J. C. Ling

HEAO 3 discovery of Al-26 in the interstellar medium

A new analysis technique was used with HEAO 3 satellite gamma ray data to quantify the galactic 1809 keV emission. A method of bins was applied in summing for specific scan directions, followed by fitting the source and background components. The 1809 keV line of Al-26 was significant because of the recent discovery that Al-26 was overabundant in meteoritic material and implanted during solar system formation. The HEAO 3 flux indicated a current decay of 3 solar masses of Al-26 per year in the interstellar medium, possibly occurring in red giants and massive main sequence stars. The ratio of the observed Al-26 to the Al-27 flux was similar to that measured in meteorites, which suggests that the ratio is normal for the galactic medium and that no supernova triggered the protosolar nebular collapse.

Long-term gamma-ray spectral variability of Cygnus X-1

Data from HEAO 3 observations of 0.05-10-MeV gamma-ray emission from Cyg X-1 during two 90-d periods (in fall 1979 and spring 1980) are compiled in tables and graphs and analyzed statistically to determine the temporal and spectral variability. It is found that a steady increase in 100-keV emission is accompanied by a decrease (and eventual disappearance) of MeV emission. The mechanisms which could theoretically be responsible for these phenomena are discussed. 28 references.

HEAO 3 observations of the Crab pulsar

The Crab pulsar (PSR 0531+21) is the only pulsar which has been observed throughout almost the entire electromagnetic spectrum from 0.5 keV to over 2000 GeV. In general, the emission from the Crab nebula and its pulsar has been remarkably constant in time (except for the pulsations). However, several recent observations, especially of gamma-ray line emission, indicate possible time variations. The present investigation is concerned with the data obtained with the HEAO C-1 experiment. The data have been studied with the hope of finding information regarding a number of questions, taking into account the existence of line emission from the pulsar, the variation of the shape of the light curve with time, and the spectral variations as a function of pulsar phase. The spectrum of the total phase-averaged pulsed emission is found to be consistent with a single power-law spectrum from 50 keV to 10 MeV.

The states of Cygnus X-1

HEAO 3 High Resolution Gamma-Ray Spectrometer observations of Cygnus X-1 in the 48-300 keV range, along with correlated 3-6 keV Ariel 5 data, have led to the identification of a new state for this object with low flux in both the soft and hard X-ray regions. This is followed by a slow increase in the hard X-ray flux to the normal low state of the source, characterized by a low intensity soft X-ray emission and a high intensity hard X-ray emission. The new superlow state is interpreted in terms of the Compton model, where the X-ray emission arises from the scattering of low energy photons and one-billion K hot electrons. The superlow state may be seen as one extreme of a continuum of low state behavior, given that Compton cooling should begin to quench the hot electrons at higher soft X-ray fluxes, giving rise to the softer X-ray spectrum and to the anticorrelation in changes in the soft and hard X-ray fluxes.

Gamma-ray observations of Co-56 in SN 1987A

During observations of SN 1987A 286 days after the supernova explosion using a JPL high-resolution gamma-ray spectrometer, a line feature was found in the net supernova spectrum at an energy of 1240.8 + or - 1.7 keV, with an intrinsic width of 8.2 + or - 3.4 keV FWHM, and a net flux of 0.0021 + or - 0.0007 photons/sq cm per s. This feature is interpreted as the 1238 keV line from the decay of Co-56 in the supernova remnant. The blueshift and intrinsic broadening of the 1238 keV line are consistent with models incorporating mixing of the radioactive cobalt in the expanding eject. 26 references.

