the rise time may be related to the source size, < 60 fan, by causality arguments, and is also consistent with dynamical time scales near the surface of a neutron star (Lamb et al. 1973~. The periodicity implies a minimum density of about 5 x 106 gm/cm3 to withstand centrifugal breakup, and is in the period range of X-ray pulsars, which are known to involve neutron stars. In addition, the rise times of other bursts, in the 500 ms range, are consistent with the free-fall times from the magnetopause of a neutron star to its surface (Lamb et al. 1973), suggesting that some GRBs may be powered by accretion, like some steady X-ray sources. Thus there is circumstantial evidence in the time histories that GRBs may originate on or near neutron stars. ENERGY SPECTRA GRB energy spectra tend to be almost as diverse in some respects as the time histories. However, a composite spectrum illustrating their most important features is shown ~ Figure 2. The continuum has been observed from X-rays of several keV (Laros et al. 1984) to gamma rays of 100 MeV (Share et al. 1986~. Superimposed upon this continuum, two types of line features are sometimes detected (Mazets et al. 1981; Teegarden and Cline 1980~. The first is an absorption line in the 2~70 keV range, with line-to- continuum ratios reaching 0.8 and line widths of about 20%. The second is an emission feature ~ the 350-500 keV range, with line-to-continuum ratios up to 0.3 and line widths around 30%. The former are observed in 15-20% of all GRB energy spectra, and the latter in about 10-20%. In all, lines have been observed in the spectra of some 100-bursts. Until very recently, all observations of the absorption features were of single lines. Although it was generally felt that these were due to Cyclotron scattering or absorption in a strong magnetic field, other interpretations were possible. New evidence from Ginga satellite data, however, have strengthened this conclusion considerably (Murakami et al. 1988; Fenimore et al. 1988~: double absorption lines, at 20 and 40 keV, have been observed in the spectra of two bursts. The line energies provide very convincing evidence that the features are indeed the first and second harmonics in a

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HIGH-ENERGY ASTROPHYSICS 1 10-2 > Y _4 t`, 1 0 Cot 0 10-6 o I 10-8 10 - 209 I ~ l I I I I i I T I - \ \ SOFT GAkl~lA REPEATER - X-RAY \ \ SGR 1806- 20 BURSTER \ \ kT~35 keV ~ kT~lkeV \ \ 3UR~ 20 a 40 keV ~_ - CREATURES \ - CRAB \\ \ ~400 keV (TOTAL) \ \ EM ISSIObJ -FEATURE ~ \ \ E-1 e-E/kT - \ \ kT~200 keV \ \P0~4ER LAW - \ \ TAIL lO ~ 1 ~ 1 ~ ~ ~ ~ ~ ~ 10 100 0.001 0.0 1 .1 ENERGY, MeV FIGURE 2 Apical photon number spectra of some astrophysical objects. The GRB spectrum is a composite, illustrating some of the important features which have been found in numerous observations. B field whose strength is about 2 x 1012 gauss. In fact, a detailed analysis (Wang et al. 1989) shows that one can derive not only the field strength, but also the angle of view with respect to the field, the temperature of the emitting region, the ratio of the line intensities, and numerous other parameters from careful modeling of these observations.

