The different emission components at hard X-ray and soft gamma-ray energies
This graph shows the various contributions to the total emission observed at hard X-ray and soft gamma-ray energies with the Spectrometer on board INTEGRAL (SPI) in the central radian of the Galaxy (|l| < 30° and |b| < 15°).
The data points shown in black correspond to the contribution due to point sources, which is mostly dominant at the lowest energies probed by INTEGRAL; the data points shown in blue, instead, correspond to the diffuse emission detected with INTEGRAL, which becomes dominant at energies above 100 keV. The solid black line and the dotted blue curve are fits to the point source and diffuse emission data points, respectively.
The coloured curves show model predictions for the various components into which the diffuse emission is broken down. Each component corresponds to a different physical mechanism and contributes most strongly at a typical range of energies.
The red curve at low energies identifies the contribution due to the superposition of many unresolved faint sources: stars with very hot coronae and cataclysmic variable stars.
The green line shows the emission produced via Inverse Compton (IC) scattering: collisions between highly energetic electrons (or positrons) and low-energy photons present in interstellar space, which result in the electrons transferring part of their energy to the photons, thus 'boosting' them to X- and gamma-ray wavelengths.
The magenta line shows the characteristic emission feature due to the annihilation of positrons. When positrons and electrons collide, two things may happen: they may destroy each other immediately, releasing a pair of photons each with an energy of 511 keV (visible as a pronounced peak); alternatively, they may create an unstable and short-lived two-particle system called positronium, which soon decays into two or more photons, producing a distinctive continuum emission spectrum up to 511 keV.
At energies above 1000 keV, the data also exhibit three peaks that are characteristic emission features of the decay of two unstable isotopes: iron (60Fe), at energies of 1173 keV and 1332 keV, and aluminium (26Al), at 1809 keV. This indicates the presence of these radioactive nuclei – the products of recent nucleosynthesis in supernova explosions – throughout the diffuse interstellar medium of the Milky Way. As a visual aid, the energies corresponding to these three emission lines are marked with vertical lines in the graph.