GINGAAll-Sky Monitor Observations of Cygnus X-1

We report 1-20 keV monitoring of the black hole candidate Cyg X-1 by the Ginga all-sky monitor (ASM), in 339 observations spanning from 1987 February to 1991 October. Cyg X-1 was in the X-ray low state in the entire 4.5 yr period. Flux modulation at the 5.6 days orbital period was observed clearly (if we assume no periodic variation, reduced χ2 value is 7.1) in the low-energy (1-2.1 keV) band. In the higher energy band, the modulation amplitude was small and less significant. The 1-2.1 keV folded light curve is a smooth function of the 5.6 days phase. These facts suggest that the modulation is caused by the orbital variation in the column density of photoelectrically absorbing circumstellar matter, as might be expected from the wind of the companion O9.7 Iab star, HDE 226868. The analysis of the spectral modulation suggests that the absorbing wind is not homogeneous. The ~300 days periodicity, reported previously by the Vela 5B investigators, was studied. Approximately 150 days periodicity, rather than ~300 days, was suggested in our data. Also, a broad peak around ~210-230 days period was found in both the power spectral density and the epoch-folding analysis. These periodicities seem to be unstable in their periods. The time-averaged 1-20 keV spectrum from the Ginga ASM alone can be characterized by a power law. However, the combined spectrum measured by Ginga in the 1-20 keV range and by the Solar Maximum Mission (SMM) hard X-ray and burst spectrometer (HXRBS), in the 25-400 keV range in 1987-1988 would require a more complex model, such as the unsaturated Comptonization model.

The gamma-ray spectrum of the galactic center region

The Galactic center region was observed with the HEAO 3 High Resolution Gamma-Ray Spectrometer during the fall of 1979 and the spring of 1980. Between these epochs there was observed (1) a statistically significant decrease in the high-energy (511 keV to about 3 MeV) luminosity, (2) a decrease in the positron annihilation line intensity, reported previousy, and (3) a low positronium annihilation fraction f = 0.38 + or 0.19 during the fall of 1979. If positrons are generated by photon-photon collisions of high-energy photons, then the absence of a detected flux above 511 keV in the spring of 1980 may indicate a time delay between positron production and annihilation. 26 references.

Search for a narrow annihilation feature correlated with the MeV emission of Cygnus X-1

Strong gamma-ray emission near 1 MeV from Cygnus X-1 observed in the fall of 1979 during the gamma 1 period has been interpreted by Liang and Dermer (1988) as thermal emission from a relativistic, pair-dominated region surrounding the black hole. The observed gamma-rays should interact with each other and with X-rays to form electron/positron pairs inside and outside the accretion disk. Because the Eddington limit is reduced by a factor of 1836 in the presence of positrons, some pairs formed inside the disk may also escape the system under the influence of radiation pressure. If these pairs stop in the cold surrounding medium, a narrow 511 keV annihilation feature should be produced. The high resolution gamma 1 spectrum has been examined for such a feature, and some evidence at 1.9 sigma statistical significance for the line is observed with a flux which is generally consistent with theoretical expectations. 16 refs.

HEAO 3 limits on the Ti-44 yield in Galactic supernovae

Data fron the high-resolution gamma-ray spectroscopy experiment on HEAO 3 have been searched for line emission from the decay of Ti-44 created in recent, as yet unobserved, Galactic supernova explosions, where the ages and locations are unknown. Because the 78 yr mean life of Ti-44 is comparable to the average time between Galactic supernovae, the gamma-ray line emission from its decay should appear as Galactic point sources. No evidence was found for such emission from a point source anywhere in the Galactic plane, with a 1-sigma limit of 8.3 x 10 exp -5 photons/sq cm per sec. Detailed models were developed to simulate the Galactic gamma-ray emission from the decay of Ti-44 produced in both type I and type II supernovae. These models were used with the measured gamma-ray line limits to constrain the supernova yields and recurrence periods.

SMM/HXRBS observations of Cygnus X-1 from 1986 December to 1988 April

The Solar Maximum Missions Hard X-ray Burst Spectrometer made 30 measurements of Cygnus X-1 from December, 1986 to April, 1988, yielding a data set of broad synoptic coverage but limited duration for each data point. The hard X-ray intensity was found to be between the gamma(2) and gamma(3) levels, with a range of fluctuations about the average intensity level. The shape of the photon spectrum was found to be closest to that reported by Ling et al. (1983, 1987) during the time of the gamma(3) level emission, although the spectral shapes reported for the gamma(2) and gamma(1) levels were not precluded. 30 refs.