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210 AMERICAN AND SOVIET PERSPECTIVES The emission features are attributed to 511 keV electron-positron annihilation radiation, gravitationally redshifted by about 20% to around 400 keV, implying that the radiation originates near the surface of a 1 solar mass, 10 km radius object. The source of the positrons is presumably in photon-photon or photon-B field interactions. This interpretation raises the interesting question of how the lines can remain relatively narrow in the presence of a hot continuum. In the case of the cyclotron features, the answer appears to be that the absorbing medium is optically thin to the continuum but, due to the magnetic field, thick to photons at resonant energies (Wang et al. 1989~. There is, however, no accepted explanation in the case of the emission lines. Figure 3 summarizes what is known about GRB emission at other en- ergies during the bursting phase. The ordinate is energy flux, or differential of power by log energy, and the normalization is such that the integral gamma ray energy flux above 30 keV is arbitrarily set to 1, so that the ratio of the gamma ray luminosity to that in any other range may be read off the ordinate directly. Practically all points at energies other than X- and gamma rays are upper limits. The exceptions are first, optical transient sources detected in and near 6 GRB regions, but not in conjunction with a burst (Schaefer 1981; Barat et al. 1984b; Cline e! al. 1984; Pedersen e! al. 1984; Hudec et al. 1988; Moskalenko et al. 1989~; for the purposes of this graph, it has been assumed that each was associated with an undetected GRB. A second exception is a radio burst detected in near-coincidence with a GRB whose location is somewhat uncertain (Mandolesi et al. 1977; Ciapi et al. 1979~. Where GRB emission is detectable, from about 2 keV to 100 MeV, the energy flux spectrum has a very unusual shape. First, it is X-ray deficient (X-ray to gamma ray luminosity ratios of about O.OQj, and second, the power output peaks in the MeV range (which justifies the name gamma ray burst). These characteristics have some important consequences for source models (Epstein 1986). Simply to cite one, it means that if gamma radiation is generated near the surface of a neutron star (which is what is suggested by the observations of gravitationally redshifted emission lines), a large fraction of the gamma rays may be directed towards the surface, where they will Compton scatter and reemerge as X-rays, violating the observed X-ray deficiency. Beaming the gamma rays away from the surface, or generating Hem far from the neutron star can solve this problem. QUIESCENT COUNTERPART SEARCHES With only a few possible exceptions (e.g., the March 5, 1979, event [Cline et al. 1982], discussed below with the Soft Gamma Repeaters), deep searches at numerous energies for quiescent burster counterparts have been

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HIGH-ENERGY ASTROPHYSICS RAC10 OPTICAL x-RAr r RAY 211 UHE Y RAYS ~ S ~ ~ ZO: ~ ~ ~S ~ -5 . J h: O: ~; i ~ -10 _ _ O 1 . , I , , 1 ' 1 ~1 , . ! -5 0 5 10 15 LOG PHOTON ENERGY, eV FIGURE 3 1be energy flux spectrum of gamma-bursters Although some of the radio upper limits (Baird et al. 1975; Mandolesi et al. 1977; Ciapi et al. 1979; Cortiglioni et al. 1981; Inzani a al. 19823 were originally published for GRBs whose locations were unknown, subsequent analysis has retained only points above the horizon of the stations used, with one possible exception (Mandolesi et al. 1977; Ciapi et al. 1979~. Optical (Schaefer 1981; Barat et al. 1984b; Cline et al. 1984; Pedersen et al.1984; Hudec e! al. 1988, Moskalenko et al. 1989; Gnndlay et al. 1974; Hudec et al. 1987), VHE gamma-ray (O'Brien and Porter 1976; Bhat et al. 1981), and UHE gamma-ray (Clay et al. 19823 upper limits are also shown. fruitless. Quiescent emission searches have been carried out in the radio range using the VISA (Schaefer et al. 1988~; in the infrared using {RAS and ground-based telescopes (Schaefer e! al. 1987~; in the optical (e.g. Schaefer et al. 1987; Motch e! al. 1985) to mB = 25; in soft X-rays using the Einstein (Pizzichini e! al. 1986) and EXOSAT (Boer et al. 1988) observatories; in the hard X-ray range with HEAO A-4 (Hueter 1987); and in the gamma ray range with COS-B (Sumner e! al. 1987~. The lack of positive identification with any known objects makes it practically impossible to establish the distances to gamma-bursters, even if one accepts the galactic neutron star hypothesis. Space Telescope time has been granted for gamma-ray burst optical counterpart studies. This will eventually allow U magnitudes of about 27 to be reached, and it is quite likely that we will begin to understand better the nature of the objects which are detected optically in GRB error boxes.

