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NASA NTRS1983

Active galaxies and the diffuse Gamma-Ray background

A model for the origin of relativistic particles and gamma rays in active galactic nuclei and quasars, together with recent HEAO-1 observations of the spectra of active galaxies from 2 to 165 keV, provide the basis for a reexamination of the nature of the extragalactic gamma ray background. Active galaxies account for the observed background if their X-ray spectra steepen to E.021 above 100 keV, as observed in Cen-A, together with a further steepening to E.021 as a result of absorption of gamma rays by photon-photon pair production interactions with X-ray photons. The compactness of active galaxies required to give this steepening is consistent with estimates of their typical luminosity and radius.

Kazanas, D.↗
NASA NTRS1982

Persistent X-ray emission from a gamma-ray burst source

A quiescent X-ray source detected with the Einstein X-ray Observatory in a location consistent with that of an intense gamma ray burst is shown to be also consistent with the location of the 1928 optical transient, the likely optical counterpart of the gamma ray burst source GBS0117-29. The system appears to be underluminous in X-rays by a factor of 10; possible reasons for this are discussed. The observed X-ray flux would require an accretion rate of about 10 to the -14th (d/1 kpc/)-squared solar masses per year, which is probably too low to be consistent with published nuclear flash models for gamma bursts, unless the distance is substantially greater than about 1 kpc or the burst recurrence time is greater than about 50 yrs, or the accretion rate is highly variable. Such a long recurrence time appears to be inconsistent with the detection of the optical burst.

Grindlay, J. E.↗
NASA NTRS1982

Comments on the gamma-ray burst catalog of Mazets et al. /1981a/

A correlation has been discovered between the Venera spacecraft locations and the gamma-ray burst positions reported in the KONUS catalog (Mazets et al., 1981a). The reason for the correlation is not clear, but it could be due to spatial selection effects and/or large localization errors for weak or soft bursts. Whatever the cause, it seems likely that this systematic bias might significantly affect catalog results pertaining to gamma burst locations, intensity distributions, and spectra. For example, it can explain why the KONUS galactic latitude distribution is peaked significantly south of the galactic equator. Apparent discrepanices between the KONUS and Los Alamos gamma burst data bases are noted.

Laros, J. G.↗
NASA NTRS1983

Status and future of high energy diffuse gamma-ray astronomy

There are two distinctly different high energy diffuse gamma-ray components, one well correlated with broad galactic features and the other apparently isotropic and presumably extragalactic. The observed diffuse galactic high energy gamma radiation is generally thought to be produced in interactions between the cosmic rays and the interstellar matter and photons. It should then ultimately be possible to obtain from the diffuse galactic emission a detailed picture of the galactic cosmic-ray distribution, a high contrast view of the general structure of the galaxy, and further insight into molecular clouds. Two of the candidates for the explanation of the extragalactic diffuse radiation are the sum of emission from active galaxies and matter-antimatter annihilation. A major advancement in the study of the properties of both galactic and extragalactic gamma radiation should occur over the next decade.

Fichtel, C. E.↗
NASA NTRS1983

Gamma-ray spectroscopy - Status and prospects

Contemporary gamma-ray spectroscopy instruments and their results are reviewed. Sensitivities of 10 to the -4th to 10 to the -3rd ph/sq cm-sec have been achieved for steady sources and 10 to the -2nd to 1 ph/sq cm-sec for transient sources. This has led to the detection of gamma-ray lines from more than 40 objects representing 6 classes of astrophysical phenomena. The lines carry model-independent information and are of fundamental importance to theoretical modeling and our understanding of the objects. The objectives and anticipated results of future instruments are discussed. Several instruments in development will have a factor of 10 sensitivity improvement to certain phenomena over contemporary instruments. A factor of 100 improvement in sensitivity will allow the full potential of gamma-ray spectroscopy to be realized. Instrument concepts which would achieve this with both present and advanced techniques are discussed.

Matteson, J. L.↗
NASA NTRS1984

High resolution X- and gamma-ray spectroscopy of solar flares

A balloon-borne X- and gamma-ray instrument was developed, fabricated, and flown. This instrument has the highest energy resolution of any instrument flown to date for measurements of solar and cosmic X-ray and gamma-ray emission in the 13 to 600 keV energy range. The purpose of the solar measurements was to study electron acceleration and solar flare energy release processes. The cosmic observations were to search for cyclotron line features from neutron stars and for low energy gamma-ray lines from nucleosynthesis. The instrument consists of four 4 cm diameter, 1.3 cm thick, planar intrinsic germanium detectors cooled by liquid nitrogen and surrounded by CsI and NaI anti-coincidence scintillation crystals. A graded z collimator limited the field of view to 3 deg x 6 deg and a gondola pointing system provided 0.3 deg pointing accuracy. A total of four flights were made with this instrument. Additional funding was obtained from NSF for the last three flights, which had primarily solar objectives. A detailed instrument description is given. The main scientific results and the data analysis are discussed. Current work and indications for future work are summarized. A bibliography of publications resulting from this work is given.

