Search

Radioactive nuclei, especially those created in SN explosion, have long been suggested to be important contributors of galactic positrons. In this paper we describe the findings of three independent OSSE/SMM/TGRS studies of positron annihilation radiation, demonstrating that the three studies are largely in agreement as to the distribution of galactic annihilation radiation. We then assess the predicted yields and distributions of SN-synthesized radionuclei, determining that they are marginally compatible with the findings of the annihilation radiation studies., Comment: 7 pages, accepted for publication in New Astronomy Reviews (Astronomy with Radioactivites III)

The Advanced Compton Telescope (ACT) has been suggested to be the optimal next-generation instrument to study nuclear gamma-ray lines. In this work, we investigate the potential of three hypothetical designs of the ACT to perform SN science. We provide estimates of 1) the SN detection rate, 2) the SN Ia discrimination rate, and 3) which gamma-ray lines would be detected from specific supernova remnants. We find that the prompt emission from a SN Ia is such that it is unlikely that one would be within the range that an INTERMEDIATE ACT would be able to distinguish between explosion scenarios, although such an instrument would detect a handful of SNRs. We further find that the SUPERIOR ACT design would be a truly breakthrough instrument for SN science. By supplying these estimates, we intend to assist the gamma-ray astrophysics community in deciding the course of the next decade of gamma-ray SN science., Comment: 10 pages, accepted for publication in New astronomy Reviews (Astronomy with Radioactivities III)

Positron annihilation radiation from the Galaxy has been observed by the OSSE, SMM and TGRS instruments. Improved spectral modeling of OSSE observations has allowed studies of the distribution of both positron annihilation radiation components, the narrow line emission at 511 keV and the positronium continuum emission. The results derived for each individual annihilation component are then compared with each other. These comparisons reveal approximate agreement between the distribution of these two emissions. In certain regions of the sky (notably in the vicinity of the previously reported positive latitude enhancement), the distribution of the emissions differ. We discuss these differences and the methods currently being employed to understand whether the differences are physical or a systematic error in the present analysis., Comment: 5 pages, to appear in the proceedings of the Gamma 2001 Symposium (Baltimore, April 2001)

By combining OSSE, SMM and TGRS observations of the galactic center region, Purcell et al. (1997) and Cheng et al. (1997) produced the first maps of galactic positron annihilation. That data-set has been augmented with additional data, both recent and archival, and re-analyzed to improve the spectral fitting. The improved spectral fitting has enabled the first maps of positronium continuum emission and the most extensive maps of 511 keV line emission. Bulge and disk combinations have been compared with the 511 keV line data, demonstrating that extended bulges are favored over a GC point source for every disk model tested. This result is independent of whether OSSE-only, OSSE/SMM, or OSSE/SMM/TGRS data-sets are used. The estimated bulge to disk ratio (and to a lesser extent the total flux) is shown to be dependent upon the assumption of bulge shape. A positive latitude enhancement is shown to have an effect upon the B/D ratio, but this effect is secondary to the choice of bulge shape., Comment: 10 pages, to appear in "Proceedings of the 5th COMPTON Symposium"

We report the results of extracting the hard X-ray continuum spectrum of Cas A SNR from RXTE/PCA Target of Opportunity (TOO) observations and CGRO/OSSE observations. The data can rule out the single thermal bremsstrahlung model for Cas A continuum between 2 and 150 keV. The single power law model gives a mediocre fit (~5%) to the data with a power-law index, $\Gamma$ = 2.94$\pm$0.02. A model with two component (bremsstrahlung + bremsstrahlung or bremsstrahlung + power law) gives a good fit. The power law index is quite constrained suggesting that this continuum might not be the X-ray thermal bremmstrahlung from accelerated MeV electrons at shock fronts (Asvarov et al. 1989) which would have $\Gamma\simeq$2.26. With several SNRs detected by ASCA showing a hard power-law nonthermal X-ray continuum, we expect a similar situation for Cas A SNR which has $\Gamma$=2.98$\pm$0.09. We discuss the implication of the hardest nonthermal X-rays detected from Cas A to the synchrotron radiation model., Comment: 5 pages, 2 postscript figures, 1 postscript table, latex uses epsfig.sty, aipproc.sty, to appear in the proc. of the 4th COMPTON symposium, held in Williamsburg, VA, April 27-30, 1997. Minor correction for SN1006's power-law index in Table 2