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212 AMERICAN AND SOVIET PE~PECT~S STATISTICAL PROPERTIES OF GAMMA-BURSTERS While there are numerous subjects which could be treated under this heading, only three will be singled out here. They are: 1. the recurrence time between bursts from a single source, which tells us something about the neutron star population involved; 2. the spatial distribution (e.g. in galactic coordinates, or in mul- tipole moment expansions, or the angular correlation function), which Forms us about the geometrical source distribution; and 3. the number-intensity relation (e.g., log N-log S. or V/Vma2 ), which gives us Formation on the geometrical distribution of sources about us and how it is sampled. These three properties are not unrelated, but for simplicity they will be decoupled from one another for the following discussion. With the exception of three Soft Gamma Repeaters (discussed below), no GRB source has yet been observed to repeat. The best lower limit to the recurrence time between bursts from a single source is about 10 years (Atteia et al. 1987; Schaefer and Cline 1985~. Yet it is clear that GRB sources must repeat on some time scale from the following argument. Suppose that a fraction f of the bursters are due to galactic neutron stars. The total number of such objects is about 1063e We detect about 0.5 - 1 GRB/day, and there are probably many more which we do not detect due to sensitivity considerations, imperfect sky coverage, etc. Thus if each source burst only once, there would only be enough to last 5 x 105If years. Taldng the age of the galaxy to be 10~ years, this means that if f > 5 x 10-5, recurrence must occur on some time scale. In fact, a value f = 0.2 is suggested by the line observations, implying a maximum recurrence time of 2.5 x 106 years. Even if the number of neutron stars in the galaxy is a factor of 10 greater, burster recurrence is necessary. Since the lack of GRB counterparts leaves the distance scale undeter- mined, much effort has been focused on understanding the spatial distri- bution of bursters, in the hope that, for example, an underlying galactic distribution might become evident at some level. It is possible to quantity the isotropy or anisotropy in a coordinate system-independent fashion, by first expanding the source distinction in spherical harmonics, and then calculating moments of the distribution (Hartmann and Epstein 1989~. The dipole and quadrupole moments have values about equal to those expected for an isotropic distribution, indicating no point or plane concentrations. The distance to which sources are sampled may be estimated calculating the angular correlation function introduced by Peebles (1973~. The appli- cation of this method to GRB source catalogs has shown that if the sources are extragalactic, they are sampled out to a distance of at least 140 Mpc, while if they are distracted in the disk of our galaxy with a scale height

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HIGH-ENERGY ASTROPHYSICS 213 H. they are sampled out to a distance of at most HE (Hartmann and Blu- menthal 1989~. Thus these studies allow one to set limits on the observed distribution, although they do not settle the question of the distance scale. Yet another approach to the distance scale is that of source counts, or the log N-log S relation (e.g. Ryle 1968~. Here the data simply indicate, once more, that we are in the presence of an apparently isotropic distri- bution, but give no information on the distance scale. Such information is likely to leave its signature on the log N-log S curve at low S. where the weak and presumably distant sources are counted. Although instrumental effects distort this portion of the curve, the same data may be used in a different formulation to test the hypothesis that GRB sources are drawn from a uniform space distribution This is the V/Vma~ test, first used to test the distribution of quasars (Schmidt 1968), and recently revised for GRB sources (Schmidt et al. 1988~. While the V/Vma~ test eliminates the detector-induced biases of the log N-log S test, it does introduce other biases (e.g. the Malmquist bias) which may become important when large data sets are employed (Hartmann et al. 1989~. 1b date, however, the test has only been carried out on small data sets, where it confirms, once more, that the GRB source distribution is uniform. THE SOFT GAMMA REPEATERS While the vast majority of the gamma-bursters have not been observed to repeat, three unusual repeating sources are known (Atteia et al. 1987b; Laros et al. 1987; Mazets et aL 1982; Golenetskii et al. 1987~. They are unusual not simply because they repeat, but also because their energy spectra are soft (kT~25 keV), hence the name "Soft Gamma Repeaters", or SGR. In addition, all have relativeh,r short time histories. Although three events obviously do not constitute a distribution, each source has what might be termed a "distance indicator" associated with it. The March 5, 1979, event has a location consistent with the N49 supernova remnant in the LMC (Cline e! al. 19823 (distance 55 kpc). SGR 1806-20 may be located in the bulge, near the galactic center (Atteia et al. 1987b) (distance 8.5 kpc). And B1900+14 is apparenth,r near the galactic plane, suggesting that its distance may be several kpc. Thus a trend may be emerging for these sources to be relatively distant (Cline et al. 1987~. This, of course, may only be confirmed by observations of many new repeating sources. However, a suggestion as to why distant sources might be soft has been proposed (Kazanas 1988~: the distant sources are inherently the most luminous, and thus convert their gamma-radiation to an optically thick electron-positron pair plasma which thermalizes and softens the emerging spectrum.

