INTEGRAL supersedes COMPTEL in measuring the Galactic diffuse MeV emission
For more than 20 years, the Compton telescope, COMPTEL, onboard NASA's
Compton Gamma-Ray Observatory, CGRO, set the standard for measurements
of the extended emission of the Milky Way in the MeV range. This photon
energy range harbours many emission mechanisms, for example, the
annihilation of positrons in the interstellar medium, nuclear decays of
radioactive isotopes, or the Inverse Compton scattering of electrons off
the interstellar radiation field. These components are indicated in the
top panel, showing the diffuse Galactic emission spectrum with its
characteristic spectral features [1].
Cosmic-ray GeV electrons scatter off photons from star light in the
optical and infrared (IR), as well as the cosmic microwave background
(CMB). This scattering produces a smooth spectrum which peaks around 0.1
MeV for the CMB, around 0.5 MeV for IR, and potentially above 10 MeV for
optical light. Together this makes the smooth spectrum shown in the
zoomed-in panel [2]. Measuring the range between 0.5 MeV up to the limit
of INTEGRAL/SPI at 8 MeV therefore constrains the transport properties
of electrons as they scatter in IR and optical light.
With more than 16 years of data, SPI was able to set a new record in
determining the diffuse emission spectrum in the 0.5-8.0 MeV band. The
four coloured data points shown have a signal-to-noise ratio of 6 or
more, superseding the data quality and precision of COMPTEL (green). The
projections of the measured photons above the strong instrumental
background [3] is shown for the four data points surrounding the bottom
panel: there is clearly flux originating from the Galactic plane.
In this study, it was possible to test extreme assumptions on the
propagation of GeV electrons: it appears that one common diffusion
coefficient of 0.5 for the entire cosmic-ray electron spectrum best
matches the INTEGRAL/SPI data. Another important finding in the work is
that the absolute normalisation of most propagation models is a factor
of 2-3 below the measurements, opening up the possibility of even more
emission components such as unresolved point sources.
