Search

COMPTEL as part of a solar target of opportunity campaign observed the sun during the period of high solar activity from 7-15 Jun. 1991. Major flares were observed on 9 and 11 Jun. Although both flares were large GOES events (greater than or = X10), they were not extraordinary in terms of gamma-ray emission. Only the decay phase of the 15 Jun. flare was observed by COMPTEL. We report the preliminary analysis of data from these flares, including the first spectroscopic measurement of solar flare neutrons. The deuterium formation line at 2.223 MeV was present in both events and for at least the 9 Jun. event, was comparable to the flux in the nuclear line region of 4-8 MeV, consistent with Solar-Maximum Mission (SSM) Observations. A clear neutron signal was present in the flare of 9 Jun. with the spectrum extending up to 80 MeV and consistent in time with the emission of gamma-rays, confirming the utility of COMPTEL in measuring the solar neutron flux at low energies. The neutron flux below 100 MeV appears to be lower than that of the 3 Jun. 1982 flare by more than an order of magnitude. The neutron signal of the 11 Jun. event is under study. Severe dead time effects resulting from the intense thermal x-rays require significant corrections to the measured flux which increase the magnitude of the associated systematic uncertainties.

The Gamma Ray Spectrometer (GRS) on the SMM completed close to 10 years of highly successful operation when the spacecraft reentered the atmosphere on December 2, 1989. During this period the GRS detected 177 events above 300 keV which have been classified as cosmic gamma-ray bursts. A catalog of these events is in preparation which will include time profiles and spectra for all events. Visual inspection of the spectra indicates that emission typically extends into the MeV range, without any evidence for a high-energy cutoff; 17 of these events are also observed above 10 MeV. We find no convincing evidence for line-like emission features in any of the time-integrated spectra.

A Big Flare Syndrome (BFS) test is used to substantiate earlier reports of a statistically significant association between nuclear gamma-ray-line (GRL) flares and metric type II bursts from coronal shocks. The type II onset characteristically follows the onset of gamma-ray emission with a median delay of two minutes. It is found that 70-90 percent of GRL flares for which coronagraph data were available were associated with coronal mass ejections (CMEs). Gradual and impulsive GRL flares were equally well associated with CMEs. The CMEs were typically fast, with a median speed greater than 800 km/s. possible `non-BFS' explanations for the GRL-type II association are discussed.

Gamma-ray line (GRL) and solar energetic proton (SEP) events observed from February 1980 through January 1985 are compared in order to substantiate and better characterize the lack of correlation between GRL fluences and SEP event peak fluxes. The scatter plot of SEP event peak flux vs. GRL fluence is presented, and the ratio of 'solar' to 'interplanetary', about 10 MeV protons, is presented. It is shown that, while even large SEP events can originate in flares lacking detectable GRL emission, the converse case of flares with a significant GRL line fluence by lacking protons in space is rare. The ratio R of the number of about 10 MeV protons that produce GRL emission at the flare site to the number of about 10 MeV protons detected in space can vary from event to event by four orders of magnitude. There is a clear tendency for impulsive flares to have larger values of R than long-duration flares, where the flare time scale is given by the e-folding decay time of the associated soft X-ray emission.

A balloon-borne coded-aperture telescope, measuring gamma-ray photons in the 160 keV to 9.3 MeV range, was used to observe the Cygnus region of the sky on October 1 and 2, 1984. In the 2-9.3-MeV band, evidence is found for a hard spectral component with a mean flux level at the top of the atmosphere of 7.4 + or - 2.5 x 10 to the -7th photons/sq cm per s per keV, inconsistent with the predictions of the inverse Compton models normally used to describe the X-ray emission. Both Cyg X-1 and Cyg X-3 could be observed simultaneously with the telescope. The results are used to establish 1-sigma upper flux limits on the spectral emission from Cyg X-3.

The backgrounds in the OSO-7 Gamma-Ray Monitor and the Solar Maximum Mission Gamma-Ray Spectrometer are compared. After scaling to the same volume, the background spectra agree to within 30 percent. This shows that analyses which successfully describe the background in one detector can be applied to similar detectors of different sizes and on different platforms. The background produced in the SMM spectrometer by a single trapped-radiation belt passage is also studied. This background is found to be dominated by a positron-annihilation line and a continuum spectrum with a high energy cutoff at 5 MeV.

