摘要:

A review of observations of the positron annihilation 511 keV line from the galactic center and plane is presented. Emphasis is given to the new data from balloon observations in 1988 and 1989. The positron annihilation source in the direction of the center was found to be turned on again in 1988 October after being observed in a quiescent state since the early 1980’s. There is evidence for variability between May and October in 1988. The new observations give the first data in which the line is spectrally resolved and in which the galactic center and plane components of the annihilation radiation are independently measured. These data support the two‐component model of the emission with a variable point source near the center and a steady‐state source distributed along the galactic plane. Comparing measurements of narrow and wide field instruments in the 1980’s gives a flux of ∼8×10−4photons cm−2s−1for the point source in its high state and a flux of ∼1.2×10−3photons cm−2s−1rad−1for the galactic plane component with a relatively flat distribution over the central radian. The line width for the point source component is ∼3 keV FWHM.

ISSN:0094-243X    

DOI:10.1063/1.40912

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Series of observations of the Galactic Center region in the soft gamma‐ray regime (up to 1.3 MeV) have been performed by theSIGMAtelescope aboard theGRANATspace observatory on March, April, September and October 1990. Analysis of the March and April data has revealed the presence of two prominent point sources: 1E 1740.7–2942, a bright soft gamma‐ray source 0.9° away from the galactic nucleus, and a previously unknown source, GRS 1758–258, 0.7° apart from GX 5–1. This paper reports on all the observational material so far analyzed, including spectral evidence for transient electron−positron annihilation radiation from the soft gamma‐ray source 1E 1740.7–2942.

ISSN:0094-243X    

DOI:10.1063/1.40933

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The most important results of the Galactic Center observations by GRANAT during 1990 are: discovery of new very hard X‐ray source GRS1758–258 in 40 arcmin from GX5‐1, strong variability of Sgr A* in 4–30 keV energy range, unambiguous detection of 1E1740.7–2942 as the brightest hard X‐ray source in the vicinity of the Galactic Center and discovery of the variable hard component in its spectrum. The shape of the latter allows one to suppose the relation of this component with electron‐positron annihilation. The hardness of the spectra of both 1E1740.7–2942 and GRS1758–258 gives strong arguments to include them to the list of black hole candidates.

ISSN:0094-243X    

DOI:10.1063/1.40947

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The x‐ray pulsar GX1+4, located near the galactic center, is a candidate for the 511 keV line source observed from this region of sky. Recent results suggest that GX1+4 is returning to a state similar to that observed during the seventies, when also the 511 keV point source in the galactic center was active. Here we present the results of SIGMA observations of GX1+4 in the energy range 35 keV–1.3 MeV, indicating a decrease in the spin‐down rate. The SIGMA unprecedented angular resolution in this energy range allows us to place an upper limit on the 511 keV flux from this particular source.

ISSN:0094-243X    

DOI:10.1063/1.40917

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The galactic center region was observed for 6 hours on 22 May 1989 from a high altitude balloon with the HEXAGONE high resolution gamma‐ray spectrometer. The instrument had a 285 cm2array of cooled germanium detectors with an energy resolution of 2.2 keV at 511 keV and an 18° FWHM field of view. 511 keV gamma‐rays from electron‐positron annihilation and 1809 keV gamma‐rays from the radioactive decay of26Al were observed to have fluxes of 8.9×10−4and 1.9×10−4ph/cm2‐s, respectively. Continuum emission was detected from 20 to 800 keV and preliminary results have been obtained for the spectrum. Below 120 keV this is well described by power law with a slope of −2.6. In the 120–250 keV band the spectrum contains a broad line‐like feature with a flux of (2 to 6)×10−3ph/cm2‐s, depending on the assumed underlying continuum. This is interpreted as the result of Compton backscattering of ∼511 keV photons from a compact source of electron‐positron annihilation radiation.

