203 research outputs found
Positron Annihilation Gamma Rays from Novae
The potential for observing annihilation gamma rays from novae is investigated. These gamma rays, a unique signature of the thermonuclear runaway models of novae, would result from the annihilation of positrons emitted by beta(+)-unstable nuclei produced near the peak of the runaway and carried by rapid convection to the surface of the nova envelope. Simple models, which are extensions of detailed published models, of the expansion of the nova atmospheres are evolved. These models serve as input into investigations of the fate of nearby Galactic fast novae could yield detectable fluxes of electron-positron annihilation gamma rays produced by the decay of N-13 and F-18. Although nuclear gamma-ray lines are produced by other nuclei, it is unlikely that the fluxes at typical nova distances would be detectable to present and near-future instruments
A Search for Gamma-Ray Lines from the Decay of Fe-59 in Supernova 1987A
We have searched spectra of Supernova (SN) 1987A, accumulated during several 35-day intervals after the explosion by the Solar Maximum Mission (SMM) Gamma Ray Spectrometer (GRS), for gamma-ray lines at 1.099 and 1.292 MeV from the decay of Fe-59 which may have been produced in the progenitor\u27s helium shell. We find no evidence for these lines, down to 3-sigma upper limits approximately = 7 x 10-4 gamma/sq cm/s for the 1.099 MeV line, or approximately = 4.5 x 10-4 gamma/sq cm/s for the 1.292 MeV line, in any 35-day interval. We derive a conservative 3-sigma upper limit on the mass fraction of Fe-59 in the helium shell of 2.9 x 10-3
Gamma ray constraints on the galactic supernova rate
Monte Carlo simulations of the expected gamma-ray signatures of galactic supernovae of all types are performed in order to estimate the significance of the lack of a gamma-ray signal due to supernovae occurring during the last millenium. Using recent estimates of nuclear yields, we determine galactic supernova rates consistent with the historic supernova record and the gamma-ray limits. Another objective of these calculations of galactic supernova histories is their application to surveys of diffuse galactic gamma-ray line emission
Gamma ray constraints on the Galactic supernova rate
We perform Monte Carlo simulations of the expected gamma ray signatures of Galactic supernovae of all types to estimate the significance of the lack of a gamma ray signal due to supernovae occurring during the last millenium. Using recent estimates of the nuclear yields, we determine mean Galactic supernova rates consistent with the historic supernova record and the gamma ray limits. Another objective of these calculations of Galactic supernova histories is their application to surveys of diffuse Galactic gamma ray line emission
Search for Gamma-Ray Transients using the SMM Spectrometer
Observations for transient radiation made by the Gamma Ray Spectrometer on the SMM satellite are summarized. Spectra were obtained from 215 solar flares and 177 gamma-ray bursts. No narrow or moderately broadened lines were observed in any of the bursts. The rate of bursts is consistent with a constant over the mission but is weakly correlated with solar activity. No evidence was found for bursts of 511 keV line emission, unaccompanied by a strong continuum, at levels not less than 0.05 gamma/sq cm s for bursts lasting not more than 16 s. No evidence was found for broad features near 1 MeV from Cyg X-1, the Galactic center, or the Crab in 12-d integrations at levels not less than 0.006 gamma/sq cm s. No evidence was found for transient celestial narrow-line emission from 300 keV to 7 MeV on min-to-hrs-long time scales from 1984 to 1989
Three Dimensional Simulation of Gamma Ray Emission from Asymmetric Supernovae and Hypernovae
Hard X- and -ray spectra and light curves resulting from radioactive
decays are computed for aspherical (jet-like) and energetic supernova models
(representing a prototypical hypernova SN 1998bw), using a 3D energy- and
time-dependent Monte Carlo scheme. The emission is characterized by (1) early
emergence of high energy emission, (2) large line-to-continuum ratio, and (3)
large cut-off energy by photoelectric absorptions in hard X-ray energies. These
three properties are not sensitively dependent on the observer's direction. On
the other hand, fluxes and line profiles depend sensitively on the observer's
direction, showing larger luminosity and larger degree of blueshift for an
observer closer to the polar () direction. Strategies to derive the degree
of asphericity and the observer's direction from (future) observations are
suggested on the basis of these features, and an estimate on detectability of
the high energy emission by the {\it INTEGRAL} and future observatories is
presented. Also presented is examination on applicability of a gray effective
-ray opacity for computing the energy deposition rate in the aspherical
SN ejecta. The 3D detailed computations show that the effective -ray
opacity cm g reproduces the
detailed energy-dependent transport for both spherical and aspherical
(jet-like) geometry.Comment: 24 pages, 13 figures. Figure 7 added in the accepted version. ApJ,
644 (01 June 2006 issue), in press. Resolution of figures lower than the
published versio
Comparative Studies of Line and Continuum Positron Annihilation Radiation
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
Hard X-ray Emission from Cassiopeia A SNR
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, =
2.940.02. 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 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 =2.980.09. 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
SMM Observations of Gamma-Ray Transients. I. A Search for Variable Emission at MeV Energies from Five Galactic and Extragalactic Sources
Transient emission at energies near 1 MeV has been reported by previous experiments on time scales of weeks to months from the Galactic center, the Crab Nebula, and Cyg X-1, and on shorter time-scales from NGC 4151 and Cen A. The spectra of these events fall into two broad classes: a broad line-like feature centered near 1 MeV, and continuum emission (or a very broad feature) extending from ˜600 keV up to several MeV. These features have been interpreted theoretically in terms of emission from hot pair-dominated plasmas, which may be the necessary positron source implied by reports of narrow e- e+ annihilation lines from Cyg X- 1 and the Galactic center. In this paper, data accumulated by the Solar Maximum Mission Gamma Ray Spectrometer (GRS) between 1980 and 1989 have been searched for evidence of these two types of features. We find no compelling evidence for transient 1 MeV broad-line emission on time scales of order 12 d or longer when any of the sources are in the GRS field of view; upper limits on the transient line flux during any 12 d period are typically ˜4.5 × 10-3gamma (cm2 s)-1. The same is true of variability of the continuum between 0.6-7 MeV, for which the upper limits are characteristically ˜2 times the nominal flux from the Crab for any 12 d period. Our analysis was not sensitive enough either to confirm or to reject several reports from other experiments of transient emission in the 0.6-7 MeV continuum during 1980-1989. We withdraw the statement in Share et al. (1993) that our upper limit for one such event, a transient from the Crab Nebula in 1980 Spring, is inconsistent with the measurement of Ling & Dermer (1991). We did not detect any transient emission in the 1 MeV feature preceding or coinciding with reported emission of the 0.511 MeV annihilation line from the Galactic center in 1988-1989. We discuss briefly the implications of this result for models of the annihilation of positrons produced in the Galactic center source 1E 1740.7-2942
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