W. R. Purcell

OSSE Mapping of Galactic 511 keV Positron Annihilation Line Emission

The Oriented Scintillation Spectrometer Experiment (OSSE) observations of the Galactic plane and Galactic center region have been combined with scanning observations by the Transient Gamma-Ray Spectrometer (TGRS) and Solar Maximum Mission (SMM) instruments to produce maps of the Galactic narrow 511 keV positron annihilation line radiation. Two different mapping methods, singular value decomposition and maximum entropy, have been applied to the data. In both cases, the resulting maps show evidence for three distinct features: (1) a central bulge, (2) emission in the Galactic plane, and (3) an enhancement or extension of emission at positive latitudes above the Galactic center. Modeling of the data confirmed the existence of these features. The derived distribution is found to be in good qualitative agreement with nearly all of the historical observations of narrow 511 keV line emission from the Galactic center region. No evidence of time variability is found. Various possible production mechanisms for the observed positrons, including the positive-latitude enhancement, are presented. It is found that supernovae are capable of producing positrons at the required rate to account for the intensity and morphology of the observed 511 keV line emission.

OSSE Observations of Blazars

Abstract : Results are reported on observations obtained with the Oriented Scintillation Spectrometer Experiment (OSSE) instrument on the Compton Gamma Ray Observatory (CGRO) of 17 active galactic nuclei known to exhibit blazar properties at other wavelengths. These observations span the period from 1991 June through 1994 May. Of the 33 high-confidence EGRET detections of blazars during CGRO observing phases 1 and 2 (1991 May 16 - 1993 Sept. 7), OSSE has observed 8 and detected 5, namely 3C 273, 3C 279, PKS 0528+134, CTA 102, and 3C 454.3. Additionally, OSSE has detected the BL LAC sources H 1517+65.6 and PKS 2155-304, which were not detected with EGRET. Variablility in the energy band 50 -150 keV is observed for all of the detected AGN. The OSSE blazar sources are all well described by simple power law models with photon number indices,,, varying from 1.0 to 2.1 among sources. When combined with available, although not necessarily contrmporaneous, COMPTEL and EGRET observations, 4 out of 5 detected blazars show clear evidence for spectral breaks between the hard X-ray and medium energy gamma-ray bands. The exception is the combined OSSE/EGRET data for 3C 279 during 1991 October, where a simple power-law with, approximately equal 1.9 works equally well. Gamma-ray evidence for beaming in CTA 102, PKS 0528+134, and 3C 454.3 is presented.

Oriented Scintillation Spectrometer Experiment observations of Co-57 in SN 1987A

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.

Positron Annihilation Radiation from the Inner Galaxy

The OSSE instrument on the Compton Gamma Ray Observatory (CGRO) was used to measure the one-dimensional latitudinal and longitudinal distributions of positron annihilation radiation (annihilation line and positronium continuum components) along the inner Galactic ridge. Intensity measurements near the Galactic center (differential relative to background fields offset by 9°-12°) show a symmetrical and spatially narrow bulge around the center (effective FWHMs of 49 ± 07 in Galactic latitude and 63 ± 15 in longitude) with significant disk contributions to at least 35° in longitude. The latitude width of the Galactic plane emission away from the Galactic center is only weakly constrained in the current analysis. The longitudinal distribution is well fitted by either a model comprising a narrow (~63 FWHM ) Gaussian bulge plus ~35° FWHM Gaussian and CO-like disk components, or by a center-truncated R1/4 spheroid plus exponential disk model. For the latter model, the observed narrow latitudinal distribution near the Galactic center suggests that the bulge is ellipsoidal, although the fits are consistent with a spherical bulge as well. The positronium fraction estimate from the Galactic center direction is 0.93 ± 0.04. This, combined with the existing narrow line width measurements for the 511 keV annihilation line, suggests that the bulk of the positron annihilation from the Galactic center direction occurs after positronium formation in a warm environment. The inner Galaxy total annihilation radiation flux intensities of ~10.6 × 10-3 to ~13.7 × 10-3 cm-2 s-1 implied by the fitted R1/4 ellipsoid plus exponential disk model (narrow and broad disks, respectively) are in agreement with intensities implied by the 511 keV line flux observed by the broad-field SMM instrument. The double-Gaussian plus CO disk model gives similar agreement if a broad (~12° FWHM) disk component is assumed.