The background rate in any gamma ray detector on a spacecraft in near-earth orbit is strongly influenced by the primary cosmic ray flux at the spacecraft's position. Although the direct counting of the primary cosmic rays can be rejected by anticoincident shields, secondary production cannot be. Secondary production of gamma rays and neutrons in the instrument, the spacecraft, and the earth's atmospheric are recorded as background. A 30 day data base of 65.5 second records has been used to show that some of the background rates observed on the Gamma Ray Spectrometer can be ordered to a precision on the order of 1 percent This ordering is done with only two parameters, namely the cosmic ray vertical cutoff rigidity and the instrument's pointing angle with respect to the earth's center. This result sets limits on any instrumental instability and also on any temporal or spatial changes in the background radiation field.

An overview is presented for a set of simple and robust X-ray and gamma ray instruments which have both cosmic and solar objectives. The primary solar scientific objective is the study of the beaming of energetic electrons and ions in solar flares. The instrument will measure spectra and polarization of flare emissions up to 10 MeV. At X-ray energies both the directly emitted flux and the reflected albedo flux will be measured with a complement of six X-ray sensors. Each of these detectors will have a different high Z filter selected to optimize both the energy resolution and high rate capabilities in the energy band 10 to 300 keV. At energies greater than 100 keV seven 7.6 x 7.6 cm NaI and a set of 30 concentric plastic scattering detectors will record the spectra and polarization of electron bremsstrahlung and nuclear gamma rays. All of the components of the instrument are in existence and have passed flight tests for earlier space missions. The instrument will use a spinning solar oriented Scout spacecraft. The NaI detectors will act as a self-modulating gamma ray detector for cosmic sources in a broad angular band which lies at 90 degrees to the Sun-Earth vector and hence will scan the entire sky in 6 months.

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.

The region of the Galactic anticenter, including the Crab Nebula, was observed during a balloon flight of the University of New Hampshire Directional Gamma-Ray Telescope employing the coded-aperture imaging technique to image celestial gamma-radiation between 160 keV and 9.3 MeV. The background systematics are treated with a simple and relatively straightforward correction procedure. The results demonstrate that the coded-aperture procedure is a viable approach for imaging not only point sources of radiation, but also extended sources of emission. The results for the Crab's photon spectrum are consistent with a power-law spectrum. Upper limits on the flux levels of line emission at 405 keV and 1050 keV and on the flux from the X-ray binary source A0535 + 26 and diffuse Galactic emission from the anticenter region are derived.

For the solar neutron event on June 3, 1982, it is shown here that the combined SMM Gamma Ray Spectrometer and Jungfraujoch neutron monitor data require a time-extended emission of neutrons at the sun with energies of 100 MeV to about 2 GeV. The solar neutron emissivity spectrum is shown to have a strong downward curvature or truncation between 2 and 4 GeV. A Bessel function and truncated power law give acceptable fits to the observational data, but only the power law can explain the rapid rise of the neutron monitor count rate. The integrated emissivity of neutrons above E(n) of 100 MeV is strongly constrained at 8 x 10 to the 28th neutrons/sr and is essentially independent of neutron spectral shape. At neutron energies of about 100 MeV, good agreement is found for both spectral forms with observations of neutron decay protons.

We compared flare gamma ray line (GRL) events and solar energetic proton (SEP) events for the period from Feb. 1980 - Jan. 1985 and substantiated earlier results showing a lack of correlation between gamma-ray-producing ions and interplanetary protons. This poor correlation results primarily from several large SEP events that originated in flares without detectable gamma ray emission. The converse case of GRL events unassociated with SEP events is rare. We present evidence which suggests that the ratio of trapped to escaping protons in GRL/SEP flares depends on the spatial scale size of the flare. We affirm the result of Bai and Dennis (1985) that GRL flares are generally accompanied (75 percent) by metric Type 2 bursts.

Hard X-ray and gamma-ray measurements of the February 8, 1982 (1250 UT) solar flare made with spectrometers aboard the ISEE 3 and SMM spacecraft show that bursts of photons from 40 keV to 40 MeV are coincident within + or - 1 s. This indicates that the acceleration of particles to relativistic velocities can occur promptly (within 1 s). As far as the energetic particles interacting at the sun are concerned, the 'two-phase' process, which requires several minutes for particle acceleration to relativistic velocities, cannot account for the present gamma-ray observations. Even the 'two-step' acceleration process, which is assumed to occur in a small magnetic loop, is severely constrained by the present observations.

