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Gamma-ray line emission from radioactive decay of 60Fe provides constraints on nucleosynthesis in massive stars and supernovae. The spectrometer SPI on board INTEGRAL has accumulated nearly three years of data on gamma-ray emission from the Galactic plane. We have analyzed these data with suitable instrumental-background models and sky distributions to produce high-resolution spectra of Galactic emission. We detect the gamma-ray lines from 60Fe decay at 1173 and 1333 keV, obtaining an improvement over our earlier measurement of both lines with now 4.9 sigma significance for the combination of the two lines. The average flux per line is (4.4 pm 0.9) times 10^{-5} ph cm^{-2} s^{-1} rad^{-1} for the inner Galaxy region. Deriving the Galactic 26Al gamma-ray line flux with using the same set of observations and analysis method, we determine the flux ratio of 60Fe/26Al gamma-rays as 0.148 pm 0.06. The current theoretical predictions are still consistent with our result.
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It is believed that core-collapse supernovae (CCSN), occurring at a rate about once per century, have seeded the interstellar medium with long-lived radioisotopes such as 60Fe (half-life 1.5 Myr), which can be detected by the gamma rays emitted when they beta-decay. Here we report the detection of the 60Fe decay lines at 1173 keV and 1333 keV with fluxes 3.7 +/- 1.1 x 10(-5) ph cm(-2) s(-1) per line, in spectra taken by the SPI spectrometer on board INTEGRAL during its first year. The same analysis applied to the 1809 keV line of 26Al yielded a line flux ratio 60Fe/26Al = 0.11 +/- 0.03. This supports the hypothesis that there is an extra source of 26Al in addition to CCSN.
Aims: We present a study of the diffuse X-ray emission in the halo and the disc of the starburst galaxy NGC 253. Methods: After removing point-like sources, we analysed XMM-Newton images, hardness ratio maps and spectra from several regions in the halo and the disc. We introduce a method to produce vignetting corrected images from the EPIC pn data, and we developed a procedure that allows a correct background treatment for low surface brightness spectra, using a local background, together with closed filter observations. Results: Most of the emission from the halo is at energies below 1 keV. In the disc, also emission at higher energies is present. The extent of the diffuse emission along the major axis of the disc is 13.6 kpc. The halo resembles a horn structure and reaches out to ~9 kpc perpendicular to the disc. Disc regions that cover star forming regions, like spiral arms, show harder spectra than regions with lower star forming activity. Models for spectral fits of the disc regions need at least three components: two thermal plasmas with solar abundances plus a power law and galactic foreground absorption. Temperatures are between 0.1 and 0.3 keV and between 0.3 and 0.9 keV for the soft and the hard component, respectively. The power law component may indicate an unresolved contribution from X-ray binaries in the disc. The halo emission is not uniform, neither spatially nor spectrally. The southeastern halo is softer than the northwestern halo. To model the spectra in the halo, we needed two thermal plasmas with solar abundances plus galactic foreground absorption. Temperatures are around 0.1 and 0.3 keV. A comparison between X-ray and UV emission shows that both originate from the same regions.
The isotopes $^{60}$Fe and $^{26}$Al originate from massive stars and their supernovae, reflecting ongoing nucleosynthesis in the Galaxy. We studied the gamma-ray emission from these isotopes at characteristic energies 1173, 1332, and 1809 keV with over 15 years of SPI data, finding a line flux in $^{60}$Fe combined lines of $(0.31pm 0.06) times 10^{-3}$ ph cm$^{-2}$ s$^{-1}$ and the $^{26}$Al line flux of $(16.8pm 0.7) times 10^{-4}$ ph cm$^{-2}$ s$^{-1}$ above the background and continuum emission for the whole sky. Based on the exponential-disk grid maps, we characterise the emission extent of $^{26}$Al to find scale parameters $R_0 =7.0^{+1.5}_{-1.0}$ kpc and $z_0=0.8^{+0.3}_{-0.2}$ kpc, however the $^{60}$Fe lines are too weak to spatially constrain the emission. Based on a point source model test across the Galactic plane, the $^{60}$Fe emission would not be consistent with a single strong point source in the Galactic center or somewhere else, providing a hint for a diffuse nature. We carried out comparisons of emission morphology maps using different candidate-source tracers for both $^{26}$Al and $^{60}$Fe emissions, and suggests that the $^{60}$Fe emission is more likely to be concentrated towards the Galactic plane. We determine the $^{60}$Fe/$^{26}$Al $gamma$-ray flux ratio at $(18.4pm4.2),%$ , when using a parameterized spatial morphology model. Across the range of plausible morphologies, it appears possible that $^{26}$Al and $^{60}$Fe are distributed differently in the Galaxy. Using the best fitting maps for each of the elements, we constrain flux ratios in the range 0.2--0.4. We discuss its implications for massive star models and their nucleosynthesis.
We show that the well-known discrepancy between the radial dependence of the Galactic cosmic ray (CR) nucleon distribution, as inferred most recently from EGRET observations of diffuse gamma-rays above 100 MeV, and of the most likely CR source distribution (supernova remnants, pulsars) can be explained purely by PROPAGATION effects. Contrary to previous claims, we demonstrate that this is possible, if the dynamical coupling between the escaping CRs and thermal plasma is taken into account, and thus a self-consistent GALACTIC WIND calculation is carried out. Given a dependence of the CR source distribution on Galactocentric radius, r, our numerical wind solutions show that the CR outflow velocity, V(r,z) depends both on r, and on vertical distance, z, at reference level z_C. The latter is defined as the transition boundary from diffusion to advection dominated CR transport and is therefore also a function of r. In fact, the CR escape time averaged over particle energies decreases with increasing CR source strength. Such an increase is counteracted by a reduced average CR residence time in the gas disk. Therfore pronounced peaks in the radial source distribution result in mild radial gamma-ray gradients at GeV energies, as it has been observed. This effect is enhanced by anisotropic diffusion, assuming different radial and vertical diffusion coefficients. We have calculated 2D analytic solutions of the stationary diffusion-advection equation, including anisotropic diffusion, for a given CR source distribution and a realistic outflow velocity field V(r,z), inferred from self-consistent numerical Galactic Wind simulations. At TeV energies the gamma-rays from the sources are expected to dominate the observed diffuse flux from the disk. Its observation should allow an empirical test of the theory presented.
The INTEGRAL observatory has been performing a deep survey of the Galactic central radian since 2003, with the goal of both extracting a catalog of sources and gaining insight into the Galactic diffuse emission. This paper concentrates on the estimation of the total point sources emission contribution. It is now clear that unresolved point sources contribute to the observed diffuse emission; the increasing sensitivity of instruments with time has lead to a steady decrease in estimates of this ``diffuse emission. We have analysed the first year data obtained with the spectrometer and imager SPI on board INTEGRAL. First, a catalog of 63 hard X-ray sources detected, time-averaged, during our 2003 Galactic plane survey, is derived. Second, after extracting the spectra of the sources detected by SPI, their combined contribution is compared to the total (resolved and unresolved) emission from the Galactic ridge. The data analysis is complex: it requires us to split the total emission into several components, as discrete sources and diffuse emission are superimposed in SPI data. The main result is that point source emission dominates in the hard X-ray/soft $gamma$-ray domain, and contributes around 90 % of the total emission around 100 keV, while above 250 keV, diffuse electron-positron annihilation, through its three-photon positronium continuum with a positronium fraction $sim$ 0.97 and the 511 keV electron-positron line, dominates over the sources.
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