We present the results of OSSE observations of the soft gamma ray continuum emission from the Galactic plane at longitude 95 degrees. Emission is detected between 50 and 600 keV where the spectrum is fit well by a power law with photon index -2.6+-0.3 and flux (4.0+-0.5) 10^{-2} photons/s/cm^2/rad/MeV at 100 keV. This spectral shape in this range is similar to that found for the continuum emission from the inner Galaxy but the amplitude is lower by a factor of four. This emission is either due to unresolved and previously unknown point sources or it is of diffuse origin, or a combination of the two. Simultaneous observations with OSSE and smaller field of view instruments operating in the soft gamma ray energy band, such as XTE or SAX, would help resolve this issue. If it is primarily diffuse emission due to nonthermal electron bremsstrahlung, as is the >1 MeV Galactic ridge continuum, then the power in low energy cosmic ray electrons exceeds that of the nuclear component of the cosmic rays by an order of magnitude. This would have profound implications for the origin of cosmic rays and the energetics of the interstellar medium. Alternatively, if the emission is diffuse and thermal, then there must be a component of the interstellar medium at temperatures near 10^9 K., Comment: 11 pages, Latex, requires AASTEX macros and psfig.tex, 2 postscript figures, Accepted for publication in the Astrophysical Journal Letters

We present progress toward using scanned OSSE observations for mapping and sky survey work. To this end, we have developed a technique for detecting pointlike sources of unknown number and location, given that they appear in a background which is relatively featureless or which can be modeled. The technique, based on the newly developed concept and mean field annealing, is described, with sample reconstructions of data from the OSSE Virgo Survey. The results demonstrate the capability of reconstructing source information without any a priori information about the number and/or location of pointlike sources in the field-of-view., Comment: 15 pages, 4 Postscript figs, AASTeX, uses psfig and epsf. To appear in the Astrophysical Journal, vol. 484

We report the observation by the Compton Gamma Ray Observatory of a spectacular flare of radio source PKS 1622-297. A peak flux of 17E-6 cm^-2 s^-1 (E > 100 MeV) was observed. The corresponding isotropic luminosity is 2.9E49 erg/s. We find that PKS 1622-297 exhibits gamma-ray intra-day variability. A flux increase by a factor of at least 3.6 was observed to occur in less than 7.1 hours (with 99% confidence). Assuming an exponential rise, the corresponding doubling time is less than 3.8 hours. A significant flux decrease by a factor of ~2 in 9.7 hours was also observed. Without beaming, the rapid flux change and large isotropic luminosity are inconsistent with the Elliot-Shapiro condition (assuming that gas accretion is the immediate source of power for the gamma-rays). This inconsistency suggests that the gamma-ray emission is beamed. A minimum Doppler factor of 8.1 is implied by the observed lack of pair-production opacity (assuming x-rays are emitted co-spatially with the gamma-rays). Simultaneous observation by EGRET and OSSE finds a spectrum adequately fit by a power law with photon index of -1.9. Although the significance is not sufficient to establish this beyond doubt, the high-energy gamma-ray spectrum appears to evolve from hard to soft as a flare progresses., Comment: 14 pages, 4 figures, 1 table

Observations from the oriented scintillation spectrometer experiment of the gamma ray lines originating from a variety of Galactic center sources are reviewed. Extensive observations were acquired of the Galactic center region, including the 0.511 MeV positron annihilation line and associated positronium continuum and Al-26 emission. The results reviewed include: Co-57 from SN 1987A; limits on Co-56 from SN 1991T; gamma ray lines from solar flares; searches for Ti-44 emission from Cas A, and searches for C-12 and O-16 lines from the Orion region.

