Abstract:Abstract. We present a map of 511 keV electron-positron annihilation emission, based on data accumulated with the SPI spectrometer aboard ESA's INTEGRAL gamma-ray observatory, that covers approximately ∼95% of the celestial sphere. Within the exposed sky area, 511 keV line emission is significantly detected towards the galactic bulge region and, at a very low level, from the galactic disk. The bulge emission is highly symmetric and is centred on the galactic centre with an extension of ∼8• (FWHM). The emission… Show more
“…The use of the GEDSAT in our modelling is an hypothesis on the time variability of the real background and, as such, it could introduce some bias in the results. Yet, many analyses of SPI gamma-ray line observations have been performed under this prescription (see Knödlseder et al 2005;Wang et al 2007;Diehl et al 2006, for the annihilation, 60 Fe and 26 Al lines respectively) and produced successful results, so that adopting a similar strategy for the study of the 44 Ti lines seems justified; in addition we will ensure that the results are not affected by systematic effects through a thorough analysis of the residuals.…”
Context. The medium-lived 44 Ti isotope is synthesised by explosive Si-burning in core-collapse supernovae. It is extremely sensitive to the dynamics of the explosion and therefore can be used to indirectly probe the explosion scenario. The young supernova remnant Cassiopeia A is to date the only source of gamma-ray lines from 44 Ti decay. The emission flux has been measured by CGRO/COMPTEL, BeppoSAX/PDS and INTEGRAL/IBIS. Aims. The high-resolution spectrometer SPI on-board the INTEGRAL satellite can provide spectrometric information about the emission. The line profiles reflect the kinematics of the 44 Ti in Cassiopeia A and can thus place constraints on its nucleosynthesis and potentially on the associated explosion process. Methods. Using 4 years of data from INTEGRAL/SPI, we have searched for the gamma-ray signatures from the decay of the 44 Ti isotope. The overwhelming instrumental background noise required an accurate modelling and a solid assessment of the systematic errors in the analysis. Results. Due to the strong variability of the instrumental background noise, it has not been possible to extract the two lines at 67.9 and 78.4 keV. Regarding the high-energy line at 1157.0 keV, no significant signal is seen in the 1140-1170 keV band, thereby suggesting that the line signal from Cassiopeia A is broadened by the Doppler effect. From our spectrum, we derive a ∼500 km s −1 lower limit at 2σ on the expansion velocity of the 44 Ti ejecta. Conclusions. Our result does not allow us to constrain the location of 44 Ti since the velocities involved throughout the remnant, derived from optical and X-ray studies, are all far above our lower limit.
“…The use of the GEDSAT in our modelling is an hypothesis on the time variability of the real background and, as such, it could introduce some bias in the results. Yet, many analyses of SPI gamma-ray line observations have been performed under this prescription (see Knödlseder et al 2005;Wang et al 2007;Diehl et al 2006, for the annihilation, 60 Fe and 26 Al lines respectively) and produced successful results, so that adopting a similar strategy for the study of the 44 Ti lines seems justified; in addition we will ensure that the results are not affected by systematic effects through a thorough analysis of the residuals.…”
Context. The medium-lived 44 Ti isotope is synthesised by explosive Si-burning in core-collapse supernovae. It is extremely sensitive to the dynamics of the explosion and therefore can be used to indirectly probe the explosion scenario. The young supernova remnant Cassiopeia A is to date the only source of gamma-ray lines from 44 Ti decay. The emission flux has been measured by CGRO/COMPTEL, BeppoSAX/PDS and INTEGRAL/IBIS. Aims. The high-resolution spectrometer SPI on-board the INTEGRAL satellite can provide spectrometric information about the emission. The line profiles reflect the kinematics of the 44 Ti in Cassiopeia A and can thus place constraints on its nucleosynthesis and potentially on the associated explosion process. Methods. Using 4 years of data from INTEGRAL/SPI, we have searched for the gamma-ray signatures from the decay of the 44 Ti isotope. The overwhelming instrumental background noise required an accurate modelling and a solid assessment of the systematic errors in the analysis. Results. Due to the strong variability of the instrumental background noise, it has not been possible to extract the two lines at 67.9 and 78.4 keV. Regarding the high-energy line at 1157.0 keV, no significant signal is seen in the 1140-1170 keV band, thereby suggesting that the line signal from Cassiopeia A is broadened by the Doppler effect. From our spectrum, we derive a ∼500 km s −1 lower limit at 2σ on the expansion velocity of the 44 Ti ejecta. Conclusions. Our result does not allow us to constrain the location of 44 Ti since the velocities involved throughout the remnant, derived from optical and X-ray studies, are all far above our lower limit.