SMM detection of diffuse Galactic 511 keV annihilation radiation

Observations of the 511 keV annihilation line from the vicinity of the Galactic center from October to February for 1980/1981, 1981/1982, 1982/1983, 1984/1985, and 1985/1986 are presented. The measurements were made with the gamma-ray spectrometer on the SMM. The design of the instrument and some of its properties used in the analysis are described, and the methods used for accumulating, fitting, and analyzing the data are outlined. It is shown how the Galactic 511 keV line was separated from the intense and variable background observed in orbit. The SMM observations are compared with previous measurements of annihilation radiation from the Galactic center region, and the astrophysical implications are discussed. It is argued that most of the measurements made to date suggest the presence of an extended Galactic source of annihilation radiation. 37 references.

The pulsed hard X-ray spectrum of PSR B1509-58

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.

Gamma Ray Observations of Cygnus X-1 With the Oriented Scintillation Spectrometer Experiment

Abstract : We report on ~120 days of observations of Cygnus X-1 with OSSE onboard the Compton Observatory. Emission is detected in the range 50 keV to 1 MeV and we find evidence for a continuum of hard X-ray flux levels rather than the existence of distinct flux states. Comparisons of the source spectra with various theoretical models show that an exponentially truncated power law best describes the average spectrum in the OSSE energy band. Although we have measured a new minimum in the hard X-ray flux from the source, no evidence was found for either a broad 1 MeV feature or a narrow 511 keV line previously reported in association with a low flux state. Upper limits on such emission features are an order of magnitude lower than earlier reported detections. The 5.6-day periodicity of the source measured at optical wavelengths was not detected with a sensitivity to the rms modulation fraction of 5% in the 60-140 keV energy band.

Evidence for Shock Acceleration in the Binary Pulsar System PSR B1259-63

The PSR B1259-63 system (Johnston et al. 1992, 1994) was observed near periastron by the Compton Gamma-Ray Observatory in 1994 January. This system contains a rapidly rotating pulsar and a Be star in a highly eccentric binary orbit. We report the discovery by the OSSE instrument of unpulsed emission with a hard power-law spectrum between 50 and 200 keV from the direction of this system. Neither diffuse Galactic background emission nor nearby X-ray binaries contribute significantly to the detected flux. Our results are particularly important for the theory of interaction of pulsars with gaseous environments. We interpret the hard X-ray emission as synchrotron radiation from relativistic particles of the PSR B1259-63 wind being shocked and accelerated within the binary. Our results indicate, for the first time in a binary pulsar, that shock acceleration can increase the original energy of pulsar wind particles by a factor 10, despite the high synchrotron and inverse Compton cooling rates near periastron. The derived shock properties (efficiency, radiation spectrum, timescale) are relevant for a broad class of high-energy astrophysical sources characterized by shocked relativistic plasmas subject to strong radiative cooling.

Limits on a Variable Source of 511 keV Annihilation Radiation near the Galactic Center

The Gamma Ray Spectrometer (GRS) on the Solar Maximum Mission satellite (SMM) has observed a strong Galactic source of 511 keV annihilation radiation from its launch in 1980 to its reentry in 1989. These observations are consistent with an extended source having an intensity of about 0.002 gamma/sq cm/s averaged over the central radian of Galactic longitude. These data are searched for evidence of the variable Galactic center source of 511 keV line radiation which was reported to have reappeared in 1988 by Leventhal et al. The SMM data are consistent with, but do not require, a compact source emitting a time-averaged flux of about 0.0004 gamma/sq cm/s during about 3 month transits in 1987 and 1988; they are inconsistent with a compact source flux in excess of 0.0008 gamma/sq cm/s for each year. 14 refs.

Hard X-rays from SN 1993J

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.