INTEGRAL/IBIS search for e-e+ annihilation radiation from the Galactic Center Region

Electron-positron annihilation radiation from the Galactic Center region has been detected since the seventies, but its astrophysical origin is still a topic of a scientific debate. We have analyzed data of the gamma-ray imager IBIS/ISGRI onboard of ESA's INTEGRAL platform in the e$^{-}$e$^{+}$ line. During the first year of the missions Galactic Center Deep Exposure no evidence for point sources at 511 keV has been found in the ISGRI data; the $2 \sigma$ upper limit for resolved single point sources is estimated to be $1.6\times 10^{-4} ph cm^{-2} s^{-1}$.Comment: 6 pages, 3 figures; Cospar 2004. To be published in: Advances in Space Researc

Experimental Results Obtained with the Positron-Annihilation Radiation Telescope of the Toulouse-Argonne Collaboration

We present laboratory measurements obtained with a ground-based prototype of a focusing positron-annihilation-radiation telescope developed by the Toulouse-Argonne collaboration. This balloon-borne telescope has been designed to collect 511-keV photons with an extremely low instrumental background. The telescope features a Laue diffraction lens and a detector module containing a small array of germanium detectors. It will provide a combination of high spatial and energy resolution (15 arc sec and 2 keV, respectively) with a sensitivity of {approximately}3{times}10{sup {minus}5} photons cm{sup {minus}2}s{sup {minus}1}. These features will allow us to resolve a possible narrow 511-keV line both energetically and spatially within a Galactic center ``microquasar`` or in other broad-class annihilators. The ground-based prototype consists of a crystal lens holding small cubes of diffracting germanium crystals and a 3{times}3 germanium array that detects the concentrated beam in the focal plane. Measured performances of the instrument at different line energies (511 keV and 662 keV) are presented and compared with Monte-Carlo simulations. The advantages of a 3{times}3 Ge-detector array with respect to a standard-monoblock detector have been confirmed. The results obtained in the laboratory have strengthened interest in a crystal-diffraction telescope, offering new perspectives for die future of experimental gamma-ray astronomy

A space bourne crystal diffraction telescope for the energy range of nuclear transitions

Recent experimental work of the Toulouse-Argonne collaboration has opened for perspective of a focusing gamma-ray telescope operating in the energy range of nuclear transitions, featuring unprecedented sensitivity, angular and energy resolution. The instrument consists of a tunable crystal diffraction lens situated on a stabilized spacecraft, focusing gamma-rays onto a small array of Germanium detectors perched on an extendible boom. While the weight of such an instrument is less than 500 kg, it features an angular resolution of 15 in., an energy resolution of 2 keV and a 3 {sigma} narrow line sensitivity of a few times 10{sup {minus}7} photons s{sup {minus}1} cm{sup {minus}2} (10{sup 6} sec observation). This instrumental concept permits observation of any identified source at any selected line-energy in a range of typically 200 keV to 1300 keV. The resulting ``sequential`` operation mode makes sites of explosive nucleosynthesis natural scientific objectives for such a telescope: the nuclear lines of extragalactic supernovae ({sup 56}Ni, {sup 44}Ti, {sup 60}Fe) and galactic novae (p{sup {minus}}p{sup +} line, {sup 7}Be) are accessible to observation, one at a time, due to the erratic appearance and the sequence of half-lifes of these events. Other scientific objectives, include the narrow 511 keV line from galactic broad class annihilators (such as 1E1740-29, nova musca) and possible redshifted annihilation lines from AGN`s

Crystal diffraction lens telescope for focusing nuclear gamma rays

A crystal diffraction lens was constructed at Argonne National Laboratory for use as a telescope to focus nuclear gamma rays. It consisted of 600 single crystals of germanium arranged in 8 concentric rings. The mounted angle of each crystal was adjusted to intercept and diffract the incoming gamma rays with an accuracy of a few arc sec. The performance of the lens was tested in two ways. In one case, the gamma rays were focused on a single medium size germanium detector. In the second case, the gamma rays were focused on the central germanium detector of a 3 x 3 matrix of small germanium detectors. The efficiency, image concentration and image quality, and shape were measured. The tests performed with the 3 x 3 matrix detector system were particularly interesting. The wanted radiation was concentrated in the central detector. The 8 other detectors were used to detect the Compton scattered radiation, and their energy was summed with coincident events in the central detector. This resulted in a detector with the efficiency of a large detector (all 9 elements) and the background of a small detector (only the central element). The use of the 3 x 3 detector matrix makes it possible to tell if the source is off axis and, if so, to tell in which direction. The crystal lens acts very much like a simple convex lens for visible light. Thus if the source is off to the left then the image will focus off to the right illuminating the detector on the right side: telling one in which direction to point the telescope. Possible applications of this type of crystal lens to balloon and satellite experiments will be discussed

Review of Crystal Diffraction and its Application to Focusing Energetic Gamma Rays

The basic features of crystal diffraction and their application to the construction of a crystal diffraction lens for focusing energetic gamma rays are described using examples from the work preformed at the Argonne National Laboratory. Both on-axis and off-axis performance are discussed. The review includes of normal crystals, bent crystals, and crystals with variable crystal-plane spacings to develop both condenser-type lenses and point-to-point imaging lenses