The oriented scintillation spectrometer experiment (OSSE) observations of the Galactic plane and the Galactic center region were combined with observations acquired with other instruments in order to produce a map of the Galactic 511 keV annihilation radiation. Two mapping techniques were applied to the data: the maximum entropy method, and the basis pursuit inversion method. The resulting maps are qualitatively similar and show evidence for a central bulge and a weak galactic disk component. The weak disk is consistent with that expected from positrons produced by the decay of radioactive Al-26 in the interstellar medium. Both maps suggest an enhanced region of emission near l = -4 deg, b = 7 deg, with a flux of approximately 50 percent of that of the bulge. The existence of this emission appears significant, although the location is not well determined. The source of this enhanced emission is presently unknown.

An emission feature near 170 keV, interpreted as Compton-backscattered 511 keV positron-annihilation radiation, has been reported twice by balloon-borne germanium spectrometers from within approximately 15 deg of the Galactic center (Leventhal, MacCallum, & Stang 1978; Smith et al. 1993). Upper limits on this feature set by HEAO 3 (Mahoney, Ling, & Wheaton 1993) and other instruments indicate that it must be transient. We have searched data from the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory (CGRO) for this feature, using daily spectral accumulations from all pointings near the Galactic center up to 1993 August, and covering most of the region viewed by the balloon instruments. We find no evidence for backscatter emission. Under the hypothesis that the source is 1E 1740.7-2942, the OSSE data set (186 days) disagrees with the balloon measurements with 99.3% confidence. The average daily 3 sigma OSSE upper limit on bakscatter flux from 1E 1740.7-2942 is 6.8 x 10(exp -4) photons/sq cm/s, compared to the 1.3 x 10(exp -3) photons/sq cm reported by the balloon observations. We also saw no evidence in 186 days for linelike emission from the point source EXS 1737.9-2952 recently discovered by Grindlay, Covault, & Manandhar (1993). This source exhibited bright emission from 83-111 keV, which has been interpreted as doubly backscattered 511 keV radiation. The average daily 3 sigma upper limit from OSSE for this line is 9.8 x 10(exp -4) photons/sq cm/s, or approximately 8% of the reported flux.

We report results of multiple observations of the quasar 3C 273 with the Oriented Scintillation Spectrometer Experiment (OSSE) instrument on the Compton Gamma Ray Observatory. These observations span the period from 1991 June through 1993 January and represent the most sensitive observations to date in low-energy gamma rays. The source was detected at historically weak 100 keV fluxes compared with previous measurements. Variability by factors of approximately 3 on timescales of approximately equal 2 months was observed in the energy band 50-150 keV. The data are well described by a single power law with a proton number index Gamma = 1.7 +/- 0.1. No significant change of Gamma was observed during changes in intensity. Thermal models do not provide acceptable fits to the data. When the OSSE data are combined with contemporaneous measurements by COMPTEL and EGRET, the spectrum is seen to break at an energy of 1.0(+0.9, -0.4) MeV to a softer power law with Delta Gamma = 0.7(+0.06, -0.11), forming a power law with Gamma = 2.4 between approximately 1 MeV and several GeV.

Cassipeia A, the youngest known supernova remnant in the Galaxy and a strong radio and X-ray source, was observed by OSSE 1992 July 16-August 6. Its close distance (approximately 3 kpc) and its young age (approximately 300 yr) make Cas A the best candidate among known supernova remnants for detecting Ti-44 gamma-ray lines. We find no evidence of emission at 67.9 keV, 78.4 keV, or 1.157 MeV, the three strongest Ti-44 decay lines. From simultaneous fits to the three lines our 99% confidence upper limit to the flux in each line is 5.5 x 10(exp -5) gamma/sq cm s. We also report upper limits for the 4.44 MeV C-12 nuclear de-excitation line, which could be produced by interactions of acclerated particles in the supernova remnant, and for the hard X-ray continuum.

The Oriented Scintillation Spectrometer Experiment (OSSE) on the COMPTON Observatory was designed to undertake a broad range of high energy astronomical observations. Many of these have been realized during the first three years of the mission. In this paper we present several of the recent results from OSSE. We highlight recent OSSE results on the galactic center region and on transient galactic X-ray sources. In addition, we present OSSE results on the average spectrum of Seyfert galaxies, the search for Ti-44 emission from Cas A, and hard X-ray emission from SN1993j.