“…INTEGRAL has imaged the positron annihilation gamma-rays across the sky in great detail, and confirmed the diffuse nature of annihilation across our Galaxy [70]. The scientific surprise of this emission had already been apparent in earlier results from the Compton Observatory [103], and was consolidated by SPI measurements: The annihilation emission predominantly arises in the inner Galaxy in an extended region of size 10 • .…”
Section: Positron Annihilationmentioning
confidence: 55%
“…The scientific surprise of this emission had already been apparent in earlier results from the Compton Observatory [103], and was consolidated by SPI measurements: The annihilation emission predominantly arises in the inner Galaxy in an extended region of size 10 • . By comparison, the disk of the Galaxy is much fainter, wit a bulge-to-disk intensity ratio of 1.4 from a total luminosity of 2 10 −3 ph cm −2 s −1 [70,144].…”
Penetrating gamma-rays require complex instrumentation for astronomical spectroscopy measurements of gamma-rays from cosmic sources. Multiple-interaction detectors in space combined with sophisticated postprocessing of detector events on ground have lead to a spectroscopy performance which is now capable to provide new astrophysical insights. Spectral signatures in the MeV regime originate from transitions in the nuclei of atoms (rather than in their electron shell). Nuclear transitions are stimulated by either radioactive decays or high-energy nuclear collisions such as with cosmic rays. Gamma-ray lines have been detected from radioactive isotopes produced in nuclear burning inside stars and supernovae, and from energetic-particle interactions in solar flares. Radioactive-decay gamma-rays from 56 Ni directly reflect the source of supernova light. 44 Ti is produced in core-collapse supernova interiors, and the paucity of corresponding 44 Ti gamma-ray line sources reflects the variety of dynamical conditions herein. 26 Al and 60 Fe are dispersed in interstellar space from massive-star nucleosynthesis over millions of years. Gamma-rays from their decay are measured in detail by gamma-ray telescopes, astrophysical interpretations reach from massive-star interiors to dynamical processes in the interstellar medium. Nuclear de-excitation gamma-ray lines have been found in solar-flare events, and convey information about energetic-particle production in these events, and their interaction in the solar atmosphere. The annihilation of positrons leads to another type of cosmic gamma-ray source. The characteristic annihilation gamma-rays at 511 keV have been measured long ago in solar flares, and now throughout the interstellar medium of our Milky Way galaxy. But now a puzzle has appeared, as a surprising predominance of the central bulge region was determined. This requires either new positron sources or transport processes not yet known to us. In this paper we discuss instrumentation and data processing for cosmic gamma-ray spectroscopy, and the astrophysical issues and insights from these measurements.
“…6): the rate of supernovae of type core-collapse is about one supernova every 50 years [47], and radioactive lifetime of 26 Al is 1.04 mission years. Also 60 Fe (τ=3.8±0.05 My) supposedly is a diffuse gamma-ray line source in our Galaxy, and it seems that the lifetime of positrons between their injection and annihilation at rest seems of the order of 10 5 to 10 7 years, as emission in the 511 keV line apparently is diffuse in nature [48,49]. The spatial distribution of candidate sources therefore is only available indirectly, i.e.…”
Section: Diffuse Nuclear Line and Positron Annihilation Gamma Raysmentioning
confidence: 99%
“…INTEGRAL with its spectrometer SPI then allowed us to obtain a more detailed view of the positron annihilation gamma-ray emission [86,49,87,88,89,90,91]. The inner Galaxy with its bulge region was the dominant source, while the disk of the Galaxy was faint, contrasting expectations that positron sources would be related to active star forming regions where massive stars and binary interactions would conspire to create high-energy sources.…”
Section: Positron Sources and Their Annihilation Gamma Raysmentioning
Abstract. Gamma ray lines from cosmic sources convey the action of nuclear reactions in cosmic sites and their impacts on astrophysical objects. Gamma rays at characteristic energies result from nuclear transitions following radioactive decays or highenergy collisions with excitation of nuclei. The gamma-ray line from the annihilation of positrons at 511 keV falls into the same energy window, although of different origin. We present here the concepts of cosmic gamma ray spectrometry and the corresponding instruments and missions, followed by a discussion of recent results and the challenges and open issues for the future. Among the lessons learned are the diffuse radioactive afterglow of massive-star nucleosynthesis in 26 Al and 60 Fe gamma rays, which is now being exploited towards the cycle of matter driven by massive stars and their supernovae; large interstellar cavities and superbubbles have been recognised to be of key importance here. Also, constraints on the complex processes making stars explode as either thermonuclear or core-collapse supernovae are being illuminated by gamma-ray lines, in this case from shortlived radioactivities from 56 Ni and 44 Ti decays. In particular, the three-dimensionality and asphericities that have recently been recognised as important are enlightened in different ways through such gamma-ray line spectroscopy. Finally, the distribution of positron annihilation gamma ray emission with its puzzling bulge-dominated intensity disctribution is measured through spatially-resolved spectra, which indicate that annihilation conditions may differ in different parts of our Galaxy. But it is now understood that a variety of sources may feed positrons into the interstellar medium, and their characteristics largely get lost during slowing down and propagation of positrons before annihilation; a recent microquasar flare was caught as an opportunity to see positrons annihilate at a source.
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