A number of observations from the SMM Gamma-Ray Spectrometer are presented that altogether strongly indicate that the high-energy emission from flares is anisotropic. They are: (1) the fraction of events detected at energies above 300 keV near the limb is significantly higher than is expected for isotropically emitting flares; (2) there is a statistically significant center-to-limb variation in the 300-1000-keV spectra of flares; and (3) nearly all of the events detected at above 10 MeV are located near the limb.

The Gamma-Ray Spectrometer on the SMM satellite has recorded high-energy gamma-ray and neutron emission from the flare on June 3, 1982. During the 65-s impulsive phase the gamma rays above 10 MeV contain emissions from both primary electron bremsstrahlung and nuclear pion decay. Hence the impulsive phase acceleration process must produce both primary electrons with energies above 60 MeV and ions above 500 MeV. This flare also has a extended emission phase lasting more than 1000 s which is most easily observed at gamma-ray energies above 10 MeV. After removing the counting rates from the more slowly moving neutrons produced at earlier times, the resulting gamma ray spectrum can be entirely explained by nuclear pion production. Over 70 percent of the pions were produced in the extended emission phase. In contrast, more than 70 percent of the high-energy primary electron bremsstrahlung and the nuclear line emission produced by ions below 30 MeV occurred in the 65-s impulsive phase. This represents the first clear observation of a new acceleration process which produces an electron-deficient, very hard ion spectrum extending beyond 1000 MeV.

The most energetic neutral emissions expected from solar flares are gamma rays (10 MeV) from relativistic primary and secondary electron bremsstrahlung,from approx 0 meson decay, and from neutrons ( 50 MeV). Bremsstrahlung photon energies extend to that of the highest energy electron present, but the shape of the pi sup 0 gamma ray spectrum, peaking at 69 MeV, does not depend strongly on the proton spectrum above threshold, which is approx. 292 MeV for meson production on protons. The highest energy neutrons observed indicate directly the highest energy ions which interact at the Sun, and the presence or absence of anergy cutoff in the acceleration process. The high-energy proton spectrum shape can be determined from the neutron spectrum.

Data obtained with the Gamma Ray Spectrometer (0.3 to 9 MeV) aboard the Solar Maximum Mission satellite from 1980 to 1985 for evidence of the reported Doppler shifted lines from SS433 were examined. The data base covers a total of 468 days when SS433 was in the field of view and includes times of quiescent and flaring radio activity. In 9 day integrations of the SMM data no evidence is found for gamma ray line emission from SS433. The 99% confidence upper limits for 9 day integrations of the shifted 1.37 and 6.1 MeV lines are 0.0013 gamma/sq cm-s and 0.0007 gamma/sq cm-s, respectively. The 360 day time averaged upper limits are 0.0002 gamma/sq cm-s x 0.0001 gamma/sq cm-s for both lines.

The techniques and the results of solar elemental abundance determinations using observations of gamma ray lines from the April 27 1981 olar flare were outlined. The techniques are elaborated on and observed and the best-fitting theoretical spectra are presented. Numerical values for the photon fluences and the total number of protons involved in the thick-target production of these gamma rays are derived.

The Gamma Ray Spectrometer (GRS) on the Solar Maximum Mission spacecraft was primarily designed and calibrated for nuclear gamma ray line measurements, but also has a high energy mode which allows the detection of gamma rays at energies above 10 MeV and solar neutrons above 20 MeV. The GRS response has been extrapolated until now for high energy gamma rays from an early design study employing Monte Carlo calculations. The response to 50 to 600 MeV solar neutrons was estimated from a simple model which did not consider secondary charged particles escaping into the veto shields. In view of numerous detections by the GRS of solar flares emitting high energy gamma rays, including at least two emitting directly detectable neutrons, the calibration of the high energy mode in the flight model has been recalculated by the use of more sophisticated Monte Carlo computer codes. New results presented show that the GRS response to gamma rays above 20 MeV and to neutrons above 100 MeV is significantly lower than the earlier estimates.

The gamma ray spectrometer on the Solar Maximum Mission Satellite has detected the interstellar Al-26 line when the Galactic center traversed its aperture. The center of the emission is consistent with the location of the Galactic center, but the spatial distribution is presently not well defined. The total flux in the direction of the Galactic center is 4.3 + or - 0.4) x .0001 gamma/sq cm-s-rad for an assumed population I distribution.

Elemental abundances of the ambient gas at the site of gamma ray line production inthe solar atmosphere are deduced using gamma ray line observations from a solar flare. The resultant abundances are different from local galactic abundances which are thought to be similar to photospheric abundances.