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory (CGRO) observed the Galactic black hole candidate GX 339-4 as a target of oppurtunity in 1991 September, in response to the outburst reported by Burst and Transient Source Experiment (BATSE). We report here on energy spectra in the 50 keV-10 MeV range obtained by OSSE. The source was detected from 50 to 400 keV at a level relative to the Crab Nebula of approximately 30%. The observed spectrum was prescribed reasonably well by a power law with an exponential cutoff; a least-squares fit yielded a photon index of 0.88 +/- 0.05 and a cutoff energy of 68 +/- 2 keV. The addition of a Compton reflection component did not significantly improve the overall fit. An optically thin thermal bremsstrahlung spectrum also provides a good fit, and the thermal Comptonization model of Sunyaev & Titarchuk, while deficient in describing the data above approximately 200 keV, cannot formally be ruled out. A pure power law with reflection does not fit the observed spectrum. During a follow-up observation made in 1991 November the intensity of the source below 100 keV had dropped by more than a factor of 40, and it was no longer detected above approximately 100 keV. The energy spectrum during the November observation could be characterized by a power law with a photon index of 2.3 +/- 0.3; the spectrum was fitted equally well with the same exponentially cutoff power-law model applied to the September observation, reduced in intensity by a factor of approximately 40. During the 1991 September observation, the luminosity in the 50-400 keV band was approximately 2 x 10(exp 37) ergs/s (assuming a distance of 4 kpc), no more than a factor of 5 below the soft X-ray luminosity of GX 339-4 observed in its X-ray high state. The luminosity during the 1991 November observation was approximately 5 x 10(exp 35) ergs/s. Extrapolations of both the exponentially cutoff power-law and Sunyaev-Titarchuk models to the approximately 5-20 keV X-ray band yield flux levels very close to that observed by Ginga during an overlapping interval in 1991 September, when GX 339-4 was reported to be in its low state. This may be one of the strongest indications to date of a direct correspondence between the low X-ray state and gamma-ray outbursts of GX 339-4.

We present soft gamma-ray observations by the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory (GRO) of the transient X-ray binary pulsar A0535+26. The observations were made 1994 February 8-17, immediately prior to the peak of a giant outburst. The phase averaged spectrum is complex and cannot be described by a single-component model. We find that structure in the spectrum above 100 keV can best be modeled by an absorption feature near 110 keV, which we interepret as the signature of cyclotron resonant scattering. Because of OSSE's 45 keV threshold, we are unable to make a definitive statement on the presence of a 55 keV absorption line; however, we can conclude that if this line does exist, it must have a smaller optical depth than the line at 110 keV. A first harmonic (=fundamental) cyclotron resonance at 110 keV corresponds to a magnetic field strength at the surface of the neutron star of approximately 1 x 10(exp 13) G (approximately 5 x 10(exp 12) G if the first harmonic is at 55 keV).

Gamma-ray observations of the nearby starburst galaxies NGC 253 and M82 over the energy range (0.05-10) MeV have been obtained with the Oriented Scintillation Spectrometer Experiment (OSSE) spectrometer on the Compton Gamma-Ray Observatory (CGRO). The priority of these galaxies as OSSE targets had been established on the grounds that the average supernova rate may be high in starbursts as indicated by infrared and radio observations, and at distances of approximately 3 Mpc a significant chance of supernova gamm-ray line detection exists. NGC 253 was detected in continuum emission up to 165 keV with a total significance of 4.4 sigma and an estimated luminosity of 3 x 10(exp 40) ergs/s. The spectrum is best fit by a power law of photon index approximately 2.5. We consider the possible contribution of different emission mechanisms, including inverse Compton scattering, bremsstrahlung, discrete sources, and Type Ia/Ib supernova continuum to the measured flux. No significant continuum flux was observed from M82. A search for the gamma-ray line from the decay of the most abundant radioactive element produced in supernovae (Ni-56 yields Co-56 yields Fe-56) yielded no significant detection: the 3 sigma upper limits to the line fluxes at 0.158, 0.812, 0.847, and 1.238 MeV for both galaxies are obtained.

