P. Wallyn

The Distance of the Soft Gamma Repeater SGR 1806–20

We present 12CO (J = 1-0) observations in the direction of the soft gamma repeater SGR 1806-20 with the SEST telescope. We detected several molecular clouds, and we discuss in this paper the implications of these observations for the distance to the X-ray counterpart AX 1805.7-2025, the supernova remnant G10.0-0.3 and the very luminous O9-B2 star detected in the line of sight. The distance of SGR 1806-20 is estimated to be 14.5 ± 1.4 kpc and this soft gamma repeater is very likely associated with one of the brightest H II regions in the Galaxy, W31. The large size of G10.0-0.3 (25 × 38 pc) for a young supernova remnant possibly powered by a central pulsar (AX 1805.7-2025) indicates that G10.0-0.3 could be expanding in the very low density region produced by the wind of the blue star.

Gamma-Ray Spectra and Variability of Cygnus X-1 Observed by BATSE

We present new BATSE Earth occultation observations of the 25 keV-1.8 MeV spectrum and variability of Cyg X-1 made between 1993 August and 1994 May. We observed that the normal soft γ-ray spectrum (γ2) of Cyg X-1 has two components: a Comptonized part seen below 300 keV and a high-energy tail in the 0.3-2 MeV range that was only hinted at in previous γ2 spectrum observed by HEAO 3. The source went through an extended sequence of changes between 1993 August and 1994 May; the 45-140 keV flux first decreased steadily from ~γ2 to below the γ1 flux level seen previously by HEAO 3 in 1979 to a new level, γ0, roughly one-quarter of its intensity over a period of ~140 days. The flux remained at this low level for about 40 days before returning swiftly (~20 days) to approximately the initial γ2 level. The γ2 spectrum may be interpreted in terms of an interacting two-region model, consisting of a high-temperature (~210-250 keV) core embedded in an ~50 keV corona. In this scenario, the observed 25-300 keV photons were produced by Compton scattering of soft photons (~0.5 keV) by the hot electrons in the outer corona. These same hard X-rays were further upscattered by a population of energetic electrons in the inner core, producing the spectral tail above 300 keV. During the excursion of the 45-140 keV flux from the γ2 to the γ0 level, the spectrum evolved to a form consistent with either a power law with a photon index of ~2.6 or a single-temperature Compton model with an electron temperature, kT, of 108 ± 11 keV and an optical depth, τ, of 0.40 ± 0.06 and then returned essentially to the original γ2 spectrum at the end of the active period. The overall cooling of the system during the low-flux period may be due to an increase in the soft photon population that effectively quenched the hot electrons in these regions through Compton scattering.

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...

BATSE Search for the ~540 keV Feature in the Crab Spectrum Observed by Granat-SIGMA

We report the first simultaneous observations of the γ-ray spectrum of the Crab in the energy range 25-1.7 MeV made by the BATSE and SIGMA experiments. These observations were made on 1992 March 10-11. We report specifically results of a search in the JPL BATSE Earth Occultation database for the transient high-energy feature at ~540 keV reported by Granat-SIGMA during this period. We obtain a null result for this feature on both March 10 and March 11. The time-averaged continuum spectrum in the 25 keV-MeV range for the 2 day average spectrum is consistent with a single power law, with photon index of 2.25 ± 0.02. The best-fit line flux for either a narrow (0 keV FWHM) feature at 541 keV or a broad (44 keV FWHM) feature at 536 keV of the same 2 day spectrum is (-1.2 ± 0.8) × 10-3 and (-1.3 ± 0.8) × 10-3 photons cm-2 s-1, compared to (4.3 ± 1.2) × 10-3 and (5.1 ± 1.7) × 10-3 photons cm-2 s-1, respectively, reported by the SIGMA investigators. The difference of the line fluxes measured simultaneously by the BATSE and SIGMA experiments is estimated to be 3.8 σ significance for the narrow feature and 3.4 σ significance for the broad feature.

Millimeter and infrared observations of SGR 1806-20

We present CO(1-0) observations toward the soft gamma-ray repeater SGR 1806-20. We discuss the implications on the distance to the X-ray counterpart: AX 1805.7-2025. We also present an upper limit at λ = 1.3 mm for the thermal emission from dust and high resolution IRAS maps of the region. SGR 1806-20 is very likely associated with the H II complex W31. The G10.0-0.3 supernova remnant (SNR) could be expanding in the very low density region produced by the wind of the bright O9-B2 star recently detected.

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)

How many positron sources in the galactic center region

On 1989 May 22, the HEXAGONE experiment observed a broad region (19° FWHM) in the direction of the center of the Galaxy, and detected a narrow 511 keV line (Durouchoux et al. 1993) as well as a broad emission feature around 170 keV (Smith et al. 1993)