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214 AMERICAN AND SOVIET PERSPECTIVES DISCUSSION AND CONCLUSIONS What are the distances to the gamma-bursters? Bible 1 summarizes the energetics for a relatively intense source (10-4 erg/cm2 at earth); the intrinsic energy is then 1034RpC2 erg (isotropic emission assumed). The last column in the liable gives an example of a known steady X- or gamma ray source at the distance indicated, along with its X- or gamma ray luminosity, simply to show that energetics may become more of a problem at large distances. What causes gamma-bursts? In the framework of the galactic neutron star hypothesis, it may be thermonuclear explosions (Woosley and Wallace 1982), accretion of cometary matter Maine and Zytkow 1986), star- quakes (Schklovskii and Mitrofanov 1985), or accretion disk instabilities (Epstein 1985), simply to name a few ideas. If all GRBs are indeed due to galactic neutron stars, they will yield a considerable amount of information on a number of topics of current interest: the behavior of matter, including its radiation mechanisms, under extreme conditions of gravitational and magnetic fields and temperatures, the evolutionary path followed by the neutron star which leads to the burster phase, and the retention of a strong magnetic field as it evolves are just a few examples. Future missions will deploy new, sensitive, high resolution and multi- wavelength experiments to resolve the issues surrounding ORB ongin. "High resolution" means not just in energy (to better resolve the emission and absorption features) but also in time (because the features are time variable, and because very short GRBs may be going undetected) and in space (to assist in the deep searches for counterparts which will take place using advanced instrumentation such as HST, RO SAT, and AXAF). Three examples of future experiments which address these needs will be discussed bnefly. The Burst and Transient Source Eiperunent (Fishman 19~33 aboard the Gamma Ray Observatory, to be launched in 1991, will survey the sky to a sensitivity level about an order of magnitude better than current detectors and pronde coarse source localizations. If GRB sources are distributed in the galactic disk it is possible that the localization data from this experiment for weak sources will reveal this. In addition, the instrument has a fast timing capability for intense events, and an array of scintillators for moderate energy resolution studies of energy spectra. The Transient Gamma Ray Spectrometer on the WIND spacecraft may be launched at the end of 1992 (Ttegarden 1986~. It will utilize a large, passively cooled Germanium detector to study the energy spectra of bursts from 10 keV to 10 MeV with an energy resolution of several keV This wd1 be essentially the first dedicated, high resolution GRB experiment to fly in