The Gamma-Ray Spectrometer (GRS) on SMM has detected more than 130 flares with emission approx 300 keV. More than 10 of these flares were detected at photon energies 10 MeV. Although the majority of the emission at 10 MeV must be from electron bremsstrahlung, at least two of the flares have spectral properties 40 MeV that require gamma rays from the decay of neutral pions. It is found that pion production can occur early in the impulsive phase as defined by hard X-rays near 100 keV. It is also found in one of these flares that a significant portion of this high-energy emission is produced well after the impulsive phase. This extended production phase, most clearly observed at high energies, may be a signature of the acceleration process which produces solar energetic particles (SEP's) in space.

The Solar Maximum Mission satellite's gamma-ray spectrometer has detected a line near 1.81 MeV in each of the three years (1980-1982) over which the Galactic center traversed the broad aperture of that instrument. No significant intensity variation is noted over this period. The Galactic center/anticenter intensity ratio is greater than 2.5, and the center of the emission is noted to be consistent with the location of the Galactic center. For an assumed source distribution which follows the more than 100 MeV Galactic gamma radiation, the total flux in the direction of the Galactic center and the measured energy of the line are consistent with the detection of a narrow gamma-ray line from interstellar Al-26 by HEAO 3 in 1979-1980.

The photofraction of a 5.1 cm x 2.0 cm bismuth germanate (BGO) scintillator was measured over a gamma-ray energy range of 0.2 to 6.1 MeV. Several methods, used to minimize the effect of room scattering on the measurement, are discussed. These include a gamma-gamma coincidence technique, a beta-gamma coincidence technique, and the use of sources calibrated with a standard 7.6 cm x 7.6 cm sodium iodide scintillator.

Between February 1980 and August 1983 the Gamma-Ray Spectrometer on the Solar Maximum Mission Satellite (SMM) detected 72 events identified as being of cosmic origin. These events are an essentially unbiased subset of all gamma-ray bursts. The measured spectra of these events show that high energy (greater than 1 MeV) emission is a common and energetically important feature. There is no evidence for a general high-energy cut-off or a distribution of cut-offs below about 6 MeV. These observations imply a limit on the preferential beaming of high energy emission. This constraint, combined with the assumption of isotropic low energy emission, implies that the typical magnetic field strength at burst radiation sites is less than 1 x 10 to the 12th gauss.

Measurements of a gamma ray burst at 01:37 UT on December 31, 1981 using the SMM gamma ray spectrometer (GRS) are compared with those made by the Konus instruments on Veneras 11-14. Burst time profiles, photon spectra, and detector energy loss spectra for three time intervals are compared for the GRS and the Konus instruments. It is concluded that the SMM spectra exhibit no evidence for the presence of emission features reported by the Konus group.

An analysis of the temporal distribution of 139 solar flares monitored by the Gamma Ray Spectrometer aboard the Solar Maximum Mission is reported. It is found that, instead of being randomly distributed in time, these events have a tendency to occur in groups with a mean spacing of about 154 days (75 nHz) over the observing interval. A larger sample of flares with an X-ray classification of M 2.5 or larger recorded by the GOES satellite showed a similar regularity.

The abundance of electrons of 5-50 MeV energies relative to protons of similar energy has been determined in a total of 49 particle events of apparently solar origin during the year 1980. Most events with high electron abundances appear to have been produced by flares which also produced observable fluxes of gamma-rays. The overall distribution of abundances is consistent with that measured in 1968-1970 at similar energies, but is systematically lower in electron content than abundance distributions measured at lower energies. Electron-rich particle events have harder electron spectra than more normal events, and are never accompanied by strong interplanetary shocks. Therefore, an investigation is conducted to decide whether observations are consistent with models in which energetic particles are accelerated by a shock produced at the flare site. It is concluded that, in such models, the variable nature of the interplanetary particle events must reflect some fundamental but variable property of the shocks - possibly their direction of propagation. An outward moving shock may produce a particle event in interplanetary space whereas a shock directed toward the sun may produce a flux of particles in the lower solar atmosphere which generate the gamma-radiation.

Data from the observations of two gamma ray and electron bremsstrahlung X-ray emitting solar flares, on June 7 and 21, 1980, were used to examine the relative starting times of the energetic ion and electron interactions. The data were gathered by the SMM, which was viewing the 2.223 MeV line from H-1(n, gamma)H-2, the 0.511 MeV line from electron-positron annihilation, the prompt nuclear lines at 4.44 and 6.13 MeV from excited C-12 and excited O-16, and the energetic solar neutrons at 1 AMU. Excess gamma ray and X-ray emissions were detected at relatively the same times during both flares in all energy bands, and were attributed to ion-induced emissions. The timing of the peaks of electron and ion activity indicate that both electrons and ions are accelerated to their peak activity in under 5 sec.