We made observations of NGC 1275 with the Compton Gamma-Ray Observatory's Oriented Scintillation Spectrometer Experiment (OSSE) between 1991 November 28 and December 12. We did not detect the source during this viewing period. Our 3 sigma upper limit to a detection in the approximately 50-90 keV range is 6 x 10(exp -6) photons/sq cm/s/keV. This flux is more than 10 times lower than the 3 sigma detection in the same energy range reported by Rothschild et al. for their OSO 7 observations. Our results are discussed in comparison with radio observations and models for the X-ray emission, and we show that it is likely that most of the approx. greater than 10 keV photons come from the nuclear region of NGC 1275. We find no significant correlation between the variable radio intensity and the hard X-ray flux of the entire NGC 1275 source. Simultaneous Very Large Baseline Array and gamma-ray observations are needed to test the self-Compton synchrotron models for gamma-ray emission from the core of NGC 1275. Our results also provide a lower limit to the magnetic field of approximately 2 x 10(exp -7) gauss for the approximately 5 min radio source centered on NGC 1275.

Oriented Scintillation Spectrometer Experiment (OSSE) observed the 150 ms X-ray pulsar PSR B1509-58 m the supernova remnant MSH 15-52 for 4 weeks in 1992. The pulsed spectrum from 50 keV to 5 MeV is well represented by a single-power-law photon spectrum of the form (3.14 +/- 0.16) x 10(exp -6) x (E/118.5 keV)(exp -1.68 +/- -0.09) photons cm(exp -2)s(exp -1)keV(exp -1). This is significantly harder than the Crab pulsar spectrum in this energy range. The Ginga soft X-ray spectrum (2-60 keV) reported by Kawai et al. is significantly harder than the observed OSSE spectrum and predicts a flux 2 times higher than we observe in the approximately 55-170 keV energy band. This requires a break to a steeper spectrum somewhere in the intermediate energy range (approximately 20-80 keV). The spectrum must soften again at higher energies or the pulsar would have easily been detected by EGRET, COS B, and SAS 2.

We present results of the Compton Gamma-Ray Observatory (CGRO) Oriented Scintillation Spectrometer Experiment (OSSE) observations of the Crab pulsar, made during MJD 48373-48406 (1991 April 27 - 1991 May 30) and MJD 48798-48804 (1992 June 25 - 1992 July 1). Pulsar light curves and spectra over the approximately 0.05 to 10 MeV range are presented. The arrival time of the gamma-ray peak and the radio peak agree to within 30 microsec which is better than the approximately 300 microsec accuracy of the measurements. The overall pulse phase averaged spectrum in the 0.1 - 10 MeV range is well-fit by a power law of the form 0.05 x (E/0.13 MeV)exp(-(1.99 +/- 0.03)) photons/sq cm/s. The outer-gap model (with gap parameter equal to 0.46) provided to us by Ho agrees with the data to better than 20%. The spectra of the bridge and second peak are slightly harder than the first peak as measured by the hardness ratio (approximately 110 - 220 keV)/(approximately 50 - 105 keV): P1 = 0.54 +/- 0.01, P2 = 0.63 +/- 0.01, bridge = 0.68 +/- 0.03. The phase of the two peaks in the light curve is constant over the 50 - 550 keV range to within the accuracy of the measurements (better than 0.02 in phase). No evidence was found for variability of the light curve on timescales from 2 minutes (less than a factor of 1.8) to 1 year (less than a factor of 1.06), where these are 3 sigma upper limits. However, when we examine the historical database, we find, in agreement with Nolan et al. (1993), that there is evidence for a 13 year variation in the ratio of the intensity of peak 2 to peak 1. We show that if this is interpreted as being due to precession (which changes the relative view of the intrinsic gamma-ray pulse as seen on earth), the variation is consistent with models of neutron star structure. The optical data may be in conflict with the interpretation however. We found no statistically significant lines in the 50 - 550 keV range in the spectrum. The average 3 sigma upper limits in 10(exp -3) photons/sq cm/s for lines at 0.073, 0.078, 0.4, 0.44, 0.511, and 0.545 MeV are 0.3, 0.5, 0.6, 0.5, and 0.1.