Lin, R. P.↗
NASA NTRS1983

Frequency of Fast, Narrow Gamma-ray Bursts and Burst Classification

Evidence from the Vela satellites that very brief, approximately 0.1 s, gamma-ray bursts constitute a class distinct from the longer, highly structured bursts has been strengthened by the results of the Venera 11 and 12 KKONUS experiments. The Goodard ISEE-3 gamma-ray burst spectrometer, utilizing a trigger criterion which is more likely to be independent of duration than previous experiments, detected a sample of events which enhances this bimodal distribution. The ISEE-3 result is corroborated by an increase in the frequency of detection of short bursts in the KONUS 13/14 database over KONUS 11/12, an effect attributable to the use of a shorter trigger integration time in the later experiments. Considerations such as repeating bursters complicate a simple dichotomous classification of gamma-ray bursts.

Norris, J. P.↗
NASA NTRS1983

A Note on the Ratio of Positively Correlated Gamma Variates

The density function and corresponding moments for the ratio of correlated gamma distributed variates were derived. A class of bivariate gamma distributions was also considered, and additional distributional results which use this class of functions were also derived. Similar results, by using a different class of bivariate gamma distributions, are presented.

Tubbs, J. D.↗
NASA NTRS1983

A large-area gamma-ray imaging telescope system

The concept definition of using the External Tank (ET) of the Space Shuttle as the basis for constructing a large area gamma ray imaging telescope in space is detailed. The telescope will be used to locate and study cosmic sources of gamma rays of energy greater than 100 MeV. Both the telescope properties and the means whereby an ET is used for this purpose are described. A parallel is drawn between those systems that would be common to both a Space Station and this ET application. In addition, those systems necessary for support of the telescope can form the basis for using the ET as part of the Space Station. The major conclusions of this concept definition are that the ET is ideal for making into a gamma ray telescope, and that this telescope will provide a substantial increase in collecting area.

Koch, D. G.↗
NASA NTRS1983

Gamma-rays from active galactic nuclei

Gamma-ray observations of active galactic nuclei (AGNs) are reviewed and the best candidate mechanisms for gamma ray production in AGNs are examined. Both nonthermal and thermal mechanisms are considered. Hot accretion disks around massive black holes in the centers of AGNs could provide the required thermal electrons, pions, and relativistic lectrons and positrons to explain both the X-ray and gamma emission.

Kafatos, M.↗
NASA NTRS1984

Frequency of fast, narrow gamma-ray bursts

A duration histogram for 24 gamma-ray burst events detected by an ISEE-3 experiment is presented which suggests the existence of two gamma-ray burst populations. A distribution is also shown of 123 Venera 13 and 14 events which suggests two such populations in each experimental sample, the domains separated with a minimum near 1 or 2 sec. It is pointed out that the results of the Goddard ISEE-3 gamma-ray burst spectrometer actually enhance the appearance of two burst populations suggested in the Venera data.

Norris, J. P.↗
NASA NTRS1983

Lack of correlation between mycoplasma induced IFN-gamma production in vitro and natural killer cell activity against FLD-3 cells

The role of interferon (IFN) in the normal-killer-cell (NK) mediated lysis of tumor cells in vitro is investigated experimentally. Normal mouse spleen cells and spleen cells treated with anti-Thy-1.2 serum are cultured for 24 h with Friend erythroleukemia (FLD-3) cells in RPMI 1640 medium; supernatant fluid from cultures with FLD-3 lysis are assayed for IFN-gamma, and it is found that pretreatment with anti-Thy-1.2 suppresses IFN-gamma generation without affecting the ability of NK to mediate the lysis of FLD-3. Further tests indicate that the generation of IFN-gamma is stimulated by the presence of Mycoplasma arginini in the FLD-3 cells.

Kumar, V.↗
NASA NTRS1985

HEAO-1 observations of gamma ray bursts

A search of data from the High Energy X-Ray and Low Energy Gamma Ray Experiment on HEAO-1 uncovered 14 gamma ray bursts. Nine of these events are reported for the first tiome. Except for the faintest events, all of the bursts detected by this experiment have been measured above an MeV, thereby confirming the hard spectral character of gamma ray burst spectra reported by SMM. Results give a burst rate of at least 105 per year above 6 times 10 to the minus 7th power ergs, which is consistent with previous measurements of burst frequency.

