INTEGRAL results on the electron-positron annihilation radiation and X-ray & Gamma-ray diffuse emission of the Milky Way

Abstract

This review summarizes INTEGRAL results on two topics: the electron-positron annihilation line and X-ray & Gamma-ray diffuse emission of the Milky Way.

The electron-positron annihilation line at 511 keV is the most prominent spectral feature in the gamma-ray spectrum of the Milky Way. From the observational perspective, INTEGRAL has already provided constraints on the total flux, morphology of the annihilation line distribution, the spectral shape of the line and the strength of the three-photon annihilation continuum. In particular, the most salient morphological feature in the all-sky map of the annihilation emission based on INTEGRAL data is the so-called ”Bulge” component, with the characteristic size of $\sim 6-{10}^{\circ }$ and the positrons’ annihilation rate of $\sim {10}^{43}{s}^{-1}$. A more extended ”Disc” component is also present, although its total luminosity is model dependent. The brightness of the Bulge component compared to the Disc is in contrast with other multi-wavelength images of the Milky Way. The annihilation spectrum consists of a line centered at 511 keV and the ortho-positronium continuum. The strength of the latter indicates that the majority of annihilations go via the positronium formation channel. The shape of the annihilation spectrum is consistent with the assumption that most of the positrons annihilate in a warm and partly ionized medium, although more complicated scenarios are also possible. From the theoretical point of view, a successful model should answer three main questions: (i) physical mechanism(s) responsible for production of positrons, (ii) positrons spatial migration (if any) from the production sites, and (iii) physics of annihilation. Remarkably, despite significant progress provided by INTEGRAL in the characterization of the Milky Way annihilation emission, the origin of positrons remains an open question. The essence of the problem is the abundance of positron production channels and the uncertainty in the distance positrons can travel before annihilation.

The spectral-imaging mapping of the Milky Way by INTEGRAL provides important constraints on the nature of the Galactic diffuse continuum hard X-rays and soft gamma-rays in the 20 keV – 2  MeV band. Below  ~ 60 keV, numerous unresolved objects (accreting white dwarfs) dominate the flux, but their contribution fades away at higher energies. Models of cosmic-ray induced emission suggest that the dominant diffuse component above  ~ 60 keV (excluding annihilation emission) is inverse Compton scattering from GeV electrons on interstellar radiation fields. Non-thermal bremsstrahlung contributes at a lower level. These models are consistent with the continuum spectrum observed by INTEGRAL and COMPTEL.