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Observatory observed SN 1993J during three intervals centered approximately 12, 30, and 108 days after its outburst. Hard X-ray emission was detected in the first two of these intervals. No emission was seen in the third observation or in two earlier observations in 1991 and 1992. The coincidence of the observed excess with the outburst of SN 1993J and the consistency of the spectra and time evolution with those seen at lower energies by ROSAT and ASCA (Astro-D) argue that the observed emission is indeed from SN 1993J. It is probably due to the interaction of the fast supernova ejecta with circumstellar material. The luminosity, 5 x 10(exp 40) ergs/sec (50-150 keV) in the first interval, is significantly larger than predicted. Extrapolating the spectrum to a few keV accounts for most or all of the observed emission at low energy. The observed high temperature, 10(exp 9) K, is easily obtained in the shocked circumstellar matter, but a surprisingly high density is required there to give the observed luminosity, and little or no additional X-ray emission from denser shocked supernova ejecta is allowed. The hard emission might also be explained in terms of the shocked supernova ejecta itself with unexpectedly high temperature.

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma-Ray Observatory (GRO) has performed numerous observations of the Galactic plane and Galactic center region to measure the distribution of Galactic 511 keV positron annihilation radiation and to search for time variability of the emission. The initial 511 keV line fluxes for the observations performed during the first 18 months of the GRO mission are presented. The 511 keV line flux for a typical Galactic center observation is (2.5 +/- 0.3) x 10 exp -4 gamma/sq cm per sec, where the quoted uncertainty represents the 1 sigma statistical uncertainty. No statistically significant time variability of the line flux has been observed; the 3 sigma upper limit to daily variations from the mean is 3 x 10 exp -4 gamma/sq cm per sec. The distribution of Galactic 511 keV positron annihilation radiation implied by the OSSE observations is discussed and compared with observations by other instruments.

The Oriented Scintillation Spectrometer Experiment on the Arthur Holly Compton Gamma Ray Observatory satellite uses four actively shielded NaI (Tl)-CsI(Na) phoswich detectors to provide gamma-ray line and continuum detection capability in the 0.05-10 MeV energy range. The instrument includes secondary capabilities for gamma-ray and neutron detection between 10 and 250 MeV. The detectors have 3.8 deg x 11.04 deg (FWHM) fields of view defined by tungsten collimators. Each detector has an independent, single-axis orientation system which permits offset pointing from the spacecraft Z-axis for background measurements and multitarget observations. The instrument, and its calibration and performance, are described.

Observations of the Seyfert galaxy NGC 4151 with the Oriented Scintillation Spectrometer Experiment instrument on the Compton Gamma Ray Observatory are presented. A continuum spectrum is observed, which steepens exponentially with an e-folding energy of 39 +/- 2 keV over the 65-300 keV range. Source intensity variability of approximately 25 percent was seen during the observations, and no significant change in the spectral shape was detected. Simple pair models cannot explain the observed spectrum of NGC 4151, and repeated observations are required to determine whether this spectral shape is common or is even the dominant spectral state in this class of active galactic nuclei.

The Oriented Scintillation Spectrometer Experiment (OSSE) was launched on NASA's Compton Observatory on 1991 April 5. OSSE uses large area scintillation detectors to undertake gamma-ray line and continuum observations in the 0.05-10 MeV energy range. During the first months of the mission, OSSE has obtained observations on a number of high priority sources including AGNs, SN1991T, the galactic center region, and several discrete galactic sources. The capabilities and performance of OSSE are discussed and initial results for several of the early observations are presented.