Results are presented for the flare of July 1, 1980, which started at approximately 1627 UT and in which simultaneous measurements were made of X-ray, gamma-ray, and optical continuum emission for the entire duration of the flare. The X-ray and gamma-ray observations were made by the Gamma-Ray Spectrometer on the Solar Maximum Mission satellite. The optical measurements were taken at the Sacramento Peak Observatory and the Big Bear Solar Observatory (Zirin and Neidig, 1981). It is found that the major white-light emission that occurs in the late phase of the flare could not have been due to heating by electron or ion precipitation. This conclusion derives from the fact that the X-ray and gamma-ray flux peaks approximately 1 minute before the maximum of the optical continuum mission emission. It is also found that approximately 73 percent of the optical continuum emission, representing a spatially and temporally distinct bright point, follows this maximum with little or no X-ray or gamma-ray emission in the same period.

The gamma-ray spectrometer on the Solar Maximum Mission satellite has observed emissions produced by nuclear reactions in over 20 separate solar flares. The observed intensity from different flares ranges over a factor of 100, and the time scale for their production ranges from 10-s pulses to complete events lasting over 1000 s. The emissions include narrow and broadened prompt gamma-ray lines from numerous isotopes from Li-7 to Fe-56 and cover the energy range from 0.431 MeV (Be-7) to 7.12 MeV (O-16). The instrument has also observed emissions at energies greater than 10 MeV from the decay of pi0 mesons, from electron bremsstrahlung, and from the direct observation of greater-than-100-MeV solar neutrons. The intensity, temporal and spectral properties of these emissions are reviewed from the point of view that solar flares represent an astrophysical particle-acceleration site.

From February 1980 - February 1982, the Gamma Ray Spectrometer on the Solar Maximum Mission satellite observed the impulsive phases of sixteen western hemisphere flares that were associated with prompt solar proton events. Six of these flares had a detectable excess in the 4-8 MeV window and four others had detectable continuum emission above 300 keV. As indicated in earlier studies based on fewer events, a lack of correlation is found between the peak 10 MeV near-earth proton fluxes and prompt gamma-ray-line fluences. The two largest proton events in the sample did not have detectable emission above 300 keV. For the 9 December event, an upper limit for the density of the ion acceleration region of 8 billion/cu cm or less is obtained for an acceleration time constant of 1500 s.

A transient flux of high energy solar neutrons from 50 MeV to about 1 GeV has been detected by the Gamma Ray Spectrometer (GRS) on the Solar Maximum Mission (SMM) satellite following an intense burst of high energy photons (less than 100 MeV) peaking at 1143:29 UT. The neutrons were also detected by the IGY neutron monitor on Jungfraujoch (Switzerland). In this paper the SMM GRS observations are summarized and compared with the Jungfraujoch neutron monitor data, and both the time dependent neutron flux at the earth and the neutron emission spectrum at the sun are estimated.

A directional detector for gamma-ray astronomy has been developed to image sources in the energy range 0.1 to 5 MeV. An array of 35 gain stabilized bismuth germanate detectors, together with a coded aperture mask based on a uniformly redundant array allows imaging in 4 deg square sky bins over a 16 x 24 deg field-of-view. The position of a strong point source, such as the Crab Nebula, can be determined to within not more than about 1 deg. A complementary 'anti-mask' greatly reduces systematic effects arising from nonuniform background rates amongst the detectors. The telescope has an effective area of 190 sq cm and an energy resolution of 19.5 percent FWHM at 662 keV. Results of laboratory tests of the imaging system, including the ability to image multiple sources, uniformity of response over the field-of-view, and the effect of the 'anti-mask', are in good agreement with computer simulations. Features of the flight detector system are described and results of laboratory tests and computer simulations are reviewed. A balloon flight of the telescope is planned for the fall of 1982.

Over the past 15 years, a number of measurements have been made of positron-electron annihilation radiation from the Galactic Center region. The results after 1979 show a significant decrease in measured flux intensity from that previously observed with the same instruments. This is probably due to time variations; however, the contributions of a spatially extended and/or energy-broadened component should also be considered. The entire data set is consistent with a time variable point source plus a distribution along the Galactic Disk. There is no strong evidence for a component broadened in energy.