C. Chapuis

The Distance to the Soft Gamma Repeater SGR 1627-41.

We report millimeter observations of the line of sight to the recently discovered soft gamma repeater SGR 1627-41, which has been tentatively associated with the supernova remnant (SNR) G337.0-0.1. Among the eight molecular clouds along the line of sight to SGR 1627-41, we show that SNR G337.0-0.1 is probably interacting with one of the most massive giant molecular clouds (GMCs) in the Galaxy, at a distance of 11 kpc from the Sun. Based on the high extinction to the persistent X-ray counterpart of SGR 1627-41, we present evidence for an association of this new soft gamma repeater (SGR) with the SNR G337.0-0.1; they both appear to be located on the near side of the GMC. This is the second SGR located near an extraordinarily massive GMC. We suggest that SGR 1627-41 is a neutron star with a high transverse velocity ( approximately 1000 km s-1) escaping the young ( approximately 5000 yr) SNR G337.0-0.1.

Compton-backscattered annihilation radiation from the Galactic Center region

On 1989 May 22, the High Energy X-ray and Gamma-ray Observatory for Nuclear Emissions, a balloon-borne high-resolution germanium spectrometer with an 18-deg FOV, observed the Galactic Center (GC) from 25 to 2500 keV. The GC photon spectrum is obtained from the count spectrum by a model-independent method which accounts for the effects of passive material in the instrument and scattering in the atmosphere. Besides a positron annihilation line with a flux of (10.0 +/- 2.4) x 10 exp -4 photons/sq cm s and a full width at half-maximum (FWHM) of (2.9 + 1.0, -1.1) keV, the spectrum shows a peak centered at (163.7 +/- 3.4) keV with a flux of (1.55 +/- 0.47) x 10 exp -3 photons/sq cm s and a FWHM of (24.4 +/- 9.2) keV. The energy range 450-507 keV shows no positronium continuum associated with the annihilation line, with a 2-sigma upper limit of 0.90 on the positronium fraction. The 164 keV feature is interpreted as Compton backscatter of broadened and redshifted annihilation radiation, possibly from the source 1E 1740.7-2942.

The annihilation of positrons in the cold phase of the interstellar medium revisited

The positron cross sections in H and H2 media are reevaluated, taking into account new experimental results. Using a Monte Carlo simulation, we find a positronium fraction before thermalization of 0.90 for H2, in good agreement with the previous experimental result given by Brown et al. (1986). For H we obtain an upper limit of 0.98. We study the behavior of the charge exchange annihilation in a cold phase (molecular cloud). We calculate a formula for the slowing-down time t, before annihilation lasting Delta t, via charge exchange, of a positron beam with a given energy for different medium densities and initial energies. An upper limit of 0.7 MeV for the initial energy of the positrons, annihilating in the molecular cloud G0.86 - 0.08 near the gamma ray source positronium and gives new time constraints on their possible observation.

Observation of SN 1987A with the gamma-ray spectrometer HEXAGONE

The HEXAGONE balloon-borne spectrometer was flown from Alice Springs (Australia) on 1989 May 22. HEXAGONE is a high-resolution gamma-ray spectrometer and consists of an array of twelve cooled germanium detectors (field of view 19° at 511 keV). One of the observed targets was the supernova 1987A and it was seen during 9.9 hr, 818 days after the initial optical outburst. No significant hard X-ray or gamma-ray emission is detected in the final spectrum of SN 1987A

Possible origin of the diffuse galactic 6.7 keV iron line, 511 keV annihilation line and of 1809 keV Al26 line

A recent measurement of the distribution of the 6.7 keV iron line flux along the galactic plane gives the opportunity to compare a model with the experimental results. The model is based on the assumption that supernova ejecta are responsible of the excitation of the interstellar iron and so of the 6.7 keV iron line. For that, we used the current supernova remnant distribution and a recent metallicity gradient. The result of the computation is well correlated with the observed iron flux versus the galactic longitude. The supernovae are progenitors of radioactive nuclei, some of which can decay via β+ emission, for example 56Co, 44Ti and 26Al. We assume that the Galactic source of 511 keV observed, in the direction of the Galactic Center are from two different origins: a steady diffuse component, and a variable point source at or near the Galactic Center. We will consider here the diffuse source of 511 keV only. The 1809 keV gamma‐ray line was discovered in 1982 and confirmed. We propose possible distribut...

