March 2, 2017

Structure and large-scale emission of the Galaxy

SPI reveals the origin of the hard X-ray emission of the Milky Way as the product of the interaction of cosmic rays with the Galaxy's interstellar radiation field.

The Galactic ridge is known to be an intense source of continuum hard X-ray and -ray emission. This interstellar emission was discovered by a rocket experiment in 1972 and has subsequently been observed from keV to MeV energies by all major astronomy satellites.

50-100 keV intensity sky map, including both sources and diffuse emission.
50-100 keV intensity sky map, including both sources and diffuse emission.

Thanks to the analysis of six years of INTEGRAL/SPI data (108 s), it has been possible to disentangle the different components of the "diffuse" emission, which results from the superposition of several physical processes: annihilation radiation, cosmic nuclear -ray lines, diffuse continuum due to interstellar emission, and unresolved sources.

Spectra of the different emission components in the central radian of the Galaxy

Spectra of the different emission components in the central radian
of the Galaxy (|l| > 30° and |b| > 15°).
SPI measurements: diffuse continuum emission (blue crosses)
and total emission of resolved sources (black squares).

    - Black line: power-law fitted to the resolved sources emission
    - thin blue dashed line: fitted to the total "diffuse emission"
    - magenta: annihilation radiation spectrum (line + positronium)
    - red: emission of low-energy "unresolved" sources.
    - Dark green line is the deduced continuum emission thought
    to be dominated by cosmic-rays (CRs) interacting in the
    interstellar medium (ISM).

    Taking advantage of the greatly increased observational data, the Galactic diffuse continuum hard X-ray spatial distribution and spectrum between 20 keV and 2.4 MeV could have been derived. The diffuse continuum seen with the Integral/SPI spectrometer is well modelled by a hard power-law spectral component. It has been identified with inverse-Compton emission from relativistic (GeV) cosmic-ray interstellar electrons on the microwave background and Galactic interstellar radiation field.

    The good agreement between measured and predicted diffuse continuum emission, using a cosmic-ray propagation code (GALPROP, Porter, Moskalenko, Strong et al., 2008), from keV to GeV energies suggests that the correct production mechanisms have been identified. In addition, this result shows the potential of SPI data to provide an indirect probe of the interstellar cosmic-ray electron distribution, in particular for energies below a few GeV.


    • L. Bouchet, A. Strong, T.A. Porter, I.V. Moskalenko, E. Jourdain and J.-P. Roques, "Diffuse emission measurement with INTEGRAL/SPI as probe of cosmic-ray electrons and positrons", ApJ, 739, 29, 2011
    • Porter, T.A., Moskalenko, I.V., Strong, A.W., Orlando E. and Bouchet, L.,"Inverse Compton origin of the hard X-ray emission from the Galactic Ridge", ApJ, 682, 400, 2008.
    • Bouchet, L., Jourdain, E., Roques, J. P., Strong, A., Diehl, R., Lebrun, F. and Terrier, R.,"INTEGRAL SPI All-Sky View in Soft Gamma Rays: Study of Point Source and Galactic Diffuse Emissions", ApJ, 679, 1315, 2008
    • Lebrun, F.; Terrier, R.; Bazzano, A.; Bélanger, G.; Bird, A.; Bouchet, L.; Dean, A.; Del Santo, M.; Goldwurm, A.; Lund, N.; Morand, H.; Parmar, A.; Paul, J.; Roques, J.-P.; Schönfelder, V.; Strong, A. W.; Ubertini, P.; Walter, R.; Winkler, C., "Compact sources as the origin of the soft gamma-ray emission of the Milky Way", Nature, Volume 428, Issue 6980, p. 293-296, 2004.