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory has observed SN 1987A for two 2 week periods during the first 9 months of the mission. Evidence for gamma-ray line and continuum emission from Co-57 is observed with an intensity of about 10 exp -4 gamma/sq cm/s. This photon flux between 50 and 136 keV is demonstrated by Monte Carlo calculations to be independent of the radial distribution of Co-57 for models of low optical depth, viz., models having photoelectric absorption losses of 122 keV photons no greater than several percent. For such models the observed Co-57 flux indicates that the ratio Ni-57/Ni-56 produced in the explosion was about 1.5 times the solar system ratio of Fe-57/Fe-56. When compared with nearly contemporaneous bolometric estimates of the luminosity for SN 1987A, our observations imply that Co-57 radioactivity does not account for most of the current luminosity of the supernova remnant in low optical depth models. We suggest alternatives, including a large optical depth model that is able to provide the SN 1987A luminosity and is consistent with the OSSE flux. It requires a 57/56 production ratio about twice solar.

Astrophysical implications of the detection by OSSE of Co-57 gamma radiation from SN 1987A are discussed. By burying the alpha-rich-freezeout portion at deeper gamma depths than in published models, it is shown that it remains barely possible that the bolometric luminosity during days 1200-1800 could derive from Co-57 power without requiring 57/56 production ratios greater than twice solar. Alternative mechanisms which may contribute to the bolometric power at late times are proposed.

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma-Ray Observatory has performed several observations of the galactic plane and galactic center region to measure the distribution of galactic 511 keV positron annihilation radiation. Preliminary analysis of data collected during the observation of the galactic center region over the period 13-24 Jun. 1991, indicates the presence of a 511 keV line and positronium continuum superimposed on a power-law continuum. The line of flux was found to be (2.7 +/- 0.5) x 10(exp -4) gamma/sq cm sec, with a positronium fraction of (0.9 +/- 0.2). The 3(sigma) upper limit to daily variations in the 511 keV line flux from the mean during the observation interval is 3 x 10(exp -4) gamma/sq cm sec. If all of the observed annihilation radiation is assumed to originate from the x-ray source 1E 1740.7-2942, the corresponding 511 keV line flux would be (3.0 +/- 0.6) x 10(exp -4) gamma/sq cm sec. The 3(sigma) upper limit for 511 keV line emission from the x-ray binary GX1+4 is 6 x 10(exp -4) gamma/sq cm sec. Results from the galactic plane observations at galactic longitudes of 25 degrees (16-21 Aug. 1991) and 339 degrees (6-11 Sep. 1991) suggest that the emission is concentrated near the galactic center. The observations and the preliminary results are described.

Preliminary results are presented of the Compton Gamma Ray Observatory Oriented Scintillation Spectrometer Experiment (OSSE) observations of the Crab pulsar. The pulsar energy spectra and light curves are in general agreement with previous observations, validating the OSSE pulsar data acquisition modes and data analysis algorithms. The data suggest that the spectrum of the pulsar varies throughout the light curve. The 'interpulse' region has a slightly flatter spectrum in the approx. 60 to 250 keV region and a slightly steeper spectrum at higher energies than the two main pulses. No evidence was found for any lines in the spectra with a typical sensitivity of about 10(exp -4) photons/sq cm/s.

Analysis of the spectra from the Oriented Scintillation Spectrometer Experiment (OSSE) is complicated because of the typically low signal to noise (approx. 0.1 percent) and the large background variability. The OSSE instrument was designed to address these difficulties by periodically offset-pointing the detectors from the source to perform background measurements. These background measurements are used to estimate the background during each of the source observations. The resulting background-subtracted spectra can then be accumulated and fitted for spectral lines and/or continua. Data selection based on various environmental parameters can be performed at various stages during the analysis procedure. In order to achieve the instrument's statistical sensitivity, however, it will be necessary for investigators to develop a detailed understanding of the instrument operation, data collection, and the background spectrum and its variability. A brief description of the major steps in the OSSE spectral analysis process is described, including a discussion of the OSSE background spectrum and examples of several observational strategies.

The Oriented Scintillation Spectrometer Experiment (OSSE) on the Arthur Holly Compton Gamma Ray Observatory is described. An overview of the operation and control of the instrument is given, together with a discussion of typical observing strategies used with OSSE and basic data types produced by the instrument. Some performance measures for the instrument are presented that were obtained from pre-launch and in-flight data. These include observing statistics, continuum and line sensitivity, and detector effective area and gain stability.