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HIGH-ENERGY ASTROPHYSICS 215 over a decade, and it is expected to return a wealth of information on the low-energy absorption lines, the high-energy emission lines, and possibly also higher energy nuclear line contributions (as yet undetected). If, as seems plausible, the spectra of a large fraction of bursts contain line features which are presently going undetected by low resolution experiments, this instrument should identify them. The High Energy Transient Experiment (Ricker et al. 1988) (METED represents a multi-wavelength approach to the study of gamma-bursters, and one which focuses on obtaining the maximum amount of information possible during the GRB and relaying it to the ground in near-real time. It is currently scheduled for a 1994 launch. HETE will employ a complement of wide field UV CCD cameras, X-ray and gamma-ray monitors to localize GRB sources to an accuracy of several arc seconds aboard the satellite. Using a novel "message-forwarding" capability, the experiment will transmit information to simple ground receiving stations around the globe, to petit rapid searches for counterparts with short-lived emission at, e.g., optical and radio wavelengths. It is anticipated that such an experiment will revolutionize GRB studies and resolve many outstanding problems, by providing the long-sought burster identifications. REFERENCES Atteia, J.-L., C Barat, K Hurley, M. Niel, G. Vedrenne, W. Evans, E. Fenimore, R. Klebesadel, J. Laros, T. Cline, U. Desai, B. Teegarden, I. Estulin, V. Zenchenko, Kuznetsov, and V. Kurt. 1987. Ap. J. Supp. 64: 305-38Z Atteia, J.-L, M. Boer, K. Hurley, M. Niel, G. Vedrenne, E. Fenimore, R. Klebesadel, J. Laros, ~ Kuzuetsov, R Sunyaev, O. Terekhov, C. Kouveliotou, T. Cline, B. Dennis, U. Desai, and L Orwig. 1987. Ap. J. Lett. 32~): L105-L110. Baird, G., T. Delaney, B. Lawless, D. Griffiths, J. Shakeshaft, R. Drever, W. Meikle, J. Jelley, W. Charman, and R. Spencer. 1975. Ap. J. Lett. 196: L11-L13. Barat, A, R. Hayles, K Hurler, M. Niel, G. Vedrenne, I. Estulin, and V. Zenchenko. 1984a. Ap. J. 285: 791~0. Barat, C, K Hurley, M. Niel, G. Vedrenne, T. Dine, U. D~i, B. Schaefer, B. Teegarden, W. Evans, E. Fenimore, R. Klebesadel, J. Laros, I. Estulin, V. Zenchenko, Kuznetsov, V. Kurt, S. Ilovaisly, and ~ Motch. 1984b. Ap. J. Lett. 286: L6 - L9. Bhat, P., N. Gopalakrishnan, S. Gupta, P. Ramana Murthy, B. Sreekantan, and S. Tonwar. 1981. Phil Trans. R. Soc. Land. A 301: 659 660. Boer, M., J.-~ Atteia, M. Gottardi, K Hurley, M. Niel, C. Barat, G. Pizzichini, K Mason, G. Branduardi-Raymont, F. Cordova, J. Laros, W Evans, E. Fenimore, R. Klebesadel, M. Sims, and C Martin. 1988. Astron. Astrophys. 20Q: 117-123. Ciapi, An, P. Inzani, G. Sironi, S. Cortiglioni, N. Mandolesi, and G. Morigi. 1979. Prom 16th ICRC OG5-1: 209-214. Clay, R. P. Gerhardy, and A Gregory. 198Z Astrophys. Space Sal. 83: 279-286. Cline, 1:, U. Desai, B. Teegarden, C. Ba rat, K Hurley, M. Niel, G. Vedrenne, ~ Evans, R. Klebesadel, J. Laros, I. Estulin, ~ Kuznetsov, V. Zenchenko, V. Kurt, and B. Schaefer. 1984. Ap. J. Lett. 286: L15-L18. Cline, 1:, U. Desai, G. P~ichini, B. Teegarden, W. Evans, R Klebesadel, J. Laros, K Hurley, M. Niel, G. Vedrenne, I. Estulin, ~ Kuznetsov, V. Zenchenko, D. Hovestadt, and G. Gloeckler. 1980. Ap. J. Lett. 237: L1-L~5.

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