Spectral analysis of the Galactic e+e- annihilation emission

We present a spectral analysis of the e+e- annihilation emission from the Galactic Centre region based on the first year of measurements made with the spectrometer SPI of the INTEGRAL mission. We have found that the annihilation spectrum can be modelled by the sum of a narrow and a broad 511 keV line plus an ortho-Ps continuum. The broad line is detected with a flux of (0.35+/-0.11)e-3 s-1 cm-2. The measured width of 5.4+/-1.2 keV FWHM is in agreement with the expected broadening of 511 keV photons emitted in the annihilation of Ps that are formed by the charge exchange process of slowing down positrons with H atoms. The flux of the narrow line is (0.72+/-0.12)e-3 s-1 cm-2 and its width is 1.3+/-0.4 keV FWHM. The measured ortho-Ps continuum flux yields a fraction of Ps of (96.7+/-2.2)%. To derive in what phase of the interstellar medium positrons annihilate, we have fitted annihilation models calculated for each phase to the data. We have found that 49(+2,-23)% of the annihilation emission comes from the warm neutral phase and 51(+3,-2)% from the warm ionized phase. While we may not exclude that less than 23% of the emission might come from cold gas, we have constrained the fraction of annihilation emission from molecular clouds and hot gas to be less than 8% and 0.5%, respectively. We have compared our knowledge of the interstellar medium in the bulge and the propagation of positrons with our results and found that they are in good agreement if the sources are diffusively distributed and if the initial kinetic energy of positrons is lower than a few MeV. Despite its large filling factor, the lack of annihilation emission from the hot gas is due to its low density, which allows positrons to escape this phase.Comment: 12 pages, 6 figures, accepted in A&

Gamma-ray spectroscopy of positron annihilation in the Milky Way

T. Siegert, et al., “Gamma-ray spectroscopy of positron annihilation in the Milky Way”, Astronomy & Astrophysics, Vol. 586, January 2016, https://doi.org/10.1051/0004-6361/201527510. Reproduced with permission from Astronomy & Astrophysics, © ESO”.Context. The annihilation of positrons in the Galaxy's interstellar medium produces characteristic gamma-rays with a line at 511 keV. This gamma-ray emission has been observed with the spectrometer SPI on ESA's INTEGRAL observatory, confirming a puzzling morphology with bright emission from an extended bulge-like region, while emission from the disk is faint. Most known or plausible sources of positrons are, however, believed to be distributed throughout the disk of the Milky Way. Aims: We aim to constrain characteristic spectral shapes for different spatial components in the disk and bulge using data with an exposure that has doubled since earlier reports. Methods: We exploit high-resolution gamma-ray spectroscopy with SPI on INTEGRAL based on a new instrumental background method and detailed multi-component sky model fitting. Results: We confirm the detection of the main extended components of characteristic annihilation gamma-ray signatures, altogether at 58σ significance in the 511 keV line. The total Galactic 511 keV line intensity amounts to (2.74 ± 0.25) × 10-3 ph cm-2 s-1 for our assumed model of the spatial distribution. We derive spectra for the bulge and disk, and a central source modelled as point-like and at the position of Sgr A*, and discuss spectral differences. The bulge (56σ) shows a 511 keV line intensity of (0.96 ± 0.07) × 10-3 ph cm-2 s-1 together with ortho-positronium continuum equivalent to a positronium fraction of (1.080 ± 0.029). The two-dimensional Gaussian that represents the disk emission (12σ) has an extent of 60+10-5 degrees in longitude and a rather large latitudinal extent of 10.5+2.5-1.5 degrees; the line intensity is (1.66 ± 0.35) × 10-3 ph cm-2 s-1 with a marginal detection of the annihilation continuum and an overall diffuse Galactic continuum of (5.85 ± 1.05) × 10-5 ph cm-2 s-1 keV-1 at 511 keV. The disk shows no flux asymmetry between positive and negative longitudes, although spectral details differ. The flux ratio between bulge and disk is (0.58 ± 0.13). The central source (5σ) has an intensity of (0.80 ± 0.19) × 10-4 ph cm-2 s-1.Peer reviewe

Astrophysical constraints from gamma-ray spectroscopy

Gamma-ray lines from cosmic sources provide unique isotopic information, since they originate from energy level transitions in the atomic nucleus. Gamma-ray telescopes explored this astronomical window in the past three decades, detecting radioactive isotopes that have been ejected in interstellar space by cosmic nucleosynthesis events and nuclei that have been excited through collisions with energetic particles. Short-lived radioactivities have been detected in a couple of supernovae (56Co and 57Co in SN1987A, 44Ti in Cas A), the diffuse glow of long-lived 26Al has been mapped along the entire plane of the Galaxy, several excited nuclei have been detected in solar flares, and, last but not least, positron annihilation has been observed in the inner Galaxy since the 70ies. Recent imaging and line shape measurements of e-/e+ annihilation emission from the Galactic bulge can hardly be accounted for by conventional sources of positrons; recent 26Al emission and line width measurement from the inner Galaxy and from the Cygnus region can constrain the properties of the interstellar medium; a diffuse 60Fe gamma-ray line emission appears rather weak, in view of current theoretical predictions. Recent Galactic core-collapse supernovae are studied through 44Ti radioactivity, but, apart from Cas A, no other source has been found. The characteristic signature of 22Na-line emission from a nearby O-Ne-Mg novae is expected to be measured during INTEGRAL's lifetime.Comment: 30 pages, 11 figures; accepted for publication in Nucl.Phys.A, special volume on Nuclear Astrophysics, Eds. K.-H. Langanke, F.-K. Thielemann, M. Wiesche