Hueter, G. J.↗
NASA NTRS1985

1000 GeV gamma rays from Cygnus X-3: An update

Measurements of 1000 GeV gamma-rays from Cygnus X-3 made with the University of Durham facility at Dugway, Utah in 1981/82 are reviewed. The light curve of the 4.8 hour modulated emission is updated and shows evidence significant at the 4.4 sigma level for strong emission (9% of the cosmic ray rate) at phase 0.625 and less significant (1.4 sigma level) indications of weaker emission (3% of the cosmic ray rate) at phase 0.125. The effect constituting the excess on the few nights showing the strongest emission appears to arise from the smallest Cerenkov light signals suggesting a steep gamma-ray spectrum. The 1982 data have been searched unsuccessfully for evidence of emission at phase 0.2, in coincidence with the results from the ultra-high energy (extensive Air Showers (EAS) measurements in 1979-1982. A systematic investigation of a long term variation in the strength of the peak of the 4.8 hr modulated 1000 GeV gamma-ray emission has been made. We find that in addition to the approximately 34 d variation reported by us previously, a stronger effect exists at around 19d.

Chadwick, P. M.↗
NASA NTRS1985

Variability in the high energy gamma ray emission from Cyg X-3 over a two-year period (1983 - 1984) at E 4 x 10(11) eV

Cygnus X-3 is observed to emit gamma rays with energies in excess of 4 x 10 to the 11th power eV during two out of 9 observational categories over an 18 month time span. The emissions are observed at the 0.6 phase of the characteristic 4.8 hr light curve for this binary system. We estimate a peak flux at phase 0.6 of 5 x 10 to the minus 10th power photons cm-2s-1 at a software threshold of 8 x 10 to the 11th power eV for Oct/Nov 1983. A flux for the June 84 effect cannot be reliably calculated at present due to lack of Monte Carlo simulations for the energy range and spectral region. For the other 7 observational categories the observations are consistent with zero source emission. The light curve would appear to be variable on a time scale of a couple of weeks at these categories. Selection of compact images in accordance with Monte Carlo simulations combined with empirical optimization techniques have led to an enriched gamma ray light curve for the Oct/Nov 1983 data. Selection on the basis of shower orientation, however, has not led to any notable enhancement of the gamma ray content. Individual Cherenko images can be reliably sorted on an event by event basis into either proton-induced or photon-induced showers.

Cawley, M. F.↗
NASA NTRS1985

Secondary periodicities of microbursts of TeV gamma rays from the Crab pulsar

Observations were made during the past several years on the Crab pulsar using the Ooty atmospheric Cerenkov array with the aim of detecting possible emission of ultra high energy gamma rays by the pulsar. During the course of these observations, it was found that the Crab pulsar emits TeV gamma rays in bursts of short duration. The microbursts of TeV gamma rays from the Crab pulsar, which were seen in the data of at least three years, also reveal interesting secondary periodicities. It was noticed at first that some bursts could be connected with the others that occurred during the same night or during the next two nights with integral number of cycles of periods 43 + or - 1 minute. Ten possible periods in the vicinity of 43 minutes were determined for all the combinations of bursts for each year. The best values of periods thus obtained were different from year to year. But when, instead of the real time, the number of Crab cycles elapsed between the bursts was used as the unit of time, two values of burst periods - 77460 and 77770 Crab cycles - were found to be significant in the data of at least two years. A Monte Carlo simulation using 1500 trial periods chosen randomly within + or - 5 minutes of the original burst period did not reveal any value of the period as significant.

Vishwanath, P. R.↗
NASA NTRS1985

The 1000 GeV gamma ray emission from radio pulsars

Radio pulsars have concentrated on long observations of the Crab pulsar and showed that it emits short intense bursts and a persistent weak periodic flux at gamma-ray energies 1000 GeV. It was shown that the light curve of the persistent emission was dominated by a single peak, coincident with the position of the radio and low energy gamma-ray main pulse. The results of a more detailed analysis of the structure of this main pulse are reported following an appraisal of the timing system. It is shown that at energies 1000 GeV the duration of the main pulse is not greater than 0.4 ms, which is less than that seen at all frequencies other than radio. Flux limits for the emission of 1000 GeV gamma-rays by seven other radio pulsars are reported

Chadwick, P. M.↗
NASA NTRS1985

On the 1983 observations of the gamma-ray source 2CG195+4

The source 2CG 195+4 (Geminga) was observed at E = gamma 2 x 10 to the 7th power eV in December, 1983 with the Tien Shan high altitude facility. The mean gamma flux was equal to (7.0 + or - 4.5) x 10 to the -11th power quanta/sq cm sec. Probably the gamma ray flux is varying with the time with a period 59.49 s.

Zyskin, Y. L.↗