ISSN:0094-243X    

DOI:10.1063/1.40922

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The HEXAGONE balloon‐borne spectrometer has flown on 22 May 1989. HEXAGONE is a high resolution gamma‐ray spectrometer and consists of an array of twelve cooled germanium detectors. One of the observed targets was the Galactic Center and its vicinity (field of view 19° at 511 keV) and it was seen during 6.3 hours. The 511 keV annihilation line was observed with a flux of (8.88±2.67)×10−4&ggr;cm−2 s−1, a width 1.09+1.38, −1.09 keV and its centroid at 511.54±0.38 keV. The results are consistent with an upper limit of 8.3×104K for the temperature of the annihilation medium of the positrons.

ISSN:0094-243X    

DOI:10.1063/1.40897

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Using the all‐sky map obtained with the Medium Energy Detectors (80 keV–2 MeV) of the UCSD/MIT Hard X‐ray and Low‐Energy Gamma Ray Experiment on HEAO‐1, a joint determination of the point source and the galactic ridge components of 511 keV annihilation radiation has been made. The ridge determination assumes the longitudinal distribution measured from 90–270 keV, which has a strong central concentration of width about 120°. Spectral fits assumed an unresolved annihilation line centered at 511 keV and orthopositronium continuum with positronium fraction f=0.9. Detector resolution was 9% at 511 keV, and field of view was 16° FWHM. The point source flux is significant at 95% confidence, and is in reasonable agreement with that measured with HEAO‐3 one to two years later in 1980. The ridge component is not significantly measured, but is consistent with a measurement by GRIS at 25° longitude, and the derived two sigma limit lies well below the ∼2.5×10−3photon/(cm2‐s‐rad), averaged over the central radian, obtained with SMM observations.

ISSN:0094-243X    

DOI:10.1063/1.40898

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The Galactic Center region, including the high energy emitter 1E1740.7–2942, has been observed on May 17, 1989 in the energy range 15–150 keV. The measurement was performed with the balloon borne experiment ‘‘POKER’’ based on an array of three high pressure Multiwire Proportional counters (MWPC) with energy resolution of 10% at 120 keV. The total geometric area was 8,000 cm2and the experiment was passively collimated with copper hexagonal collimator with 1.9 degree FWHM field of view. Data analysis is still in progress and preliminary results are presented.

ISSN:0094-243X    

DOI:10.1063/1.40899

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Observations show that there are two components of positron annihilation radiation from the region of the Galactic Center: a variable component resulting from one or just a few compact sources at or near the Galactic Center and a steady, diffuse component resulting from positron annihilation in the galactic disk. We model the diffuse component using the observed longitude distributions of 70–150 MeV gamma rays, CO, and hot plasma revealed by Fe line emission. We review recent results on positron annihilation in the interstellar medium and discuss the implications of the annihilation processes on the fraction of positrons annihilating via positronium and on the shape of the 511 keV annihilation line. We also review the sources of diffuse galactic positrons and discuss the nature of the compact source of annihilation radiation near the Galactic Center.

ISSN:0094-243X    

DOI:10.1063/1.40900

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We show that Compton scattering of 511 keV electron‐positron annililation radiation produces a line‐like reflection feature at ∼170 keV from backscattered photons. Assuming a simple model of an accretion disk around a compact source, we explore the spectrum of Compton scattered annihilation line emission for a range of disk geometries, opacities, and observing angles, and we find that the line‐like feature is produced under a wide variety of conditions. We further show that such Compton backscattering of slightly redshifted annihilation line emission from the inner edge of an accretion disk could account for the 170 keV line emission and higher energy continuum observed1–3together with the 511 keV annihilation radiation from the direction of the Galactic center. Identification of the observed 170 keV line as an annihilation line reflection feature provides direct new evidence for the existence of an accreting stellar‐mass black‐hole source of annihilation radiation at or near the Galactic center.

ISSN:0094-243X    

DOI:10.1063/1.40901

出版商:AIP    

年代:1991

数据来源: AIP