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 pointlike 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 twodimensional 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 .

Type-Ia supernovae result from binary systems that include a carbon-oxygen white dwarf, and these thermonuclear explosions typically produce 0.5 M ! of radioactive 56 Ni. The 56 Ni is commonly believed to be buried deeply in the expanding supernova cloud. Surprisingly, in SN2014J we detected the lines at 158 and 812 keV from 56 Ni decay (τ~8.8 days) earlier than the expected several-week time scale, only ~20 days after the explosion, and with flux levels corresponding to roughly 10% of the total expected amount of 56 Ni. Some mechanism must break the spherical symmetry of the supernova, and at the same time create a major amount of 56 Ni at the outskirts. A plausible explanation is that a belt of helium from the companion star is accreted by the white dwarf, where this material explodes and then triggers the supernova event.SN2014J was discovered on January 22, 2014 (1), in the nearby starburst galaxy M82, and was classified as a supernova of type Ia (SN Ia) (2). This is the closest SN Ia since the advent of gamma-ray astronomy. It reached its optical brightness maximum on Feb 3, 20 days after the explosion on January 14.75 UT (3). At a distance of 3.5 Mpc (4), a most-detailed comparison of models to observations across a wide range of wavelengths appears feasible, including gammaray observations from the 56 Ni decay chain.Calibrated lightcurves of SNe Ia have become standard tools to determine cosmic distances and the expansion history of the universe (5), but we still do not understand the physics that drives their explosion (6,7). Their extrapolation as distance indicators at high redshifts, where their population has not been empirically studied, can only be trusted if a physical model is established (5). Unlike core-collapse supernovae, which obtain their explosion energy from their gravitational energy, SNe Ia are powered by the release of nuclear binding energy through fusion reactions.It is generally believed that carbon fusion reactions ignited in the degenerate matter inside a white dwarf star lead to a runaway. This sudden release of a large amount of nuclear energy is enough to overcome the binding energy of such a compact star, and thus causes a supernova explosion of type Ia. A consensus had been for years that the instability of a white dwarf at the Chandrasekhar-mass limit in a binary system with a main sequence or (red-) giant companion star was the most plausible model to achieve the apparent homogeneity (6). However, observations have revealed an unexpected diversity in type-Ia supernovae in recent years (8), and increasing model sophistication along with the re-evaluations of more exotic explosion scenarios have offered plausible alternatives. The consensus now leans towards a broader range of binary systems and more methods of igniting a white dwarf, independent of its mass. Destabilizing events such as accretion flow instabilities, He detonations, mergers or collisions with a degenerate companion star are being considered (9-12).# This manuscript has been accepted for publication...

Context. Feedback by massive stars shapes the interstellar medium and is thought to influence subsequent star formation. Details of this process are under debate. Aims. We exploited observational constraints on stars, gas and nucleosynthesis ashes for the closest region of recent massive-star formation, Scorpius-Centaurus OB2, and combined them with 3D hydrodynamical simulations, in order to address physics and history for the case of the Scorpius-Centaurus superbubble. Methods. We used published cold gas observations through PLANCK survey data processing, HERSCHEL and APEX, continuum and molecular line observations. We analysed the Galactic All Sky Survey (GASS) to investigate shell structures in atomic hydrogen, and used HIPPARCOS and Gaia data in combination with interstellar absorption against stars to obtain new constraints for the distance to the Hi features. Hot gas is traced in soft X-rays via the ROSAT all sky survey. Nucleosynthesis ejecta from massive stars were traced with new INTEGRAL spectrometer observations via 26 Al radioactivity. We also performed 3D hydrodynamical simulations for the Sco-Cen superbubble. Results. Soft X-rays and a now more significant detection of 26 Al confirm recent (≈ 1 Myr ago) input of mass, energy and nucleosynthesis ejecta, likely by a supernova in the Upper Scorpius (USco) subgroup. We confirm a large supershell around the entire OB association and perform a 3D hydrodynamics simulations with a conservative massive star population that reproduces the morphology of the superbubble. High resolution GASS observations of a nested supershell reveal that it is filamentary possibly related to the Vishniac clumping instability, but molecular gas (Lupus I) is only present where the shell coincides with the connecting line between the subgroups of the OB association, suggesting a connection to the cloud, probably an elongated sheet, out of which the OB association formed. Stars have formed sequentially in the subgroups of the OB association and currently form in Lupus I. To investigate the impact of massive star feedback on extended clouds, we simulate the interaction of a turbulent cloud with the hot, pressurised gas in a superbubble. The hot gas fills the tenuous regions of the cloud and compresses the denser parts. Stars formed in these dense clumps would have distinct spatial and kinematic distributions. Conclusions. The combined results from observations and simulations are consistent with a scenario where dense gas was initially distributed in a band elongated in the direction now occupied by the OB association. Superbubbles powered by massive stars would then repeatedly break out of the elongated parent cloud, surround and squash the denser parts of the gas sheet and thus induce more star formation. The expected spatial and kinematic distribution of stars is consistent with observations of Sco-Cen. The scenario might apply to many similar regions in the Galaxy and also to AGN-related superbubbles.

Context. The detection and measurement of gamma-ray lines from the decay chain of 56 Ni provides unique information about the explosion in supernovae. SN2014J at 3.3 Mpc is a sufficiently-nearby supernova of type Ia so that such measurements have been feasible with the gamma-ray spectrometer SPI on ESA's INTEGRAL gamma-ray observatory. Aims. The 56 Ni freshly produced in the supernova is understood to power the optical light curve, because it emits gamma rays upon its radioactive decay first to 56 Co and then to 56 Fe. Gamma-ray lines from 56 Co decay are expected to become directly visible through the white dwarf material several weeks after the explosion, as they progressively penetrate the overlying material of the supernova envelope, which is diluted as it expands. The lines are expected to be Doppler-shifted or broadened from the kinematics of the 56 Ni ejecta. We aim to exploit high-resolution gamma-ray spectroscopy with the SPI spectrometer on INTEGRAL toward constraining the 56 Ni distribution and kinematics in this supernova. Methods. We use the observations with the SPI spectrometer on INTEGRAL, together with an improved instrumental background method. Results. We detect the two main lines from 56 Co decay at 847 and 1238 keV, which are significantly Doppler-broadened, and at intensities (3.65 ± 1.21) × 10 −4 and (2.27 ± 0.69) × 10 −4 ph cm −2 s −1 , respectively, at their brightness maximum. We measure their rise toward a maximum after about 60-100 days and a decline thereafter. The intensity ratio of the two lines is found to be consistent with expectations from 56 Co decay (0.62 ± 0.28 at brightness maximum, the expected ratio is 0.68). We find that the broad lines seen in the late, gamma-ray transparent phase are not representative of the early gamma-ray emission, and notice instead that the emission spectrum is complex and irregular until the supernova is fully transparent to gamma rays, with progressive uncovering of the bulk of 56 Ni. We infer that the explosion morphology is not spherically symmetric, both in the distribution of 56 Ni and in the unburnt material which occults the 56 Co emission. After we compare light curves from different plausible models, the resulting 56 Ni mass is determined to be 0.49 ± 0.09 M .