New constraints on the positron annihilation media from the direction of the Galactic center based on the 511 KeV line profiles

We present here an analysis of the profiles and intensities of the 511 keV annihilation line observed in the direction of the Galactic center by high energy resolution detectors. We first investigate the HEXAGONE 1989 May data, where the point source was in a rather low state. We find that a warm medium (temperature of 8000 K) can describe the annihilation of the positrons from the diffuse component of the line. We compare these results with the Bell/Sandia 1977, and GRIS 1988 October flights during high states of the central source, and we then show that the discrepancy in the line shape and width between these two sets of data is explained if the time-variable component of the line coming form the annihilation of the positrons emitted by the central source annihilates in a cold medium (temperature around 80 K)

Observation of the galactic 1809 keV gamma‐ray line with the HEXAGONE spectrometer

We report an observation of the galactic 1809 keV gamma-ray line produced by radioactive {sup 26}Al in the interstellar medium. The measurement was performed with our high resolution germanium spectrometer HEXAGONE on a balloon flight in May 1989 from Alice Springs, Australia. Our differential spectrum of the Galactic Center region shows a narrow line at 1809 keV corresponding to a flux of (1.9 +/{minus} 0.9) {center dot} 10{sup {minus}4} photons {center dot} cm{sup {minus}2} s{sup {minus}1} assuming a source at the Galactic Center. We discuss the available observations of the 1809 keV line in the context of models that have been proposed for the origin of the galactic {sup 26}Al.

An observation of the Galactic center region with the HEXAGONE high resolution gamma‐ray spectrometer

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 cm{sup 2} array of cooled germanium detectors with an energy resolution of 2.2 keV at 511 keV and an 18{degree} FWHM field of view. 511 keV gamma-rays from electron-positron annihilation and 1809 keV gamma-rays from the radioactive decay of {sup 26}Al were observed to have fluxes of 8.9{times}10{sup {minus}4} and 1.9{times}10{sup {minus}4} ph/cm{sup 2}-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 {minus}2.6. In the 120--250 keV band the spectrum contains a broad line-like feature with a flux of (2 to 6){times}10{sup {minus}3} ph/cm{sup 2}-s, depending on the assumed underlying continuum. This is interpreted as the result of Compton backscattering of {similar to}511 keV photons from a compact source of electron-positron annihilation radiation.

Observation of the 511 keV annihilation line in the direction of the galactic center with HEXAGONE

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 γcm−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 8.3×104 K for the temperature of the annihilation medium of the positrons.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 γcm−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 8.3×104 K for the temperature of the annihilation medium of the positrons.

A possible correlation between the 511 keV annihilation radiation and the 6.7 keV iron line emission in the galactic plane

Since the 1970’s more than twenty observations of the annihilation radiation from the Galactic Plane have been performed. Previous analysis showed that there could be two components to this emission: (1) a variable, compact source of 511 keV annihilation radiation within the Galactic Centre regions, (2) a steady, diffuse interstellar 511 keV annihilation source. In order to evaluate the contributions of these two components, several models assuming extended sources have been used for the diffuse source emission. For example, the distribution of radiation is consistent with that of high energy gamma rays or molecular (CO) gas. We suppose here that the diffuse 511 keV line intensity distribution follows the iron line (6.7 keV) radiation along the galactic longitude since they could have a common progenitor: supernovae remnants. The result shows that our hypothesis can account for the observations and is consistent with an extended source having an intensity of 2.0×10(−3) ph/cm2/s/rad near the Galactic Centr...